stars - Science-Education-Research

The supernova and the quasar: the hungriest guy in the universe followed the ultimate toaster

Keith S. Taber

Communicating astronomical extremes

I was recently listening to a podcast of an episode of a science magazine programme which included two items of astronomy news, one about a supernovae, the next about a quasar. I often find little snippets in such programmes that I think work making a note of (quite a few of the analogies, metaphors and similes – and anthropomorphisms – reported on this site come from such sources). Here, I went back and listened to the items again, and decided the discussions were rich enough in interesting points to be worth taking time to transcribe them in full. The science itself was fascinating, but I also thought the discourse was interesting from the perspective of communicating abstract science. ¹

I have appended my transcriptions below for anyone who is interested – or you can go and listen to the podcast (episode 'Largest ever COVID safety study' of the BBC World Service's Science in Action).

Space, as Douglas Adams famously noted, is big. And it is not easy for humans to fully appreciate the scales involved – even of say, the distance to the moon, or the mass of Jupiter, let alone beyond 'our' solar system, and even 'our' galaxy. Perhaps that is why public communication of space science is often so rich with metaphor and other comparisons?

When is a star no longer a star (or, does it become a different star?)

One of the issues raised by both items is what we mean by a star. When we see the night sky there are myriad visible sources of light, and these were traditionally all called stars. Telescopes revealed a good many more, and radio telescopes other sources that could not detected visually. We usually think of the planets as being something other than stars, but even that is somewhat arbitrary – the planets have also been seen as a subset of the stars – the planetary or wandering stars, as opposed to the 'fixed' stars.

At one time it was commonly thought the fixed stars were actually fixed into some kind of crystalline sphere. We now know they are not fixed at all, as the whole universe is populated with objects influenced by gravity and in motion. But on the scale of a human lifetime, the fixed stars tend to appear pretty stationary in relation to one another, because of the vast distances involved – even if they are actually moving rather fast in human terms.

Wikipedia (a generally, but not always, reliable source) suggests "a star is a luminous spheroid of plasma held together by self-gravity" – so by that definition the planets no longer count as stars. What about Supernova 1987A (SN 1987A) or quasar J0529-4351?

"This image, taken with Hubble's Wide Field and Planetary Camera 2in 1995, shows the orange-red rings surrounding Supernova 1987A in the Large Magellanic Cloud. The glowing debris of the supernova explosion, which occurred in February 1987, is at the centre of the inner ring. The small white square indicates the location of the STIS aperture used for the new far-ultraviolet observation. [George Sonneborn (Goddard Space Flight Center), Jason Pun (NOAO), the STIS Instrument Definition Team, and NASA/ESA]" [Perhaps the supernova explosion did not actually occur in February 1987]

Supernova 1987A is so-called because it was the first supernova detected in 1987 (and I am old enough to remember the news of this at the time). Stars remain in a more-or-less stable state (that is, their size, temperature, mass are changing, but, in proportional terms, only very, very slowly²) for many millions of years because of a balance of forces – the extremely high pressures at the centre work against the tendency of gravity to bring all the matter closer together. (Imagine a football supported by a constant jet of water fired vertically upwards.) The high pressures inside a star relate to a very high temperature, and that temperature is maintained despite the hot star radiating (infra-red, visible, ultraviolet…) into space ³ because of the heating effect of the nuclear reactions. There can be a sequence of nuclear fusion reactions that occur under different conditions, but the starting point and longest-lasting phase involves hydrogen being fused into helium.

The key point is that when the reactants ('fuel') for one process have all (or nearly all) been reacted, then a subsequent reaction (fusing the product of a previous phase) becomes more dominant. Each specific reaction releases a particular amount of energy at a particular rate (just as with different exothermic chemical reactions), so the star's equilibrium has to shift as the rate of energy production changes the conditions near the centre. Just as you cannot run a petrol engine on diesel without making some adjustments, the characteristics of the star change with shifts along the sequence of nuclear reactions at its core.

These changes can be quite dramatic. It is thought that in the future the Earth's Sun will expand to be as large as the Earth's orbit – but that is in the distant future: not for billions of years yet.

Going nova

Massive stars can reach a point when the rate of energy conversion drops so suddenly (on a stellar scale) that there is a kind of collapse, followed by a kind of explosive recoil, that ejects much material out into space, whilst leaving a core of condensed nuclear matter – a neutron star. For even more massive stars, not even nuclear material is stable, as there is sufficient gravity to even collapse nuclear matter, and a black hole forms.

It was such an explosion that was bright enough to be seen as a 'nova' (new star) from Earth. Astronomers have since been waiting to find evidence of what was left behind at the location of the explosion – a neutron star, or a black hole. But of course, although we use the term 'nova', it was not actually a new star, just a star that was so far away, indeed in another galaxy, that it was not noticeable – until it exploded.

Dr. Olivia Jones (from the UK Astronomy Technology Centre at The Royal Observatory, Edinburgh) explained that neutron stars form from

"…really massive stars like Supernova 1987A or what it was beforehand, about 20 times the mass of a Sun…

So, what was SN 1987A before it went supernova? It was already a star – moreover, astronomers observing the Supernova were studying

…how it evolves in real time, which in astronomy terms is extremely rare, just tracing the evolution of the death of a star…

So, it was a star; and it died, or is dying. (This is a kind of metaphor, but one that has become adopted into common usage – this way of astronomers talking of stars as having births, lives, careers, deaths, has been discussed here before: 'The passing of stars: Birth, death, and afterlife in the universe.') What once was the star, is now (i) a core located where the star was – and (ii) a vast amount of ejected material now "about 20 light years across" – so spread over a much larger volume than our entire solar system. The core is now a "neutron star [which] will start to cool down, gradually and gradually and fade away".

So, SN 1987A was less a star, than an event: the collapse of a star and its immediate aftermath. The neutron star at is core is only part of what is left from that event (perhaps like a skeleton left by a deceased animal?) Moreover, if we accept Wikipedia's definition then the neutron star is not actually a star at all, as instead of being plasma (ionised gas – 'a phase of matter produced when material is too hot to exist as, what to us seems, 'normal' gas) it comprises of material that is so condensed that it does not even contain normal atoms, just in effect a vast number of atomic nuclei fused into one single object – a star-scale atomic nucleus. So, one could say that SN 1987A was no so much a star, as the trace evidence of a star that no longer existed.

And SN 1987A is not alone in presenting identity problems to astronomers. J0529-4351 is now recognised as being possibly the brightest object in the sky (that is, if we viewed them all from the same distance to give a fair comparison) but until recently it was considered a fairly unimpressive star. As doctoral researcher Samuel Lai (Research School of Astronomy and Astrophysics, Australian National University) pointed out,

this one was mis-characterised as a star, I mean it just looks like one fairly insignificant point, just like all the other ones, right, and so we never picked it up as quasar before

But now it is recognised to only appear insignificant because it is so far away – and it is not just another star. It has been 'promoted' to quasar status. That does not mean the star has changed – only our understanding of it.

But is it a star at all? The term quasar means 'quasi–stellar object', that is something that appears much like a star. But, if J0529-435 is a quasar, then it consists of a black hole, into which material is being attracted by gravity in a process that is so energetic that the material being accreted is heated and radiates an enormous amount of energy before it slips from view over the black hole's event horizon. That material is not a luminous spheroid of plasma held together by self-gravity either.

This video from the European Southern Observatory (ESO) gives an impression of just how far away (and so how difficult to detect) the brightest object in the galaxy actually is.

These 'ontological' questions (how we classify objects of different kinds) interest me, but for those who think this kind of issue is a bit esoteric, there was a great deal more to think about in these item.

"A long time ago, in a galaxy far, far away"

For one thing, it was not, as presenter Roland Pease suggested, strictly the 37th anniversary of the SN 1987A – at least not in the sense that the precursor star went supernovae 37 years ago. SN 1987A is about 170 000 light years away. The event, the explosion, actually occurred something like 170 000 years before it could be detected here. So, saying it is the 37th anniversary (rather than, perhaps, the 170 037^th anniversary ⁴) is a very anthropocentric, or, at least, geocentric take on things.

Then again, listeners are told that the supernova was in "the Large Magellanic Cloud just outside the Milky Way galaxy" – this is a reasonable description for someone taking an overview of the galaxies, but there is probably something like 90,000 light-years between what can be considered the edges of our Milky Way galaxy and this 'close by' one. So, this is a bit like suggesting Birmingham is 'just outside' London – an evaluation which might make more sense to someone travelling from Wallaroo rather than someone from Wolverhampton.

It is all a matter of scale. Given that the light from J0529-4351 takes about twelve billion years to reach us, ninety thousand light years is indeed, by comparison, just outside our own galaxy.

But the numbers here are simply staggering. Imagine something the size of a neutron star (whether we think it really is a star or not) that listeners were informed is "rotating…around 700 times a second". I do not think we can actually imagine that (rather than conceptualise it) even for an object the size of a pin – because our senses have not evolved to engage with something spinning that fast. Similarly, material moving around a black hole at tens of thousands of kilometres per second is also beyond what is ready visualisation. Again, we may understand, conceptually, that "the neutron star is over a million degrees Celsius" but this is just another very big number way that is outside any direct human experience.

Comparisons of scale

Thus the use of analogies and other comparisons to get across something of the immense magnitudes involved:

"If you think of our Sun as a tennis ball in size, the star that formed [SN] 87A was about as big as the London Eye."
"A teaspoon of this material, of a neutron star, weighs about as much as Everest"
the black home at the centre of the quasar acquires an entire Sun worth of mass every single day
the black hole at the centre of the quasar acquires the equivalent of about four earths, every single second
the quasar is about five hundred trillion times brighter than the Sun, or equivalent to about five hundred trillion suns

Often in explaining science, everyday objects (fridges, buses – see 'Quotidian comparisons') are used for comparisons of size or mass – but here we have to shift up to a mountain. The references to 'every single day' and 'every single second' include redundancy: that is, no meaning is lost by just saying 'every day' and 'every second' but the inclusion of 'single' acts a kind of rhetorical decoration giving greater emphasis.

Figurative language

Formal scientific reports are expected to be technical, and the figurative language common in most everyday discourse is, generally, avoided – but communication of science in teaching and to the public in journalism often uses devices such as metaphor and simile to make description and explanations seem more familiar, and encourage engagement.

Of course, it is sometimes a matter of opinion whether a term is being used figuratively (as we each have our own personal nuances for the meanings of words). Would we really expect to see a 'signature' of a pulsar? Not if we mean the term literally, a sign made by had to confirm identify, but like 'fingerprint' the term is something of a dead metaphor in that we now readily expect to find so-called 'signatures' and 'fingerprints' in spectra and D.N.A. samples and many other contexts that have no direct hand involvement.

Perhaps, more tellingly, language may seem so fitting that it is not perceived as figurative. To describe a supernova as an 'evolving fireball' seems very apt, although I would pedantically argue that this is strictly a metaphor as there is no fire in the usual chemical sense. Here are some other examples I noticed:

"we have been searching for that Holy Grail: has a neutron star formed or has a black hole been left behind"
"the quasar is not located in some kind of galactic desert"
there is a "storm, round the black hole"
"the galaxies are funnelling their material into their supermassive black hole"
"extraordinarily hot nuclear ember"
"a dense dead spinning cinder"
"the ultimate toaster"

Clearly no astronomer expects to find the Holy Grail in a distant galaxy in another part of the Universe (and, indeed, I recently read it is in a Museum in Ireland), but clearly this is a common idiom to mean something being widely and enthusiastically sought.⁵

A quasar does exist in a galactic desert, at least if we take 'desert' literately as it is clearly much too hot for any rain to fall there, but the figurative meaning is clear enough. The gravitational field of the black hole causes material to fall into it – so although the location, at the centre of a galaxy (not a coincidence, of course), means there is much material around, I was not sure how the galaxy was actively 'funnelling' material. This seems a bit light suggesting spilt tea is being actively thrown to the floor by the cup.

A hot ember or cinder may be left by a fire that has burned out, and one at over a million degrees Celsius might indeed 'toast' anything that was in its vicinity. So, J0529-4351 may indeed be the ultimate toaster, but not in the sense that it is a desirable addition to elite wedding lists.

Anthropomorphism

Anthropomorphism is a particular kind of metaphor that describes non-human entities as if they had the motivations, experiences, drives, etc., of people. The references to dying stars at least suggest animism (that the stars are in some sense alive – something that was once commonly believed ⁶), but there are other examples (that something is 'lurking' in the supernova remnant) that seem to discuss stellar entities as if they are deliberate agents like us. In particular, a black hole acquiring matter (purely due to its intense gravitational field) was described as feeding:

quasars are basically supermassive black holes just swallowing up all the stars and rubbish around
a quasar is feeding from the accretion disc
a monstrous black hole gobbling up anything within reach
just sat [sic] there, gobbling up everything around it
it has to have been feeding for a very, very long time
it will eat about four of those earths, every single second
in a particularly nutritious galaxy
a quasar that has been declared the hungriest object in the universe

There is clearly some kind of extended metaphor being used here.

Feeding frenzy?

The notion of a black hole feeding on surrounding material seems apt (perhaps, again, because the metaphor is widely used, and so familiar). But there seems a lot more 'negative analogy' than 'positive analogy: that is the ways in which (i) a black hole acquires matter, and (ii) an organism feeds, surely have more points of difference than similarity?

For advanced animals like mammals, birds, fish, snails and the like, feeding is a complex behaviour that usually involves active searching for suitable food, whereas the black hole does not need to go anywhere.
The animal has specialist mouth-parts and a digestive system that allows it to break apart foodstuff. The black-hole just tears all materials apart indiscriminately:"it's just getting chopped up, heated up, shredded".
The organism processes the foodstuff to release specific materials (catabolism) and then processes these is very specific ways to support is highly complex structure and functioning, including the building up of more complex materials (anabolism). The black hole is just a sink for stuff.
The organism takes in foodstuffs to maintain equilibrium, and sometimes to grow in very specific, highly organised ways. The black hole just gets more massive.

A black hole surely has more in keeping with an avalanche or the collapse a tall building than feeding?

One person's garbage…?

Another feature of the discourse that I found intriguing was the relative values implicitly assigned to different material found in distant space. There is a sense with SN 1987A that, after the explosion, the neutron star in some sense deserves to be considered the real remnant of the star, whilst the other material has somehow lost status by being ejected and dispersed. Perhaps that makes sense given that the neutron star remains a coherent body, and is presumably (if the explosion was symmetrical) located much where the former star was.

But I wonder if calling the ejected material – which is what comprises the basis of "an absolutely stunning supernova [which is] beautiful" – as 'debris' and 'outer debris"? Why is this material seen as the rubbish – could we not instead see the neutron star as the debris being the inert residue left behind when the rest of the star explored in a magnificent display? (I am not suggesting either should be considered 'debris', just playing Devil's advocate.)

Perhaps the reference to being able to "isolate the core where the explosion was from the rest of the debris" suggests all that is left is debris of a star, which seems fairer; but the whole history of the universe, as we understand it, involves sequences of matter changing forms quite drastically, and why should we value one or some of these successive phases as being the real product of cosmic evolution (stars?) and other phases as just rubbish? This is certainly suggested by the reference to "supermassive black holes in the middle of a galaxy … swallowing up all the stars and rubbish".

Let's hear it for the little guys

Roland Pease's analogy to "the muck at the bottom of your sink going down into the blender" might also suggest a tendency to view some astronomical structures and phenomenon as intrinsically higher status (the blender/black hole) than others (clouds of dust, or gas or plasma – the muck). Of course, I am sympathetic to the quest to better understand "these guys" (intense quasars already formed early in the universe), but as objectively minded scientists we should be looking out for the little guys (and gals) as well.

Appendix A: "the star hidden in the heart of [the] only supernova visible from Earth"

"If you are listening to this live on Thursday, then you're listening to the 37th anniversary of the supernova 1987A, the best view astronomers have had of an exploding star in centuries, certainly during the modern telescope era. So much astrophysics to be learned.

All the indications were, back then, that amidst all the flash and glory, the dying star should have given birth to a neutron star, a dense dead spinning cinder, that would be emitting radio pulses. So, we waited, and waited…and waited, and still there's no pulsing radio signal.

But images collected by the James Webb telescope in its first weeks of operation, peering deep into the ejecta thrown out by the explosion suggest there is something powerful lurking beneath.
Olivia Jones is a James Webb Space Telescope Fellow at Edinburgh University and she helped in the analysis."

"87A is an absolutely stunning supernova , it's beautiful, and the fact that you could see it when it first exploded with the naked eye is unprecedented for such an object in another galaxy like this.

We have been able to see how it evolves in real time, which in astronomy terms is extremely rare, just tracing the evolution of the death of a star. It's very exciting."

"I mean the main point is the bit which we see when the star initially explodes , we see all the hot stuff which is being thrown out into space, and then you've got this sort of evolving fireball which has been easiest to see so far."

"Yes, what see initially is the actual explosion of the star itself right in the centre. What happens now is then we had a period of ten years when you couldn't actually see very much in the centre. You needed these new telescopes like Webb and JWST to see the mechanics of the explosion and then, key to this is what was left behind, and we have been searching for that Holy Grail: has a neutron star formed or has a black hole been left behind at the centre of this explosion. And we've not seen anything for a very long time."

"And this neutron star, so this is the bit where the middle of the original star which at the ends of its life is mostly made of iron, just gets sort of crushed under it's own weight and under the force of the explosion to turn itself entirely into this sort of ball of neutron matter."

"Yeah, it's the very, very core of the star. So the star like the Sun, right in the centre is a very dense core, but really massive stars like Supernova 1987A or what it was beforehand, about 20 times the mass of a Sun.

If you think of our Sun as a tennis ball in size, the star that formed 87A was about as big as the London Eye. So it's a very massive star. The pressure and density right in the centre of that star is phenomenal. So, it creates this really, really, compact core. A teaspoon of this material, of a neutron star, weighs about as much as Everest. So, it's a very, a very dense, very heavy, core that is left behind."

"These were the things which were first detected in the 1960s, because they have magnetic fields and they rotate, they spin very fast and they cause radio pulsations and they're called pulsars. so When the supernova first went off I know lots of radio astronomers were hoping to see those radio pulsations from the middle of this supernova remnant."

"Yes. So, we know really massive stars will form a black hole in the centre, 30, 40, 50 solar masses will form a black hole when it dies. Something around 20 solar masses you'd expect to form a neutron star, and so you'd expect to see these signatures, like you said, in the radiowaves or in optical light of this really fastly rotating – by fastly rotating it can be around 700 times a second – but you would expect to see that signature or some detection of that. But even with all these telescopes – with the radio telescopes, X-ray observatories, Hubble – we've not seen that signature, before and so we are wondering, has a black hole been formed? We've seen neutrinos, so we thought the neutron star had formed, but we've not had that evidence before now."

"So, as I understand it, what your research is doing is showing that there's some unexplained source of heat in the middle of the debris that's been thrown out, and that's what your associating which what ought to be a neutron star in the middle, is that roughly speaking the idea?"

"So, the wonderful thing thing about the Webb telescope, you can see at high resolution both the ring, the outer debris of the star, and right at the very centre where the explosion was, but it's not just images we take, so it's not just taking a photograph, we also have this fantastic instrument or two instruments, called spectrographs, which can break down light into their individual elements, so very small wavelengths of light, it's like if you want to see the blue wavelength or the red wavelength, but in very narrow bands."

"And people may have done this at school when they threw some salt into a Bunsen burner and saw the colours, it's that kind of analysis?"

"Yes. And so what we see where the star was and where it exploded was argon and sulphur, and we know that these needed an awful lot of energy, to create these, and I mean a lot, of energy. And the only thing that can be doings this, we compared to many different kinds of scenarios, is a neutron star."

"So this is basically an extraordinarily hot nuclear ember, that's sort of sitting in the middle."

"Yes, right in the middle and you can see this, cause Supernova 1987A is about 20 light years across, in total, and we can isolate the core where the explosion was from the rest of the debris in this nearby galaxy, which I think is fantastic."

"Do you know how hot the surface of this star is and is it just sort of the intense heat, X-ray heat I imagine, that's coming off, that's causing all this radiation that you're seeing."

"I hope you are ready for a very big number."

"Go on."

"The neutron star is over a million degrees Celsius."

"And so, that's just radiating heat, is it, from, I mean this is like the ultimate toaster?"

"Yes, so what eventually will happen over the lifetime of the universe is this neutron star will start to cool down, gradually and gradually and fade away. But that'll be many, many billions of years from now.

What we currently have now is one of the hottest things you can imagine, in a very small location, heating up all its surroundings. I would not want to be anywhere nearby there."
Roland Pease interviewing Dr. Olivia Jones (Edinburgh University)

Appendix B: "possibly the brightest object in our universe"

"Now 1987A was, briefly, very bright. Southern hemisphere astronomy enthusiasts could easily spot it in the Large Magellanic Cloud just outside [sic] the Milky Way galaxy. But it was nothing like as bright as JO529-4351 [J0529-4351], try memorising that, its a quasar twelve or so billion light years away that has been declared the brightest object in the universe and the hungriest. At first sight, it's an anonymous, unremarkable spot of light of trillions on [sic] an astronomical photo. But, if you are an astronomer who knows how to interpret the light, as Samual Lai does, you will find this is a monstrous black hole gobbling up anything within reach. Close to the edge of all that we can see."

"So this quasar is a record breaking ultra-luminous object, in fact it is the most luminous object that we know of in the universe. Its light has travelled twelve billion years to reach us, so it's incredibly far object, but it's so intrinsically luminous that it appears bright in the sky."

"And as I understand it, you identified this as being a very distant and bright object pretty recently though you have gone back through the catalogues and its was this insignificant speck for quite a long time."

"Yes, indeed. In fact we were working on a survey of bright quasars, so we looked at about 80% of the sky using large data sets from space satellites. Throughout our large data sets, this one was mis-characterised as a star, I mean it just looks like one fairly insignificant point, just like all the other ones, right, and so we never picked it up as quasar before. Nowadays we are in the era of extremely astronomical, pardon the pun, data sets where in order to really filter thorough them we have these classification algorithms that we use. So, we have the computer, look at the data set, and try to learn what we are looking at, and pick out between stars and quasars."

"Now, is it also interesting, they were discovered about sixty years ago, the first quasars. These are basically supermassive black holes in the middle of a galaxy that's just swallowing up all the stars and rubbish just around it, and that's the bit that for you is quite interesting in this instance?"

"Yes, exactly, and the quasar owes its luminosity to the rate at which it is feeding from this accretion disc, this material that's swirling around, like a storm, with the black hole being the eye of the storm."

"I mean, I think of it as being a bit like the muck at the bottom of your sink going down into the blender at the bottom, it's just getting chopped up, heated up, shredded, and, I mean what sort of temperatures are you talking about? What, You know, what kind of energy are you talking about being produced in this system?"

"Yes ,so the temperatures in the accretion disc easily go up to tens of thousands of degrees, but talking about brightness, the other way that we like to measure this is in terms of the luminosity of the Sun, which gives you are sense of scale. So, this quasar is about five hundred trillion times brighter than the Sun, or equivalent to about five hundred trillion suns."

"And it's been doing this sort of constantly, or for really for a long time, I mean it's just sat there, gobbling up everything around it?"

"Yeah, I mean the mass of the quasar is about 17 billion solar masses, so in order to reach that mass it has to have been feeding for a very, very long time. We work it out to be about one solar mass per day, so that's an entire Sun worth of mass every single day. Or if you like to translate that to more human terms, if you take the Earth and everybody that's on it, and you add up all of that mass together, it will eat about four of those earths, every single second."

"I suppose what I find gob-smacking about this is (a) the forces, the gravitational forces presumably involved in sweeping up that amount of material, but (b) it must be an incredibly busy place – it can't be doing this in some kind of galactic desert."

"Yes, indeed, I mean these quasars, these super-massive black holes are parts of their galaxies, right, they're always in the nuclear regions of their host galaxies, and in some way the galaxies are funnelling their material into their supermassive black hole."

"But this one must be presumably a particularly, I don't know, nutritious galaxy, I guess. It is so far away, you can't make out those kinds of details."

"We can however make out that some of that material moving around, inside the storm, round the black hole, their dynamics are such that their velocities reach up to tens of thousands of kilometres per second."

"Why are you looking for then? Is it because you just want to break records – I'm sure it's not. Or is it, that you can see these things a long way away? Is it, it tells you about the history of galaxies?"

"I mean we can learn a lot about the universe's evolution by looking at the light from the quasars. And in fact, the quasar light it tells you a lot about not just the environment that the quasar resides in, but also in anything the quasar light passes through. So, you can think of this, lights from the quasar, as a very distant beacon that illuminates information about everything and anything in between us and the quasar."

"I mean the thing that I find striking is, if I've read the numbers right, this thing is so far away that the universe was about a billion years old. I mean I suppose what I'm wondering is how did a black hole becomes so massive so early in the universe?"

"Ah see, I love this question because you are reaching to the frontier of our current understanding, this is science going as we speak. We are running into an issue now that some of these black holes are so massive that there's not enough time in the universe, at the time that we observe them to be at, in order for them to have grown to such masses as they are seen to be. We have various hypotheses for how these things have formed, but at the moment we observe it in its current state, and we have to work backwards and look into the even older universe to try to figure out how these guys came to be."
Roland Pease interviewing Dr. Samuel Lai (Australian National University)

Notes

¹ Having been a science teacher, I find myself listening to, or reading, science items in the media at two levels

I am interested in the science itself (of course)
I am also intrigued by how the science is presented for the audience

So, I find myself paying attention to simplifications, and metaphors, and other features of the way the science is communicated.

Teachers will be familiar with this. Curriculum selects some parts of science and omits other parts (and there is always a debate to be had about wither the right choices are made about what to include, and what to omit). However, it is rare for the selected science itself to be presented in 'raw' form in education. The primary science literature is written by specialists for other specialists, and to a large extent by researchers for other researchers in the same field – and is generally totally unsuitable for a general audience.

Curriculum science is therefore an especially designed representation of the science intended to be accessible to learners at a particular stage in their education. Acids for twelve years olds or natural selection for fifteen year olds cannot be as complex, nuanced and subtle as the current state of the topic as presented in the primary literature. (And not just because of the level f presentation suitable for learners, but also because in any live field, the work at the cutting edge will by definition be inconsistent across studies as this is just where the experts are still trying to make the best sense of the available evidence.)

The teacher then designs presentations and sequences of learning activities to engage particular classes of learners, for often teaching models and analogies and the like are needed as stepping stones, or temporary supports, even to master the simplified curriculum models set out as target knowledge. Class teaching is challenging as every learner arrives with a unique nexus of background knowledge, alternative conceptions, relevant experiences, interests, vocabulary, and so forth. Every class is a mixed ability class – to some extent. The teacher has to differentiate within a basic class plan to try and support everyone.

I often think about this when I listen to or read science journalism or popular science books. At least the teacher usually knows that all the students are roughly the same age, and have followed more-or-less the same curriculum up to that point. Science communicators working with the public know very little about their audience. Presumably they are interested enough in the topic or science more generally to be engaging with the work: but likely of a very diverse age, educational level, background knowledge: the keen ten year old to the post-doctoral researcher; the retired engineer to the autistic child with an intense fascination in every detail of dinosaurs…

I often find myself questioning some of the simplifications and comparisons used on science reports in the media – but I do not underestimate the challenge of reporting on the latest findings in some specialist area of science in an 'academically honest' way (to borrow a term from Jerome Bruner) in a three minute radio slot or 500 words in a magazine. So, in that spirit, I was fascinated by the way in which the latest research into Supernova 1987A and J0529-4351 was communicated, at least as much as the science itself.

² That is, the flux of material emitted by our Sun, for example, is quite significant in human terms, but is minute compared to its total mass. Our sun has cooled considerably in the past few billions of years, but that's long time for it to change! (The Earth's atmosphere has also changed over the same time scale, which has compensated.)

³ Some very basic physics (Isaac Newton's law of cooling) tells us that objects radiate energy at a rate according to their temperature. Stars are (very large and) very hot so radiate energy at a high rate. An object will also be absorbing radiation – but the 'bath' of radiation it experiences depends on the temperature of its surroundings. A hot cup of coffee will cool as it is radiating faster than it is absorbing energy, because it is hotter than its surroundings. Eventually it will be as cool as the surroundings and will reach a dynamic equilibrium where it radiates and absorbs at the same rate. (Take the cooled cup of coffee into the sauna and it will actually get warmer. But do check health and safety rules first to see if this is allowed.)

The reference to how

"what eventually will happen over the lifetime of the universe is this neutron star will start to cool down, gradually and gradually and fade away. But that'll be many, many billions of years from now"

should be understood to mean that the cooling process STARTED as soon as there was no internal source of heating (form nuclear reactions or gravitational collapse) to maintain the high temperature; although the process will CONTINUE over a long period.

⁴ That weak attempt at humour is a variant on the story of the museum visitors who asked the attendant how old some ancient artefacts were. Surprised at the precision of the reply of "20 012 " years, they asked how the artefacts could be dated so precisely. "Well", the attended explained, "I was told they were twenty thousand years old when I started, and I've worked here for twelve years."

Many physics teachers will not find this funny at all, as it is not at all unusual for parallel mistakes to be made by students. (And not just students: a popular science book suggested that material in meteors can be heated in the atmosphere to temperatures of up to – a rather precise – 36 032 degrees! (See 'conceptions of precision').

⁵ The Holy Grail being the cup that Jesus is supposed to have used at the last supper to share wine with his disciples before he was arrested and crucified. Legend suggests it was also used to collect some of his blood after his execution – and that it was later brought to England (of all places) by Joseph of Arimathea, and taken to Glastonbury. The Knights of King Arthur's Round Table quested to find the Grail. It was seen as a kind of ultimate Holy Relic.

⁶ Greek and Roman cultures associated the planets (which for them included the Sun and Moon) with specific Gods. Many constellations were said to be living beings that have been placed in the heavens after time on earth. Personification of these bodies by referring to them in gendered ways ('he', 'she') still sometimes occurs.

Read about personification

In his cosmogony, Plato had the stars given a kind of soul. Whereas Aristlotle's notion of soul can be understood as being something that emerges from the complexity of organisation (in organisms), Plato did imply something more supernatural.

The complicated social lives of stars

Stealing, escaping, and blowing-off in space

Keith S. Taber

"After a lecture on cosmology and the structure of the solar system, James [William James] was accosted by a little old lady.

'Your theory that the sun is the centre of the solar system, and the earth is a ball which rotates around it has a very convincing ring to it, Mr. James, but it's wrong. I've got a better theory,' said the little old lady.

'And what is that, madam?' inquired James politely.

'That we live on a crust of earth which is on the back of a giant turtle.'

Not wishing to demolish this absurd little theory by bringing to bear the masses of scientific evidence he had at his command, James decided to gently dissuade his opponent by making her see some of the inadequacies of her position.

'If your theory is correct, madam,' he asked, 'what does this turtle stand on?'

'You're a very clever man, Mr. James, and that's a very good question,' replied the little old lady, 'but I have an answer to it. And it's this: The first turtle stands on the back of a second, far larger, turtle, who stands directly under him.'

'But what does this second turtle stand on?' persisted James patiently.

To this, the little old lady crowed triumphantly,

'It's no use, Mr. James – it's turtles all the way down.'
Ross, 1967, iv

"The Hindoos [sic] held the earth to be hemispherical, and to be supported like a boat turned upside down upon the heads of four elephants, which stood on the back of an immense tortoise. It is usually said that the tortoise rested on nothing, but the Hindoos maintained that it floated on the surface of the universal ocean. The learned Hindoos, however, say that these animals were merely symbolical, the four elephants meaning the four directions of the compass, and the tortoise meaning eternity." (*The Popular Science Monthly*, March, 1877; image via Wikipedia)

It's metaphors all the way down

A well-known paper in the journal 'Cognitive Science' is entitled 'The metaphorical structure of the human conceptual system' (Lakoff & Johnson, 1980). What the authors meant by this was that metaphor, or perhaps better analogy, was at the basis of much of our thinking, and so our language.

This links to the so-called 'constructivist' perspective on development and learning, and is of great significance in both the historical development of science and in science teaching and learning. Consider some of the concepts met in a science course (electron, evolution, magnetic flux, hysteresis, oxidation state, isomerism…the list is enormous) in comparison to the kind of teaching about the world that parents engage in with young children:

That is a dog
That is a tree
That is round
This is hot
This is aunty
etc.

Pointing out the names of objects is not a perfect technique – just as scientific theories are always underdetermined by the available data (it is always possible to devise another scheme that fits the data, even if such a scheme may have to be forced and convoluted), so the 'this' that is being pointed out as a tree could refer to the corpse of trees, or the nearest branch, or a leaf, or this particular species of plant, or even be the proper name of this tree, etc. ¹

Pointing requires the other person to successfully identify what is being pointed at
(Images by Joe {background} and OpenClipart-Vectors {figures} from Pixabay)

But, still, the 'this' in such a case is usually more salient than the 'this' when we teach:

This is an electron
This is reduction
This is periodicity
This is electronegativity
This is a food web
This is a ᴨ-bond
This is a neurotransmitter
etc.

Most often in science teaching we are not holding up a physical object or passing it around, but offering a 'this' which is at best a model (e.g., of a generalised plant cell or a human torso) or a complex linguistic structure (a definition in terms of other abstract concepts) or an abstract representation ('this', pointing to a slope of an a graph, is acceleration; 'this', pointing to an image with an arrangement of a few letters and lines, is a transition state…).

So, how do we bridge between the likes of dogs and trees on one hand and electrons and the strong nuclear force on the other (so to speak!)? The answer is we build using analogy and we talk about those constructions using a great deal of metaphor.² That is, we compare directly, or indirectly, with what we can experience. This refers to relationships as well as objects. We can experience being on top of, beneath, inside, outside, next to, in front of, behind, near to, a long way from (a building, say – although hopefully not beneath in that case), and we assign metaphorical relationships in a similar way to refer to abstract scenarios. (A chloroplast may be found in a cell, but is sodium found in (or on) the periodic table? Yes, metaphorically. And potassium is found beneath it!)

In a wall, the bricks on the top layer are supported by the bricks in the layer beneath – but those are in turn supported by those beneath them.

In building, we have to start at the foundations, and build up level by level. The highest levels are indirectly supported by the foundations.

(Image by OpenClipart-Vectors from Pixabay)

In science, we initially form formal concepts based on direct experience of the world (including experience mediated by our interventions, i.e., experiments), and then we build more abstract concepts from those foundational concepts, and then we build even more abstract concepts by combining the abstract ones. In the early stages we refine 'common sense' or 'life-world' categories into formal concepts so we can more 'tightly' (and operationally, through standard procedures) define what count as referents for scientific terms (Taber, 2013). So, the everyday phenomenon of burning might be reconceptualised as combustion: a class of chemical reactions with oxygen.

This is not just substituting a technical term, but also a more rigid and theoretical (abstract) conceptualisation. So, in the 'life-world' we might admit the effects of too much sunshine or contact with a strong acid within the class of 'burning' by analogy with the effect of fire (it hurts and damages the skin); but the scientific categorisation is less concerned with direct perception, and more with explanation and mechanism. So, iron burning in chlorine (in the absence of any oxygen) is considered combustion, but an acid 'burn' is not.

Combustion without oxygen: A Royal Society of Chemistry video demonstrating the reactions of iron with the halogens.

This is what science has done over centuries, and is also what happens in science education. So, one important tool for the teacher is concept analysis, where we check which prerequisite concepts need to be part of a student's prior learning before we introduce some new concept that is built upon then (e.g., do not try to teach mass spectroscopy before teaching about atomic structure, and do not teach about atomic structure before introducing the notion of elements; do not try to teach about the photoelectric effect to someone who does not know a little about the structure of metals and the nature of electromagnetic radiation.)

This building up of abstract concepts, one on another, is reflected in the density of metaphor we find in our language. (That is a metaphorical 'building', metaphorically placed one upon another, with a metaphorical 'density' which is metaphorically 'inside' the language and which metaphorically 'reflects' the (metaphorical) building process! You can 'see' (a metaphor for understand) just how extensive (oops, another metaphorical reference to physical space) this is. Hopefully, the (metaphorical) 'point' is (metaphorically) 'made', and so I am going to stop now, before this gets silly. ³

A case study of using language in science communication: the death of stars

Rather, I am going to discuss some examples of the language used in a single science programme, a BBC radio programme/podcast in the long-running series 'In Our Time' that took as its theme 'The Death of Stars'. The programme was hosted by Melvyn Bragg, and The Lord Bragg's guests were Professors Carolin Crawford (University of Cambridge), the Astronomer Royal Martin Rees (University of Cambridge) and Mark Sullivan (University of Southampton). This was an really good listen (recommended to anyone with an interest in astronomy), so I have certainly not picked it out to be critical, but rather to analyse the nature of some of the language used from the perspective of how that language communicates technical ideas.

An episode of 'In Our Time' on 'The Death of Stars'
"The image above is of the supernova remnant Cassiopeia A, approximately 10,000 light years away, from a once massive star that died in a supernova explosion that was first seen from Earth in 1690"

A science teacher may be familiar with stars being born, living, and dying – but how might a young learner, new to astronomical ideas, make sense of what was meant?
The passing of stars: birth, death, and afterlife in the universe

The lives and deaths of stars

Now there is already a point of interest in the episode title. Are stars really the kind of entities that can die? Does this mean they are living beings prior to death?

There are a good many references in the talk of these three astronomers in the episode that suggests that, in astronomy at least, stars do indeed live and die. That is, this does not seem to be consciously used as a metaphor – even if the terminology may have initially been introduced that way a long time ago. The programme offered so much material on this theme, that I have separated it out for a post of its own:

"So, in the language of astronomy, stars are born, start young, live; sometimes living alone but sometimes not, sometimes have complicated lives; have lifetimes, reach the end of their lives, and die, so, becoming dead, eventually long dead; and indeed there are generations of stars with life-cycles."
The passing of stars: birth, death, and afterlife in the universe

In this post I am going to consider some of the other language used.

Making the unfamiliar familiar

Language is used in science communication to the public, as it is in teaching, to introduce abstract technical ideas in ways that a listener new to the subject can make reasonable sense of. The constructivist perspective on learning tells us that meaning is not automatically communicated from speaker (or author or teacher) to listener (or reader or student). Rather, a text (spoken or written, or even in some other form – a diagram, a graph, a dance!) has to be interpreted, and this relies on the interpretive resources available to the learner. ⁴ The learner has to relate the communication to something familiar, and the speaker can help by using ways to make the new idea seem like something already familiar.

Read about constructivism in education

This is why it it is so common in communicating science to simplify, to use analogies and similes, to gesture, to use anthropomorphism and other narrative devices. There was a good deal of this in the programme, and I expect I have missed some examples. I have divided my examples into

simplifications: where some details are omitted so not to overburden the listener;
anthropomorphism: where narratives are offered such that non human entities are treated as if sentient actors, with goals, that behave deliberately;
analogies where an explicit comparison is made to map a familiar concept onto the target concept being introduced; ⁵
similes and metaphors: that present the technical material as being similar to something familiar and everyday.

Simplification

Simplification means ignoring some of the details, and offering a gloss on things. The details may be important, but in order to get across some key idea it is introduced as a simplification. Progress in understanding would involve subsequently filling in some details to develop a more nuanced understanding later.

In teaching there are dangers in simplification, as if the simplified idea is readily latched onto (e.g., there are two types of chemical bonds: ionic and covalent) it may be difficult later to shift learners on in their thinking. This may mean that there is a subtle balance to be judged between

giving learners enough time to become comfortable with the novel idea as introduced in a simplified form,

and

seeking to develop it out into a more sophisticated account before it become dogma.

In a one-shot input, such as a public lecture or appearance in the media, the best a scientist may be able to do is to present an account which is simple enough to understand, but which offers a sense of the science.

Simplification: all elements/atoms are formed in stars

When introducing the 'In Our Time' episode, Lord Bragg suggested that

"…every element in our bodies, every planet, was made in one of those stars, either as they burned, or as they exploded".

Clearly Melvyn cannot be an expert on the very wide range of topics featured on 'In our time' but relies on briefing notes provided by his guests. Later, in the programme he asks Professor Rees (what would clearly be considered a leading question in a research context!) "Is the sun recycled from previous dead stars?"

"Yes it is because we believe that all pristine material in the universe was mainly just hydrogen and helium, and all the atoms we are made of were not there soon after the big bang. They were all made in stars which lived and died before our solar system formed. And this leads to the problem of trying to understand more massive stars which have more complicated lives and give rise to supernovae…

The cloud from which our solar system formed was already contaminated by the debris, from earlier generations of massive stars which had lived and died more than say five billion years ago so we're literally the ashes of those long dead stars or if you are less romantic we're the nuclear waste from the fuel that kept those old stars shining."
Prof. Martin Rees

There is a potential for confusion here.

"…all the atoms we are made of were not there soon after the big bang. They were all made in stars which lived and died before our solar system formed…"

seems to be meant to convey something like

not all the atoms we are made of were there soon after the big bang.
[Some were, but the rest/others] were all made in stars which lived and died before our solar system formed…

A different interpretation (i.e., that all atoms/elements are formed in stars) might well be taken, given Lord Bragg's introductory comments.

Professor Rees referred to how "…the idea that the elements, the atoms we are made of, were all synthesised in stars…" first entered scientific discourse in 1946, due to Fred Hoyle, and to

"this remarkable discovery that we are literally made of the ashes of long dead stars"
Prof. Martin Rees

Before the first star formation, the only elements present in the universe were hydrogen and helium (and some lithium) and the others have been produced in subsequent high energy nuclear processes. Nuclear fusion releases energy when heavier nuclei are formed from fusing together lighter ones, up to iron (element 56).

Forming even heavier elements requires an input of energy from another source. It was once considered that exploding stars, supernovae, gave rise to the conditions for this, but recently other mechanisms have been considered: and Prof. Sullivan described one of these:"we think these combining neutron stars are the main sites where heavy elements like strontium or plutonium, perhaps even gold or silver, these kinds of elements are made in the universe in these neutron stars combining with each other".

A human body includes many different elements, though most of these in relatively small amounts. Well represented are oxygen, carbon, calcium, and nitrogen. These elements exist because of the processes that occur in stars. However, hydrogen is also found in 'organic' substances such as the carbohydrates, proteins, and fats found in the human body. Typically the molecules of these substances contain more hydrogen atoms than atoms of carbon or any other element.

substance	formula
glucose (sugar)	C₆H₁₂O₆
leucine (amino aid)	C₆H₁₃NO₂
leukotriene B4 (inflammatory mediator)	C₂₀H₃₂O₄
thymine (nucleobase)	C₅H₆N₂O₂
adreneline (hormone)	C₉H₁₃NO₃
insulin (hormone)	C₂₅₇H₃₈₃N₆₅O₇₇S₆
cholesterol (lipid)	C₂₇H₄₆O
cobalamin (vitamin B₁₂)	C₆₃H₈₈CoN₁₄O₁₄P

formulae of some compounds found in human bodies

The body is also said to be about 60% water, and water has a triatomic molecule: two hydrogen atoms to one of oxygen (H₂O). That is, surely MOST of "the atoms we are made of" are hydrogen, which were present in the universe before any stars were 'born'.

So, it seems here we have a simplification ("every element in our bodies…was made in one of those stars, either as they burned, or as they exploded"; "atoms we are made of … were all made in stars") which is contradicted later in the programme. (In teaching, it is likely the teacher would feel the need to draw the learner's attention to how the more detailed information was actually developing an earlier simplification, and not leave a learner to work this out for themselves.)

Simplification: mass is changed into energy

Explaining nuclear fusion, Prof. Crawford suggested that

"Nuclear fusion is when you combine nuclei of elements to form heavier elements, and when you do this there is a loss of mass, which is converted to energy which provides the thermal pressure and that is what counteracts the gravity and stalls the gravitational collapse."
Prof. Carolin Crawford

This seems to reflect a common alternative conception ('misconception') that, in nuclear processes, mass is converted to energy. This is often linked to Albert Einstein's famous equation E = mc².

Actually, as discussed before here, this is contrary to the scientific account. The equation presents an equivalence between mass and energy, but does not suggest they can be inter-converted. In nuclear fusion, the masses of the new nuclei are very slightly less than the masses of the nuclei which react to form them (the difference is known as the mass defect), but this is because this omits some details of the full description of the process. If the complete process is considered then there is no loss of mass, just a reconfiguration of where the mass can be located.

The formation of helium from hydrogen in a star

(Image source: Wikamedia Commons)

Although the ⁴He formed has slightly less mass than four ¹H; the positrons, neutrinos and gamma rays produced all have associated (energy and) mass, so that overall there is conservation of mass.

This is a bit like cooking some rice, and finding that when the rice is cooked the contents of the saucepan had slightly less weight than when we started – as some of the water we began with has evaporated and is no longer registering on our balance. In a similar way, if we consider everything that is produced in the nuclear process, then the mass overall is conserved.

As E = mc² can be understood to tell us that mass follows the energy (or vice versa) we should expect mass changes (albeit very, very small ones) whenever work is done: when we climb the stairs, or make a cup of tea, or run down a mobile 'phone 'battery' (usually a cell?) – but mass is always conserved when we consider everything involved in any process (such as how the 'phone very, very slightly warms -and so very marginally increases the mass of – the environment).

Read 'How much damage can eight neutrons do?'

Despite the scientific principles of conservation of energy and conservation of mass always applying when we make sure we consider everything involved in a process, I have mentioned on this site another example of an astrophysicist suggesting mass can be converted into energy: "an electron and the positron, and you put them together, they would annihilate…they would annihilate into energy" (on a different episode of 'In Our Time': come on Melvyn…we always conserve mass).

Read 'The missing mass of the electron'

Perhaps this is an alternative conception shared by some professional scientists, but I wonder if it sometimes seems preferably to tell the "mass into energy" narrative because it is simpler than having to explain the full details of a process – which is inevitably a more complex story and so will be more difficult for a novice to take in. After all, the "mass into energy" story is likely to seem to fit with a listener's interpretive resources, as E=mc² is such a famous equation that it can be assumed that it will be familiar to most listeners, even if only a minority will have a deep appreciation of how the equivalence works.

Anthropomorphic narratives

In science learning, anthropomorphism is (to borrow a much used metaphor) a double edged sword that can cut both ways. Teachers often find that using narratives that present inanimate entities which are foci of science lessons as if they are sentient beings with social lives and motivations engages learners and triggers mental images that a student can readily remember. So, students may recall learning about what happens at a junction in a circuit in terms of a story about an electron that had to make a decision about which way to go – perhaps she took one branch while her friend tried another? They recall that covalent bonds are the 'sharing' of electrons between atoms, and indeed that atoms want, perhaps even need, to fill their electron shells, and if they manage this they will be happy.

Read about anthropomorphism

The danger here is that for many students such narratives are not simply useful ways to get them thinking about the science concepts (weak anthropomorphism) but seem quite sufficient as the basis of explanations (strong anthropomorphism) – and so it may become difficult to shift them towards more canonical accounts. They will then write in tests that chemical reactions occur because the atoms want full shells, or that only one electron can be removed from a sodium atom because it then has a full shell. (That is, a force applied to an electron in an electric field is seen as irrelevant compared with the atom's desires. These are genuine examples reflecting what students have said.)

However, there is no doubt that framing scientific accounts within narratives which have elements of human experience as social agent does seem to help make these ideas engaging and accessible. Some such anthropomorphism is explicit, such as when gas molecules (are said to) like to move further apart, and some is more subtle by applying terms which would normally be used in relation to human experiences (not being bothered; chomping; escaping…).

What gravity did next

Consider this statement:

"All stars have the problem of supporting themselves against gravitational collapse, whether that is a star like our sun which is burning hydrogen into helium, and thus providing lots of thermal pressure to stop collapse, or whether it is a white dwarf star, but it does not have any hydrogen to burn, because it is an old dead star, fading away, so it has another method to stop itself collapsing and that is called degeneracy pressure. So, although a white dwarf is very dense, gravity is still trying to pull that white dwarf to be even denser and even denser."
Prof. Mark Sullivan

There is an explicit anthropomorphism here: from the scientific perspective gravity is not trying to pull the white dwarf to be even denser. Gravity does not try to do anything. Gravity is not a conscious agent with goals that it 'tries' to achieve.

However, there is also a more subtle narrative thread at work – that a star has the problem of supporting itself, and it seems that when its first approach to solving this problem fails, it has a fallback method "to stop itself collapsing". But the star is just a complex system where various forces act and so processes occur. A star is not the kind of entity that can have a problem or enact strategies to achieve goals. Yet, this kind of language seems to naturally communicate abstract ideas though embedding them within an accessible narrative.

Star as moral agents

In the same way, a star is not the type of entity which can carry out immoral acts, but

"A star like our sun will never grow in mass, because it lives by itself in space. But most stars in the universe don't live by themselves, they live in what are called binary systems where you have two stars orbiting each other, rather than just the single star that we have as the sun. They are probably born with different masses, and so they evolve at different speeds and one will become a white dwarf. Now the physics is a bit complicated, but what can happen, is that that white dwarf can steal material from its companion star."
Prof. Mark Sullivan

The meaning here seems very clear, but again there are elements of using an anthropomorphic narrative. For one star to steal material from another star, that material would have to first belong to that other star, and its binary 'partner' would have to deliberately misappropriate that material knowing it belongs to its 'neighbour' (indeed, "companion").

Such a narrative breaks down on analysis. If we were to accept that the matter initially belongs to the first star (leaving aside for the moment what kind of entities can be considered to own property) then given that the material in a star got to be there through mutual gravitational attraction, the only obvious basis for ownership is that that matter has become gravitationally bound as part of that star.

If we have no other justification than that (as in the common aphorism, possession is nine points of the law), then when the material is transferred to another star because its gravitational field gives rise to a net force causing the matter to become gravitationally bound to a different star, then we should simply consider ownership to have changed. There is no theft in a context where ownership simply depends on pulling with the greater force. Despite this, we readily accept an analogy from our more familiar human social context and understand that (in a metaphorical sense) one star has stolen from another!

Actually, theft can only be carried out by moral agents – those who have capacity to intend to deprive others of their property

"A person [sic] is guilty of theft if he dishonestly appropriates property belonging to another with the intention of permanently depriving the other of it; and "thief" and "steal" shall be construed accordingly"
U.K. Theft Act 1968

Generally, these days (though this was not always so), even non-human animals are seldom considered capable of being responsible for such crimes. Admittedly, the news agency Reuters reported that as recently as 2008 "A Macedonian court convicted a bear of theft and damage for stealing honey from a beekeeper", but this seems to have been less a judgement on the ability of the bear (convicted it its absence) to engage in ethical deliberation, and more a pragmatic move that allowed the bee-keeper to be awarded criminal damages for his losses.

But, according to astronomers, stars are not only involved in the petty larceny of illicitly acquiring gas, but observations of exoplanets suggests some stars may even commit more daring, large-scale, heists,

"fairly small rocky planets two or three times the mass of the earth, in quite tight orbits around their star and you can speculate that they were once giant planets like Jupiter that have had the outer gassy layers blasted off and you are left with the rocky core, or maybe those planets were stolen from another star that got too close"
Prof. Carolin Crawford

A ménage à trois?

And there were other suggestions of anthropomorphism. It is not only stars that "don't live by themselves" in this universe,

"Nickel-56 [⁵⁶Ni] is what's called an iron peak element, so it lives with iron and cobalt on the periodic table…"
Prof. Mark Sullivan

And, it is not only gravity which seems to have preferences:

"And like Mark has described with electrons not wanting to be squeezed, you have neutron degeneracy pressure. Neutrons don't like to be compressed, at some point they resist it."
Prof. Carolin Crawford

Neither electrons nor neutrons actually have any preferences: but this is an anthropomorphic metaphor that efficiently communicates a sense of the natural phenomena. 'Resist' originally had an active sense as in taking a stand, but today would not necessarily be understood that way. Wanting and liking (or not wanting and not liking), however, strictly only refer to entities that can have desires and preferences.

Navigating photons

Professor Rees explained why some imploding stars are not seen as very bright stars that fade over years, but rather observed through extremely intense bursts of high energy radiation that fade quickly,

"The energy in the form of ordinary photons, ordinary light, that's arisen in the centre of a supernova, diffuses out and takes weeks to escape, okay, but if the star is spinning, then it will be an oblate spheroid, it will have a minor axis along the spin axis, and so the easy way out is for the radiation not to diffuse through but to find the shortest escape route, which is along the spin axis, and I mention this because gamma ray bursts are … when a supernova occurs but because the original star was sort of flattened there is an easy escape route and all the energy escapes in jets along the spin axis and so instead of it diffusing out over a period of weeks, as it does in a supernova, it comes out in a few seconds."
Prof. Martin Rees

Again, the language used is suggestive. Radiation is not just emitted by the star, but 'escapes' (surely a metaphor?). The phrasing "an easy way out" implies something not being difficult. Inanimate entities like photons do not actually (literally) find anything difficult or easy. Moreover, the radiation might "find the shortest escape route": language that does not reflect a playing out of physical forces but an active search – only a being able to seek can find. Yet, again, the language supports an engaging narrative, 'softening' the rather technical story by subtly reflecting a human quest.

Professor Rees also referred to how,

"when those big stars face a crisis they blow off their outer layers"
Prof. Martin Rees

again using phrasing which seems to present the stars as deliberate actors – they actively "blow off" material when they "face a crisis". A crisis is (or at least was originally) a point where a decision needs to be made. A star does not reach the critical point where it reluctantly decides it needs to shed some material – but rather is subject to changing net forces as the rate of heat generation from nuclear processes starts to decrease.

A sense of anthropomorphic narrative also attaches to Professor Crawford's explanation of how more massive stars process material faster,

"…more massive stars … actually have shorter lifetimes …they have to chomp through their fuel supply so furiously that they exhaust it more rapidly
Prof. Carolin Crawford

'Chomping', a term for vigorous eating (biting, chewing, munching), is here a metaphor, as a star does not eat – as pointed out in the companion piece, nutrition is a characteristics feature of living things, but does not map across to stars even if they are described as being born, living, dying and so forth. To be furious is a human emotional response: stars may process their remaining hydrogen quickly, but there is no fury involved. Again, though, the narrative, perhaps inviting associated mental imagery, communicates a sense of the science.

Laid-back gas

Another example of anthropomorphism was

"…if you have a gas cloud that's been sitting out in space for billions of years and has not bothered to contract because it's been too hot or it's too sparse…"
Prof. Carolin Crawford

This is an interesting example, as Prof. Crawford explicitly explains here that the gas cloud has not contracted because of the low density of material (so weak gravitational forces acting on the particles) and/or the high temperature (so the gas comprises of energetic, so fast moving, particles), so the suggestion that the material cannot be bothered (implication: that the 'cloud' operates as a single entity, and is sentient if perhaps a little lazy) does not stand in place of a scientific explanation, but rather simply seems to be intended to 'soften' (so to speak) the technical nature of the language used.

Analogy

An analogy goes beyond a simile or metaphor because there is some kind of structural mapping to make it explicit in what way or ways the analogue is considered to be like the target concept. ⁵ (Such as when explaining mass defect in relation to the material lost from the saucepan when cooking rice!)

Read about science analogies

So, Prof. Rees suggests that scientists can test their theories about star 'life cycles' by observation, even though an individual star only moves through the process over billions of years, and uses an analogy to a more familiar everyday context:

"We can test our theories, not only because we understand the physics, but because we can look at lots of stars. It is rather like if you had never seen a tree before, and you wandered around in a forest for a day, you can infer the life cycles of trees, you'd see saplings and big trees, etcetera. And so even though our lifetime is minuscule compared to the lifetime of a stable star, we can infer the population and life cycles of stars observationally and the theory does corroborate that fairly well."
Prof. Martin Rees

This would seem to make the basis of a good teaching analogy that could be discussed with students and would likely link well with their own experiences.

The other explicit analogy introduced by Prof. Rees is one well-known to physics teachers (sometimes in an ice-skater variant),

"If a contracting cloud has even a tiny little bit of spin, if it is rotating a bit, then as it contracts, then just like the ballerina who pulls in her arms and spins faster, then the contracting cloud will start to spin faster…"
Prof. Martin Rees

Stellar similes

I take the difference between a simile and a metaphor as the presence of an explicit marker (such as '…as…',…like…') to tell the listener/reader that a comparison is being made – so 'the genome is the blueprint for the body' would be a metaphor, where 'the genome is like a blueprint for the body' would be a simile.

As if a black hole cuts itself off

So, when Professor Rees describes how a massive black hole forms, he uses simile (i.e., "…as if were…"),

"So, if a neutron star gets above that mass, then it will compress even further, and will become a black hole – it will go on contracting until it, as it were, cuts itself off from the rest of the universe, leaving a gravitational imprint frozen in the space that's left. It becomes a black hole that things can fall into but not come out."
Prof. Martin Rees

There is an element of anthropomorphic narrative (see above) again here, if we consider the choice of active, rather than passive, phrasing

…as it were, cuts itself off from the rest of the universe, compared with
…as it were, becomes cut off from the rest of the universe

This is presented as something the neutron star itself does ("it will compress…become a black hole – it will go on contracting until it, as it were, cuts itself off…") rather than a process occurring in/to the matter of which it is comprised.

As if galaxies drop over the horizon

Prof. Rees uses another simile, when talking of how the expansion of space means that in time most galaxies will disappear from view,

"All the more distant universe which astronomers like Mark [Sullivan] study, galaxies far away, they will all have expanded their distance from us and in effect disappeared over a sort of horizon and so we just wouldn't see them at all. They'd be too faint, rather like …an inside-out black hole as it were, but in this case they moved so far away that we can't see them any more …"
Prof. Martin Rees

The term horizon, originally referring to the extent of what is in sight as we look across the curved Earth, has become widely used in astronomical contexts where objects cease to be in sight (i.e., the event horizon of a black hole beyond which any light being emitted by an object will not be able to leave {'escape!'} the black hole because of the intense gravitation field), but here Prof. Rees clearly marks out for listeners ("…in effect…a sort of…") that he is making a comparison with the familiar notion of a horizon that we experience here on Earth.

There is another simile here, the reference to the expansion of space leading to an effect "rather like…an inside-out black hole as it were" – but perhaps that comparison would be less useful to a listener new to the topic as it uses a scientific idea rather than an everyday phenomenon as the analogue.

Through a glass onion darkly?

Another simile used by Professor Rees was a references to a "sort of onion skin structure". Now 'onion skin' sometimes refers to the hard, dry, outer material (the 'tunic') usually discarded when preparing the onion for a dish. To a science teacher, however, this is more likely to mean the thin layer of epithelial tissue that can be peeled from the scales inside the bulb. These scales, which are potentially the bases of leaves that can grow if the bulb is planted, are layered in the bulb.

The skin is useful in science lessons as it is a single layer of cells, that is suitable for students to dissect from the onion, and mount for microscopic examination – allowing them to observe the individual cells. There is something at least superficially analogous to this in stars. Observations of the Sun show that convection processes gives rise to structures referred to as convection 'cells'.

Convection 'cells' at the Sun's surface (Source: NASA)

Yet, when Professor Rees' simile is heard in context, it seems that this is not the focus of the comparison:

"…all the nuclear processes which would occur at different stages in the heavy stars…which have this sort of onion skin structure with the hotter inner layers"
Prof. Martin Rees

Very large stars that have processed much of their hydrogen into helium can be considered to have a layered structure where under different conditions a whole sequence of processes are occurring leading to the formation of successively heavier and heavier elements, and ultimately to a build-up of iron near the centre.

The onion model of the structure of a large star (original image by Taken from Pixabay)

When I heard the reference to the onion, this immediately suggested the layered nature of the onion bulb being like the structure of a star that was carrying out the sequence of processes where the products of one fusion reaction become the raw material for the next. Presumably, my familiarity with the layered model of a star led me to automatically make an association with onions which disregarded the reference to the skin. That is, I had existing 'interpretive resources' to understand why the onion reference was relevant, even though the explicit mention of the skin might make the comparison obscure to someone new to the science.

Metaphors – all the way back up?

Some metaphors can easily be spotted (if someone suggests mitochondria are the power stations of the cell, or a lion is King of the jungle), but if our conceptual systems, and our language, are built by layers of metaphor upon metaphor then actually most metaphors are dead metaphors.

That is, an original metaphor is a creative attempt to make a comparison with something familiar, but once the metaphor is widely taken up, and in time becomes common usage and so a part of standard language, it ceases to act as a metaphor and becomes a literal meaning.

This presumably is what has happened with the adoption of the idea that stars are born, live out their lives, and then die: originally it was a poetic use of language, but now among astronomers it reflects an expanded standard use of terms that were once more restricted (born, live, lifetime, die etc.).

"…Stars dived in blinding skies / Stars die / Blinding skies…"
Stars die, but only due to artistic license
(Artwork from 'Star's die' by *Porcupine Tree*, photographer: Chris Kissadjekian)

If you see a standard candle…

When Professor Sullivan refers to a "standard candle", this is now a widely used astronomical notion (in relation to how we estimate distances to distant stars and galaxies that are much too far away to triangulate from parallax as the earth changes its position in the solar system) – but at one time this was used as a figure of speech.

Some figures of speech are created in the moment, but never widely copied and adopted. The astronomical community adopted the 'standard candle' such that it is now an accepted term, even though most young people meeting astronomical ideas for the first time probably have very little direct experience of candles. What might once have seemed a blatantly obvious allusion may now need explaining to the novice.

When Sir Arthur Eddington (famous for collecting observations during an eclipse consistent with predictions from relativity theory about the gravitational 'bending' of starlight) gave a public lecture in 1932, he seems to have assumed that his audience would understand the analogy between an astronomer's 'standard candles' (Cepheid variables) and standard candles they might themselves use!

"If you see a standard candle anywhere and note how bright it appears to you, you can calculate how far off it is; in the same way an astronomer observes his [or her] 'standard candle' in the midst of a nebula, notes its apparent brightness or magnitude, and deduces the distance of the nebula"
Eddington, 1933/1987, pp.7-8

This ongoing development in language means that it may not always be entirely clear which terms are still engaged with as if metaphors and which have now become understood as literal. That is, in considering whether some phrase is a metaphor we can ask two questions:

did the author/speaker intend this as a comparison, or do they consider the term has direct literal meaning?
does the reader/listener understand the term to have a literal meaning, or is it experienced as some novel kind of comparison with another context which has to be related back to the focus?

In the latter case we might also think it is important to distinguish between cases where the audience member can decode the intention of the comparison 'automatically' as part of normal language processing – and cases where they would have to consciously deliberate on the meaning. (In the latter case, the interpretation is likely to disrupt the flow of reading, and when listening could perhaps even require the listener to disengage from the communication such that subsequent speech is missed.)

(Metaphorical?) hosts

So, when Prof. Crawford suggests that

"The supernovae, particularly, are of fundamental importance for the host galaxy…"
Prof. Carolin Crawford

her use of the term 'host' is surely metaphorical (at least for a listener – this term is widely used in the literature of academic astronomy ⁶). A host offers hospitality for a guest. That does not seem to obviously reflect the relationship between a supernova and the galaxy it is found in and is part of. It is not a guest: rather, in Prof. Sullivan's terms we might suggest that star has 'lived its entire life' in that galaxy – it is its galactic 'home'. Despite this comparison not standing up to much formal analysis, I suspect the metaphor can be automatically processed by anyone with strong familiarity with the concept of a host. Precise alignment may not be a strong criterion for effective metaphors.

Another meaning of host refers to a sacrificial victim (as in the host in the Christian Eucharist) which seems unlikely to be the derivation here, but perhaps fits rather well with Prof. Crawford's point. A supernova too close to earth could potentially destroy the biosphere – an unlikely but not impossible event.

(Metaphorical?) bubbles

Professor Crawford described some of the changes during a supernova,

"You have got your iron core, it collapses down under gravity in less than a second, that kind of leaves the outer layers of the star a little behind, they crash down, bounce on the surface of the core, and then there's a shockwave, that propels all this stellar debris, out into space. So, this is part of the supernova explosion we have been talking about, and it carves out a bubble within the interstellar medium."
Prof. Carolin Crawford

There are a number of places here where everyday terms are applied in an unfamiliar context such as 'core', 'bouncing', 'layers' and 'debris'. But the idea of carving a bubble certainly seems metaphorical, if only because a familiar bubble would have a physical surface, where surely, here, there is no strict interface between discrete regions of gases. But, again, the term offers an accessible image to communicate the process. (And anyone looking at the NASA image above of convection cells in the Sun might well feel that these can be perceived as if bubbles.)

(Metaphorical?) pepper

Similarly, the idea of heavy elements from exploding suns being added to the original hydrogen and helium in the interstellar medium as like adding pepper also offers a strong image,

"…this is the idea of enrichment, you start off with much more primordial hydrogen and helium gas that gets steadily peppered with all these heavy elements…"
Prof. Carolin Crawford

Perhaps 'peppered' is now a dead metaphor, as it is widely used in various contexts unrelated to flavouring food.

(Metaphorical?) imprints

When Professor Rees referred to a neutron star that has become a black hole leaving a "gravitational imprint frozen in the space that's left" this makes good sense as the black hole will not be visible, but its gravitational field will have effects well beyond its event horizon. Yet, one cannot actually make an imprint in space, one needs a suitable material substrate (snow, plater, mud…) to imprint into; and nor has anything been 'frozen' in a literal sense. Indeed, the gravitational field will change as the black hole acquires more material through gravitational capture (and in the very long term loses mass though evaporates Hawking radiation – which is said to cause the black hole to 'evaporate'). So, this is a kind of double metaphor.

(Metaphorical?) blasts and blows

I report above both the idea that rocky planet close to large stars might have derived from 'giant' planets "that have had the outer gassy layers blasted off" and how "big stars…blow off their outer layers". Can stars really blow, or is this based on a metaphor. Blasts usually imply explosions, sudden events, so perhaps these are metaphorical blasts? And it is not just larger stars that engage in blowing off,

"[The sun] will blow off its outer layers and become a red giant, expanding so it will engulf the inner planets, but then the core will settle down to what's called a white dwarf, this is a dead, dense star, about a million times denser than normal stuff…."
Prof. Martin Rees

Metaphors galore!

Perhaps those last examples are not especially convincing – but this reflects a point I made earlier. Language changes over time: it is (metaphorically-speaking) fluid. If language started from giving names to things we can directly point at, then anything we cannot directly point at needs to be labelled in terms of existing words. Most of the terms we use were metaphors at some point, but became literal as the language norms changed.

But society is not a completely homogeneous language community. The requirements of professional discourse in astronomy (or any other specialised field of human activity) drive language modifications in particular regards ahead of general language use. It is not just people in Britain and the United States who are divided by a common language – we all are to some extent. What has become literal meaning for for one person (perhaps a science teacher) may well only be a metaphor to another (a student, say).

After all, when I look up what it is to blow off, I find that the most common contemporary meaning relates to a failure to meet a social obligation or arrangement – I am pretty sure (from the context) that that is not what Professor Rees was suggesting ("…when those big stars face a crisis they [let down] their outer layers".) Once we start looking at texts closely, they seem to be 'loaded' with figures of speech. A planet is not materially constrained in space, yet we understand why an orbit might be considered 'tight'.

In the proceeding quote, the core of a star seems to need no explanation although it presumably derives by analogy with the core of an apple or similar fruit, which itself seems to derive metaphorically form an original meaning of the heart. Again, what is meant by engulf is clear enough although originally it referred to the context of water and the meaning has been metaphorically (or analogously) extended.

The terms red giant and white dwarf clearly derive from metaphor. (Sure, a red giant is gigantic, but then, on any normal scale of human experience, so is a white dwarf.) These terms might mystify someone meeting them for the first time so not already aware they are used to refer to classes of star. This might suggest the value of a completely objective language for discussing science where all terms are tightly (hm, too metaphorical…closely? rigidly? well-) defined, but that would be a project reminiscent of the logical positivist programme in early twentieth century that ultimately proved non-viable. We can only define words with more words, and there are limits to the precision possible with a usable, 'living', language.

Take the "discovery that we are literally made of the ashes of long dead stars". Perhaps, but the term ashes normally refers to the remains of burnt organic material, especially wood, so perhaps we are not literally, but only metaphorically made of the ashes of long dead stars. Just as when when Professor Sullivan noted,

"the white dwarf is made of carbon, it's made of oxygen, and the temperature and the pressure in the centre of that white dwarf star can become so extreme, that carbon detonation can occur in the centre of the white dwarf, and that is a runaway thermonuclear reaction – that carbon burns in astronomer speak into more massive elements…"
Prof. Mark Sullivan

Are we stardust, ashes or just waste?

Burning is usually seen in scientific terms as another word for combustion. So, the nuclear fusion, 'burning' "in astronomer speak" of its nuclear 'fuel' in a star represents an extension of the original meaning by analogy with combustion. ⁹ Material that is deliberately used to maintain a fire is fuel. A furnace is an artefact deliberately built to maintain a high temperature – the nuclear furnace in a star is not an artefact but a naturally occurring system (gravity holds the material in place), but is metaphorically a furnace. A runaway is a fugitive who has absconded – so to describe a thermonuclear reaction (which is not going anywhere in spatial terms) as 'runaway' adopts what was a metaphor. (Astronomers also use the term 'runaway' to label a class of star that seem to be moving especially fast compared with the interstellar medium – a somewhat more direct borrowing of the usual meaning of 'runaway'.)

To consider us to be made from 'nuclear waste' relies on seeing the star-as-nuclear-furnace as analogous to a nuclear pile in a power station. In nuclear power stations we deliberately process fissile material to allow us to generate electrical power: and material is produced as a by-product of this process (that is, it is a direct product of the natural nuclear processes, but a by-product of our purposeful scheme to generate electricity). To consider something waste means making a value judgement.

If the purpose of a star is to shine (a teleological claim) and the fusion of hydrogen is the means to achieve that end, then the material produced in that process which is no longer suitable as 'fuel' can be considered 'waste'. If the universe does not have any purpose(s) for stars then there is no more basis for seeing this material as waste than there is for seeing stars themselves as the waste products of a process that causes diffuse matter to come together into local clumps. That is, this is an anthropocentric perspective that values stars as of more value than either the primordial matter from which they formed, or the 'dead' matter they will evolve into when they no longer shine 'for us'. Nature may not have such favourites! If it has a purpose, then stars seem to only be intermediate steps towards its ultimate end.

What does support the turtle? *Surely*, it's metaphors all the way down.
(Source: Pintrest)

Sources cited:

Eddington, A. (1933/1987). The Expanding Universe. Cambridge: Cambridge University Press.
Lakoff, G., & Johnson, M. (1980). The metaphorical structure of the human conceptual system. Cognitive Science, 4(2), 195-208.
Ross, J. R. (1967). Constraints on variables in syntax. (Ph.D.), Massachusetts Institute of Technology. https://babel.ucsc.edu/~hank/ross67.pdf
Taber, K. S. (2013). Revisiting the chemistry triplet: drawing upon the nature of chemical knowledge and the psychology of learning to inform chemistry education. Chemistry Education Research and Practice, 14(2), 156-168. doi:10.1039/C3RP00012E (Manuscript version can be downloaded here).
Taber, K. S. (2019). The Nature of the Chemical Concept: Constructing chemical knowledge in teaching and learning. Cambridge: Royal Society of Chemistry.
Ulmer, M., Grace, V., Hudson, H., & Schwartz, D. (1972). Upper Limits to the X-Ray Luminosities of Five Supernovae. The Astrophysical Journal, 173, 205.

Notes:

¹It may seem fanciful that we give a specific individual tree a proper name but should a child inherently appreciate that we commonly name individual hamsters (say, or ships, or roads), but not individual trees? 'Major Oak' is a particular named Oak tree in Sherwood Forest, so the idea is not ridiculous. (It is very large, but apparently the name derives from it being described by an author with the army rank of major. Of course, this term for a soldier leading others derives metaphorically from a Latin word meaning bigger, so…)

² "So how do we bridge between dogs and trees on one hand and electrons and the strong nuclear force on the other (so to speak!)? The answer is we build using analogy and we talk about those constructions using a great deal of metaphor."

We understand what is meant by bridge here in relation to an actual bridge that physically links two places – such as locations on opposite sides of a river or railway line.
There is no actual building up of materials, but we understand how we can 'build' in the abstract by analogy.
These things are not actually at hand, but we make a metaphorical comparison in terms of distinguishing items held in 'opposite' hands. We understand what is meant by a great deal of something abstract by analogy with a great deal of something we can directly experience, e.g., sand, water, etcetera.

Justice personified, on the one hand weighing up the evidence and on the other imposing sanctions

(Image by Sang Hyun Cho from Pixabay)

We construct scientific concepts and models and theories by analogy with how we construct material buildings – we put down foundations then build up brick by brick so that the top of the structure is only very indirectly supported by the ground.

(Image by joffi from Pixabay)

³ A point is a hypothetical, infinitesimally small, location in space, which is not something a person could actually make. The 'point' of an argument is metaphorically like the point of a pencil or spear which is metaphorically an approximation to an actual point. Of course, we (adult members of the English language community) all know what is meant by the point of an argument – but people new to a language (such as young children) have to find this out, without someone holding up the point of an argument for them to learn to recognise.

⁴ In part, this means linguistic resources. Each individual person has a unique vocabulary, and even though sharing most words with others, often has somewhat unique ranges of application of those words. But it also refers to personal experiences that can be drawn upon (e.g., having cared for an ill relative, having owned a pet, having undertaken part-time work in a hospital pharmacy, having been taken to work by a parent…) and the cultural referents that are commonly discussed in discourse (cultural icons like the Mona Lisa or Beethoven's fifth symphony; familiarity with some popular television show or film; appreciating that Romeo and Juliet were tragic lovers, or that Gandhi is widely considered a moral role model, and so forth.)

"Penny, I'm a physicist. I have a working knowledge of the entire universe and everything it contains."

"Who's Radiohead?"

"I have a working knowledge of the important things in the universe."

Still from 'The Big Bang Theory' (Chuck Lorre Productions / Warner Bros. Television)

The interpretive resources are whatever mental resources are available to help make sense of communication.

⁵ I am using the term concept in an 'inclusive' sense (Taber, 2019), in that whenever a person can offer a discrimination about whether something is an example of some category, then they hold a concept (vague or detailed; simple or complex; canonical or alternative).

That is, if someone can (beyond straight guesswork) try to answer one of the questions "what is X? ", "is this an example of X?" or "can you suggests an example of X?", then they have a relevant concept – where X could be…

a beaker
a force
a bacterium
opaque
a transition metal
an isomer
distillation
neutralisation
a representation of the ideal gas law
and so forth

Read more about concepts

⁶ The earliest reference to 'host galaxies' I found in a quick search of the scientific literature was from 1972 in a paper which used the term 'host galaxy' 8 times, including,

"We estimated the distances [of observed supernovae]…by four different methods:

(1) Estimating the absolute luminosity of the host galaxy.

(2) Estimating the absolute luminosity of the supernova.

(3) Using the measured redshift of the host galaxy and assuming the Hubble constant H = 75 km (s Mpc)^-1 …

(4) Identifying the host galaxy with a cluster of galaxies for which the distance from Earth had already been estimated.

Ulmer, Grace, Hudson & Schwartz, 1972, p.209

The term 'host galaxy' was not introduced or defined in the paper, suggesting that either it was already in common use as a scientific term (and so a dead metaphor within the astronomical community) in 1972 or Ulmer and colleagues assumed it was obvious enough not to need explanation.

⁷ It should be pointed out that 'In Our Time' is not presented as succession of mini-lectures, or as a tightly scripted programme, but as a conversation between Melvyn as his guests. Of course, there is some level of preparation by those involved, but in adopting a conversational style, avoiding the sense of prepared statements, it is inevitable that a guest's language will sometimes lack the precision of a drafted and much revised account.

⁸ A supernova may appear as a new star in the sky if it is so far away that the star was not previously detectable, or as a known star quick;y becoming very much brighter.

⁹ One should be careful in making such equivalences, as in that although we may equate burning with combustion, burning is an everyday ('life world') phenomenon, and combustion is a scientific concept: often our scientific concepts are more precisely defined than the related everyday terms. (Which is why melting has a broader meaning in everyday life {the sugar melts in the hot tea; the stranger melted away into the mist} than it does in science.) But although we might say, as suggested earlier in the text, we have been burned by exposure to the sun's ultraviolet rays, or by contact with a caustic substance, in those contexts we are unlikely to consider our skin as 'fuel' for the process.

The passing of stars

Birth, death, and afterlife in the universe

Keith S. Taber

stars are born, start young, live, sometimes living alone but sometimes not, sometimes have complicated lives, have lifetimes, reach the end of their lives, and die, so, becoming dead, eventually long dead; and, indeed, there are generations of stars with life cycles

One of the themes I keep coming back to here is the challenge of communicating abstract scientific ideas. Presenting science in formal technical language will fail to engage most general audiences, and will not support developing understanding if the listener/reader cannot make good sense of the presentation. But, if we oversimplify, or rely on figures of speech (such as metaphors) in place of formal treatments of concepts, then – even if the audience does engage and make sense of the presentation – audience members will be left with a deficient account.

Does that matter? Well, often a level of understanding that provides some insight into the science is far better than the impression that science is so far detached from everyday experience that it is not for most people.

And the context matters.

Public engagement with science versus science education

In the case of a scientist asked to give a public talk, or being interviewed for news media, there seems a sensible compromise. If people come away from the presentation thinking they have heard about something interesting, that seems in some way relevant to them, and that they understood the scientist's key messages, then this is a win – even if it is only a shift to an over-simplified account, or an understanding in terms of a loose analogy. (Perhaps some people will want to learn more – but, even if not, surely this meets some useful success criterion?)

In this regard science teachers have a more difficult job to do. ¹ The teacher is not usually considered successful just because the learners think they have understood teaching, but rather only when the learners can demonstrate that what they have learnt matches a specified account set out as target knowledge in the curriculum. This certainly does not mean a teacher cannot (or should not) use simplification and figures of speech and so forth – this is often essential – but rather that such such moves can usually only be seen as starting points in moving learners onto temporary 'stepping stones' towards creditable knowledge that will eventually lead to test responses that will be marked correct.

The Death of Stars

With this in mind, I was fascinated by an episode of the BBC's radio show, 'In Our Time' which took as its theme the death of stars. Clearly, this falls in the category of scientists presenting to a general public audience, not formal teaching, and that needs to be borne in mind as I discuss (and perhaps even gently 'deconstruct') some aspects of the presentation from the perspective of a science educator.

The show was broadcast some months ago, but I made a note to revisit it because I felt it was so rich in material for discussion, and I've just re-listened. I thought this was a fascinating programme, and I think it is well worth a listen, as the programme description suggests:

"Melvyn Bragg and guests discuss the abrupt transformation of stars after shining brightly for millions or billions of years, once they lack the fuel to counter the force of gravity. Those like our own star, the Sun, become red giants, expanding outwards and consuming nearby planets, only to collapse into dense white dwarves. The massive stars, up to fifty times the mass of the Sun, burst into supernovas, visible from Earth in daytime, and become incredibly dense neutron stars or black holes. In these moments of collapse, the intense heat and pressure can create all the known elements to form gases and dust which may eventually combine to form new stars, new planets and, as on Earth, new life."
https://www.bbc.co.uk/sounds/play/m0018128

I was especially impressed by the Astronomer Royal, Professor Martin Rees (and not just because he is a Cambridge colleague) who at several points emphasised that what was being presented was current understanding, based on our present theories, with the implication that this was open to being revisited in the light (sic) of new evidence. This made a refreshing contrast to the common tendency in some popular science programmes to present science as 'proven' and so 'certain' knowledge. That tendency is an easy simplification that distorts both the nature and excitement of science.

Read about scientific certainty in the media

Presenter Melvyn Bragg's other guests were Carolin Crawford (Emeritus Member of the Institute of Astronomy, and Emeritus Fellow of Emmanuel College, University of Cambridge) and Mark Sullivan (Professor of Astrophysics at the University of Southampton).

Public science communication as making the unfamiliar familiar

Science communicators, whether professional journalists or scientists popularising their work, face similar challenges to science teachers in getting across often complex and abstract ideas; and, like them, need to make the unfamiliar familiar. Science teachers are taught about how they need to connect new material with the learners' prior knowledge and experiences if it is to make sense to the students. But successful broadcasters and popularisers also know they need to do this, using such tactics as simplification, modelling, metaphor and simile, analogy, teleology, anthropomorphism and narrative.

There were quite a few examples of the speakers seeking to make abstract ideas accessible to listeners in such ways in this programme. However, perhaps the most common trope was one set up by the episode title, and one which could very easily slip under radar (so to speak). In this piece I examine the seemingly ubiquitous metaphor (if, indeed, it is to be considered a metaphor!) of stars being alive; in a sequel I discuss some of the wide range of other figures of speech adopted in this one science programme.

Science: making the familiar, unfamiliar?

If when working as a teacher I saw a major part of my work as making the unfamiliar familiar to learners, in my research there was a sense in which I needed to make the familiar unfamiliar. Often, the researcher needs to focus afresh on the commonly 'taken-for-granted' and to start to enquire into it as if one does not already know about it. That is, one needs to problematise the common-place. (This reflects a process sometimes referred to as 'bracketing'.)

To give one obvious example. Why do some students do well in science tests and others less well? Obviously, because some learners are better science students than others! (Clearly in some sense this is true – but is it just a tautology? ²) But one clearly needs to dig into this truism in more detail to uncover any insights that would actually be useful in supporting students and improving teaching!

The same approach applies in science. We do not settle for tautologies such as fire burns because fire is the process of burning, or acids are corrosive because acids are the category of substances which corrode; nor what are in effect indirect disguised tautologies such as heavy objects fall because they are largely composed of the element earth, where earth is the element whose natural place is at the centre of the world. (If that seems a silly example, it was the widely accepted wisdom for many centuries. Of course, today, we do not recognise 'earth' as a chemical element.)

I mention this, because I would like to invite readers to share with me in making the familiar unfamiliar here – otherwise you could easily miss my point.

"so much in the Universe, and much of our understanding of it, depends on changes in stars as they die after millions or billions of stable years"
Tag line for 'the Death of Stars'

The lives of stars

The episode opens with

"Hello. Across the universe, stars have been dying for millions of years…
Melvyn Bragg introducing the episode

The programme was about the death of stars – which directly implies stars die, and, so, also suggests that – before dying – they live. And there were plenty of references in the programme to reinforce this notion. Carolin Crawford suggested,

"So, essentially, a star's life, it can exist as a star, for as long as it has enough fuel at the right temperature at the right density in the core of the star to stall the gravitational collapse. And it is when it runs out of its fuel at the core, that's when you reach the end of its lifetime and we start going through the death processes."
Prof. Carolin Crawford talking on 'In Our Time'

Not only only do stars have lives, but some have much longer lives than others,

"…more massive stars can … build quite heavy elements at their cores through their lifetimes. And … they actually have shorter lifetimes – it is counter-intuitive, but they have to chomp through their fuel supply so furiously that they exhaust it more rapidly. So, the mass of the star dictates what happens in the core, what you create in the core, and it also determines the lifetime of the star."

"The mass of the star…determines the lifetime of the star….
our sun…we reckon it is about halfway through its lifetime, so stars like the sun have lifetimes of 10 billions years or so…"

Prof. Carolin Crawford talking on 'In Our Time'

This was not some idiosyncratic way that Professor Crawford had of discussing stars, as Melvyn's other guests also used this language. Here are some examples I noted:

"this is a dead, dense star" (Martin Rees)
"the lifetime of a stable star, we can infer the … life cycles of stars" (Martin Rees)
"stars which lived and died before our solar system formed…stars which have more complicated lives" (Martin Rees)
"those old stars" (Martin Rees)
"earlier generations of massive stars which had lived and died …those long dead stars" (Martin Rees)
"it is an old dead star" (Mark Sullivan)
"our sun…lives by itself in space. But most stars in the universe don't live by themselves…" (Mark Sullivan)
"two stars orbiting each other…are probably born with different masses" (Mark Sullivan)
"when [stars] die" (Mark Sullivan)
"when [galaxies] were very young" (Martin Rees)
"stars that reach the end point of their lives" (Carolin Crawford )
"a star that's younger" (Martin Rees)

So, in the language of astronomy, stars are born, start young, live; sometimes living alone but sometimes not, sometimes have complicated lives; have lifetimes, reach the end of their lives, and die, so, becoming dead, eventually long dead; and, indeed, there are generations of stars with life cycles.

The processes that support a star's luminosity come to an end: but does the star therefore die?

(Cover art for the Royal Philharmonic Orchestra's recording of David Bedford's composition Star's End. Photographer: Monique Froese)

Are stars really alive?

Presumably, the use of such terms in this context must have originally been metaphorical. Life (and so death) has a complex but well-established and much-discussed meaning in science. Living organisms have certain necessary characteristics – nutrition, (inherent) movement, irritability/sensitivity, growth, reproduction, respiration, and excretion, or some variation on such a list. Stars do not meet this criterion. ³ Living organisms maintain a level of complex organisation by making use of energy stores that allow them to decrease entropy internally at the cost of entropy increase elsewhere.

Animals and decomposers (such as fungi) take in material that can be processed to support their metabolism and then the 'lower quality' products are eliminated. Photosynthetic organisms such as green plants have similar metabolic processes, but preface these by using the energy 'in' sunlight to first facilitate endothermic reactions that allow them to build up the material used later for their mortal imperative of working against the tendencies of entropy. Put simply, plants synthesise sugar (from carbon dioxide and water) that they can distribute to all their cells to support the rest of the metabolism (a complication that is a common source of alternative conceptions {misconceptions} to learners ⁴).

By contrast, generally speaking, during their 'lifetimes', stars only gain and lose marginal amounts of material (compared with a 70 kg human being that might well consume a tonne of food each year) – and do not have any quality control mechanism that would lead to them taking in what is more useful and expelling what is not.

As far as life on earth is concerned, virtually all of that complex organisation of living things depends upon the sun as a source of energy, and relies on the process by which the sun increases the universe's entropy by radiating energy from a relatively compact source into the diffuse vastness of space. ⁴ In other words, if anything, a star like our sun better reflects a dead being such as a felled tree or a zebra hunted down by a lion, providing a source of concentrated energy for other organisms feeding on its mortal remains!

Are the lives and deaths of stars simply pedagogical devices?

So, are stars really alive? Or is this just one example of the kind of rhetorical device I referred to above being adopted to help make the abstract unfamiliar becomes familiar? Is it the use of a familiar trope employed simply to aid in the communication of difficult ideas? Is this just a metaphor? That is,

Do stars actually die, or…
are they only figuratively alive and, so, only suffer (sic) a metaphorical death?

I do not think the examples I quote above represent a concerted targeted strategy by Professors Crawford, Rees and Sullivan to work with a common teaching metaphor for the sake of Melvyn and his listeners: but rather the actual language commonly used in the field. That is, the life cycles and lifetimes of stars have entered into the technical lexicon of the the science. If so, then stars do actually live and die, at least in terms of what those words now mean in the discipline of astronomy.

Gustav Strömberg referred to "the whole lifetime of a star" in a paper in the The Astrophysical Journal as long ago as 1927. He did not feel the need to explain the term so presumably it was already in use – or considered obvious. Kip Thorne published a paper in 1965 about 'Gravitational Collapse and the Death of a Star". In the first paragraph he pointed out that

"The time required for a star to consume its nuclear fuel is so long (many billions of years in most cases) that only a few stars die in our galaxy per century; and the evolution of a star from the end point of thermonuclear burning to its final dead state is so rapid that its death throes are observable for only a few years."
Thorne, 1965, p.1671

Again, the terminology die/death/dead is used without introduction or explanation.

He went on to refer to

deaths of stars
different types of death
final resting states

before shifting to what a layperson would recognise as a more specialist, technical, lexicon (zero point kinetic energy; Compton wavelength of an electron; neutron-rich nuclei; photodistintegration; gravitational potential energy; degenerate Fermi gas; lambda hyperons; the general relativity equation of hydrostatic equilibrium; etc.), before reiterating that he had been offering

"the story of the death of a star as predicted by a combination of nuclear theory, elementary particle theory, and general relativity"
Thorne, 1965, p.1678

So, this was a narrative, but one intended to be fit for a professional scientific audience. It seems the lives and deaths of stars have been part of the technical vocabulary of astronomers for a long time now.

When did scientists imbue stars with life?

Modern astronomy is quite distinct from astrology, but like other sciences astronomy developed from earlier traditions and at one time astronomy and astrology were not so discrete (an astronomical 'star' such as Johannes Kepler was happy to prepare horoscopes for paying customers) and mythological and religious aspects of thinking about the 'heavens' were not so well compartmentalised from what we would today consider as properly the realm of the scientific.

In Egyptian religion, Ra was both a creative force and identified with the sun. Mythology is full of origin stories explaining how the stars had been cast there after various misadventures on earth (the Greek myths but also in other traditions such as those of the indigenous North American and Australian peoples ⁵) and we still refer to examples such as the seven sisters and Orion with the sword hanging in his belt. The planets were associated with different gods – Venus (goddess of love), Mars (the god of war), Mercury (the messenger of the gods), and so on.⁶ It was traditional to refer to some heavenly bodies as gendered: Luna is she, Sol is he, Venus is she, and so on. This usage is sometimes found in scientific writing on astronomy.

Read about examples of personification in scientific writing

Yet this type of poetic license seems unlikely to explain the language of the life cycles of stars, even if there are parallels between scientific and poetic or spiritual accounts,

Stars are celestial objects having their own life cycles. Stars are born, grow up, mature and eventually die. …The author employs inductive and deductive analysis of the verses of the Quran and the Hadith texts related with the life and death of stars. The results show that the life and death of the stars from Islamic and Modern astronomy has some similarities and differences.
Wahab, 2015

After all, the heavenly host of mythology comprised of immortals, if sometimes starting out as mortals subsequently given a kind of immorality by the Gods when being made into stars. Indeed the classical tradition supported by interpretation of Christian orthodoxy was that unlike the mundane things of earth, the heavens were not subject to change and decay – anything from the moon outwards was perfect and unchanging. (This notion was held onto by some long after it was established that comets with their varying paths were not atmospheric phenomena – indeed well into the twentieth century some young earth creationists were still insisting in the perfect, unchanging nature of the heavens. ⁷)

So, presumably, we need to look elsewhere to find how science adopted life cycles for stars.

A natural metaphor?

Earlier in this piece I asked readers to bear with me, and to join with me in making the familiar unfamiliar, to 'bracket' the familiar notion that we say starts are born, live and later die, and to problematise it. In one scientific sense stars cannot die – as they were never alive. Yet, I accept this seems a pretty natural metaphor to use. Or, at least, it seems a natural metaphor to those who are used to hearing and reading it. A science teacher may be familiar with the trope of stars being born, living, and dying – but how might a young learner, new to astronomical ideas, make sense of what was meant?

Now, there is a candidate project for anyone looking for a topic for a student research assignment: how would people who have never previously been exposed to this metaphor respond to the kinds of references I've discussed above? I would genuinely like to know what 'naive' people would make of this ⁸ – would they just 'get' the references immediately (appreciate in what sense stars are born, live, and die); or, would it seem a bizarre way of talking about stars? Given how readily people accept and take up anthropomorphic references to molecules and viruses and electrons and so forth, I find the question intriguing.

Read about anthropomorphism in science

What makes a star alive or dead?

Even if for the disciplinary experts the language of living stars and their life cycles has become a 'dead metaphor 'and is now taken (i.e., taken for granted) as technical terminology – the novice learner, or lay member of the public listening to a radio show, still has to make sense of what it means to say a star is born, or is alive, or is nearing the end of its life, or is dead.

The critical feature discussed by Professors Crawford, Rees and Sullivan concerns an equilibrium that allow a star to exist in a balance between the gravitational attraction of its component matter and the pressure generated through its nuclear reactions.

A star forms when material comes together under its mutual gravitational attraction – and as the material becomes denser it gets hotter. Eventually a sufficient density and temperature is reached such that there is 'ignition' – not in the sense of chemical combustion, but self-sustaining nuclear processes occur, generating heat. This point of ignition is the 'birth' of the star.

Fusion processes continue as long as there is sufficient fissionable material, the 'fuel' that 'feeds' the nuclear 'furnace' (initially hydrogen, but depending on the mass of the star there can be a series of reactions with products from one stage undergoing further fusion to form even heavier elements). The life time of the star is the length of time that such processes continue.

Eventually there will not be sufficient 'fuel' to maintain the level of 'burning' that is needed to allow the ball of material to avoid ('resist') gravitational collapse. There are various specific scenarios, but this is the 'death' of the star. It may be a supernova offering very visible 'death throes'.

The core that is left after this collapse is a 'dead' star, even if it is hot enough to continue being detectable for some time (just as it takes time for the body of a homeothermic animal that dies to cool to the ambient temperature).

It seems then that there is a kind of analogy at work here.

Organisms are alive as long as they continue to metabolise sufficiently in order to maintain their organisation in the face of the entropic tendency towards disintegration and dispersal.

Stars are alive as long as they exhibit sufficient fusion processes to maintain them as balls of material that have much greater volumes, and lower densities than the gravitational forces on their component particles would otherwise lead to.

It is clearly an imperfect analogy.

Organisms base metabolism on a through-put of material to process (and in a sense 'harvest' energy sources).	Stars do acquire new materials and eject some, but this is largely incidental and it is essentially the mass of fissionable material that originally comes together to initiate fusion which is 'harvested' as the energy source.
Organisms may die if they cannot access external food sources, but some die of built-in senescence and others (those that reproduce by dividing) are effectively immortal. We (humans) die because the amazing self-constructing and self-repairing abilities of our bodies are not perfect, and somatic cells cannot divide indefinitely to replace no longer viable cells.	Stars 'die' because they run out of their inherent 'fuel'. Stars die when the hydrogen that came together to form them has substantially been processed.

Read about analogy in science

One person's dead star is another person's living metaphor

So, do stars die? Yes, because astronomers (the experts on stars) say they do, and it seems they are not simply talking down to the rest of us. The birth and death of stars seems to be based on an analogy: an analogy which is implicit in some of the detailed discussion of star life cycles. However, through the habitual use of this analogy, terms such as the birth, lifetimes, and death of stars have been adopted into mainstream astronomical discourse as unmarked (taken-for-granted) language such that to the uninitiated they are experienced as metaphors.

And these perspectival metaphors ⁹ become extended to describe stars that are considered young, old, dying, long dead, and so forth. These terms are used so readily, and so often without a perceived need for qualification or explanation, that we might consider them 'dead' metaphors within astronomical discourse – terms of metaphorical origin but now so habitually used that they have come to be literal (stars are born, they do have lifetimes, they do die). Yet for the uninitiated they are still 'living' metaphors, in the sense that the non-expert needs to work out what it means when a star is said to live or die.

There is a well recognised distinction between live and dead metaphors. But here we have dead-to-the-specialists metaphors that would surely seem to be non-literal to the uninitiated. These terms are not explained by experts as they are taken by them as literal, but they cannot be understood literally by the novice, for whom they are still metaphors requiring interpretation. That is, they are perspectival metaphors – zombie words that may seem alive or dead (as figures of speech) according to audience, and so may be treated as dead in professional discourse, but may need to be made undead when used in communicating to the public.

Other aspects of the In Our Time discussion of 'The death of stars' are explored as The complicated social lives of stars: stealing, escaping, and blowing-off in space

Sources cited:

Strömberg, G. (1927). The Motions of Giant M Stars. The Astrophysical Journal, 65, 238.
Thorne, K. S. (1965). Gravitational Collapse and the Death of a Star. Science, 150(3704), 1671-1679. http://www.jstor.org.ezp.lib.cam.ac.uk/stable/1717408
Wahab, R. A. (2015). Life and death of stars: an analysis from Islamic and modern astronomy perspectives. International Proceedings of Economics Development and Research, 83, 89.

Notes

¹ In this regard, but not in all regards. As I have suggested here before, the teacher usually has two advantages:

a) generally, a class has a limited spread in terms of the audience background: even a mixed ability class is usually from a single school year (grade level) whereas the public presentation may be addressing a mixed audience of all ages and levels of education.

b) usually a teacher knows the class, and so knows something about their starting points, and their interests

² Some students do well in science tests and others less well.

If we say this is because

some learners are better science students than others
and settle for defining better science students as those who achieve good results in formal science tests (that is tests as currently administered, based on the present curriculum, taught in our usual way)

then we are simply 'explaining' the explicandum (i.e., some students do better on science tests that others) by a rephrasing of what is to be explained (some students are better science students: that is, they perform well in science tests!)

Read about tautology

³ Criterion (singular) as a living organism has to satisfy the entries in the list collectively. Each entry is of itself a necessary, but not sufficient, condition.

⁴ A simple misunderstanding is that animals respire but plants photosynthesise.

In a plant in a steady state, the rates of build-up and break down of sugars would be balanced. However, plants must photosynthesise more than they respire overall in order to to grow and ultimately to allow consumers to make use of them as food. (This needs to be seen at a system level – the plant is clearly not in any inherent sense photosynthesising to provide food for other organisms, but has evolved to be a suitable nutrition source as it transpires [no pun intended] that increases the fitness of plants within the wider ecosystem.)

A more subtle alternative conception is that plants photosynthesise during the day when they are illuminated by sunlight (fair enough) and then use the sugar produced to respire at night when the sun is not available as a source of energy. See, for example, 'Plants mainly respire at night because they are photosynthesising during the day'.

Actually cellular processes require continuous respiration (as even in the daytime sunlight cannot directly power cellular metabolism, only facilitate photosynthesis to produce the glucose that that can be oxidised in respiration).

Schematic reflection of the balance between how photosynthesis generates resources to allow respiration – typically a plant produces tissues that feed other organisms.
The area above the line represents energy from sunlight doing work in synthesising more complex substances. The area below the lines represents work done when the oxidation of those more complex substances provides the energy source for building and maintaining an organism's complex organisation of structure and processes (homoestasis).

⁵ Museum Victoria offers a pdf that can be downloaded and copied by teachers to teach about how "How the southern night sky is seen by the Boorong clan from north-west Victoria":

'Stories in the Stars – the night sky of the Boorong people' shows the constellations as recognised by this group, the names they were given, and the stories of the people and creatures represented.

(This is largely based on the nineteenth century reports made by William Edward Stanbridge of information given by Boorong informants – see 'Was the stellar burp really a sneeze?')

The illustration shown here is of 'Kulkunbulla' – a constellation that is considered in the U.K. to be only part of the constellation known here as Orion. (Constellations are not actual star groupings, but only what observers have perceived as stars seeming to be grouped together in the sky – the Boorong's mooting of constellations is no more right or wrong than that suggested in any other culture.)

⁶ The tradition was continued into modern times with the discovery of the planets that came to be named Neptune and Uranus after the Gods of the sea and sky respectively.

⁷ Creationism, per se, is simply the perspective or belief that the world (i.e., Universe) was created by some creator (God) and so creationism as such is not necessarily in conflict with scientific accounts. The theory of the big bang posits that time, space and matter had a beginning with an uncertain cause which could be seen as God (although some theorists such as Professor Roger Penrose develop theories which posit a sequence of universes that each give rise to the next and that could have infinite extent).

Read about science and religion

Young earth creationists, however, not only believe in a creator God (i.e., they are creationists), but one who created the World no more than about 10 thousand years ago (the earth is young!), rather than over 13 billion years ago. This is clearly highly inconsistent with a wide range of scientific findings and thinking. If the Young Earth Creationists are right, then either

a lot of very strongly evidenced science is very, very wrong
some natural laws (e.g. radioactive decay rates) that now seem fixed must have changed very substantially since the creation
the creator God went to a lot of trouble to set up the natural world to present a highly misleading account of its past history

⁸ I am not using the term naive here in a discourteous or demeaning way, but in a technical sense of someone who is meeting something for the first time.

⁹ That is, terms that will appear as metaphors from the perspective of the uninitiated, but now seem literal terms from the perspective of the specialist. We cannot simply say they are or are not metaphors, without asking 'for whom?'

Was the stellar burp really a sneeze?

Pulling back the veil on an astronomical metaphor

Keith S. Taber

It seems a bloated star dimmed because it sneezed, and spewed out a burp.

'Pardon me!' (Image by Angeles Balaguer from Pixabay)

I was intrigued to notice a reference in Chemistry World to a 'stellar burp'.

"…the dimming of the red giant Betelgeuse that was observed in 2019…was later attributed to a 'stellar burp' emitting gas and dust which condensed and then obscured light from the star"
Motion, 2022

The author, Alice Motion, quoted astrophysics doctoral candidate and science communicator Kirsten Banks commenting that

"In recorded history…It's the first time we've ever seen this happen, a star going through a bit of a burp"
Kirsten Banks quoted in Chemistry World

although she went on to suggest that the Boorong people (an indigenous culture from an area of the Australian state Victoria) had long ago noticed a phenomena that became recorded in their oral traditions ¹, which

"was actually the star Eta Carinae which went through a stellar burp, just like Betelgeuse did"
Kirsten Banks quoted in Chemistry World

Composite image (optical appearing as white; ultraviolet as cyan; X-rays as purple) of Eta Carinae,

Source: NASA

Clearly a star cannot burp in the way a person can, so I took this to be a metaphor, and wondered if this was a metaphor used in the original scientific report.

A clump and a veil

The original report (Montargès, et al, 2021) was from Nature, one of the most prestigious science research journals. It did not seem to have any mention of belching. This article reported that,

"From November 2019 to March 2020, Betelgeuse – the second-closest red supergiant to Earth (roughly 220 parsecs, or 724 light years, away) – experienced a historic dimming of its visible brightness…an event referred to as Betelgeuse's Great Dimming….Observations and modelling support a scenario in which a dust clump formed recently in the vicinity of the star, owing to a local temperature decrease in a cool patch that appeared on the photosphere."
Montargès, et al., 2012, p.365

So, the focus seemed to be not on any burping but a 'clump' of material partially obscuring the star. That material may well have arisen from the star. The paper in nature suggests that Betelgeuse may loose material through two mechanisms: both by a "smooth homogeneous radial outflow that consists mainly of gas", that is a steady and continuous process; but also "an episodic localised ejection of gas clumps where conditions are favourable for efficient dust formation while still close to the photosphere" – that is the occasional, irregular, 'burp' of material, that then condenses near the star. But the word used was not 'burp', but 'eject'.

A fleeting veil

Interestingly the title of the article referred to "A dusty veil shading Betelgeuse". The 'veil' (another metaphor) only seemed to occur in the title. There is an understandable temptation, even in scholarly work, to seek a title which catches attention – perhaps simplifying, alliterating (e.g., '…mediating mental models of metals…') or seeking a strong image ('…a dusty veil shading…'). In this case, the paper authors clearly thought the metaphor did not need to be explained, and that readers would understand how it linked to the paper content without any explicit commentary.

Word	Frequency in Nature article
clump(s)	25 (excluding reference list)
eject(ed, etc.)	4
veil	1 (in title only)
burp	0
blob	0

There's no burping in Nature

The European Southern Observatory released a press release (sorry, a 'science release') about the work entitled 'Mystery of Betelgeuse's dip in brightness solved', that explained

"In their new study, published today in Nature, the team revealed that the mysterious dimming was caused by a dusty veil shading the star, which in turn was the result of a drop in temperature on Betelgeuse's stellar surface.

Betelgeuse's surface regularly changes as giant bubbles of gas move, shrink and swell within the star. The team concludes that some time before the Great Dimming, the star ejected a large gas bubble that moved away from it. When a patch of the surface cooled down shortly after, that temperature decrease was enough for the gas to condense into solid dust.

'We have directly witnessed the formation of so-called stardust,' says Montargès, whose study provides evidence that dust formation can occur very quickly and close to a star's surface. 'The dust expelled from cool evolved stars, such as the ejection we've just witnessed, could go on to become the building blocks of terrestrial planets and life', adds Emily Cannon, from KU Leuven, who was also involved in the study."
https://www.eso.org/public/news/eso2109/

So, again, references to ejection and a veil – but no burping.

Delayed burping

Despite this, the terminology of the star burping, seems to have been widely taken up in secondary sources, such as the article in Chemistry World

A New Scientist report suggested "Giant gas burp made Betelgeuse go dim" (Crane, 2021). On the website arsTECHNICA, Jennifer Ouellette wrote that "a cold spot and a stellar burp led to strange dimming of Betelgeuse".

On the newsite Gizmodo, George Dvorsky wrote a piece entitled "A dusty burp could explain mysterious dimming of supergiant star Betelgeuse". Whilst the term burp was only used in the title, Dvorsky was not shy of making other corporeal references,

"a gigantic dust cloud, which formed after hot, dense gases spewed out from the dying star. Viewed from Earth, this blanket of dust shielded the star's surface, making it appear dimmer from our perspective, according to the research, led by Andrea Dupree from the Centre for Astrophysics at Harvard & Smithsonian.

A red supergiant star, Betelgeuse is nearing the end of its life. It's poised to go supernova soon, by cosmological standards, though we can't be certain as to exactly when. So bloated is this ageing star that its diameter now measures 1.234 million kilometers, which means that if you placed Betelgeuse at the centre of our solar system, it would extend all the way to Jupiter's orbit."

The New York Times published an article (June 17, 2021) entitled "Betelgeuse Merely Burped, Astronomers Conclude", where author Dennis Overbye began his piece:

"Betelgeuse, to put it most politely, burped."
The New York Times

Overbye also reports the work from the Nature paper

"We have directly witnessed the formation of so-called stardust," Miguel Montargès, an astrophysicist at the Paris Observatory, said in a statement issued by the European Southern Observatory. He and Emily Cannon of Catholic University Leuven, in Belgium, were the leaders of an international team that studied Betelgeuse during the Great Dimming with the European Southern Observatory's Very Large Telescope on Cerro Paranal, in Chile.

Parts of the star, they found, were only one-tenth as bright as normal and markedly cooler than the rest of the surface, enabling the expelled blob to cool and condense into stardust. They reported their results on Wednesday in Nature."
The New York Times

So, instead of the clumps referred to in the Nature article as ejected, we now have an expelled blob (neither word appears in the nature article itself). Overbye also explains how this study followed up on earlier observations of the star

"Their new results would seem to bolster findings reported a year ago by Andrea Dupree of the Harvard-Smithsonian Center for Astrophysics and her colleagues, who detected an upwelling of material on Betelgeuse in the summer of 2019.

'We saw the material moving out through the chromosphere in the south in September to November 2019,', Dr. Dupree wrote in an email. She referred to the expulsion as 'a sneeze.'
The New York Times

'**…material moving out through the chromosphere in the south…**': Hubble space telescope images of Betelgeuse (Source: NASA) ²

Bodily functions and stellar processes

I remain unsure why, if the event was originally considered a sneeze, it became transformed into a burp. However the use of such descriptions is not so unusual. Metaphor is a common tool in science communication to help 'make the unfamiliar familiar' by describing something abstract or out-of-the-ordinary in more familiar terms.

Read about metaphors in science

Here, the body [sic] of the scientific report keeps to technical language although a metaphor (the dust cloud as a veil) is considered suitable for the title. It is only when the science communication shifts from the primary literature (intended for the science community) into more popular media aimed at a wider audience that the physical processes occurring in a star became described in terms of our bodily functions. So, in this case, it seems a bloated star dimmed because it sneezed, and spewed out a burp.

Coda

The astute reader may have also noticed that the New York Times article referred to Betelgeuse as an "ageing star" that is "nearing the end of its life": terms that imply a star is a living, and mortal, being. This might seem to be journalistic license, but the NASA website from which the sequence of Betelgeuse images above are taken also refers to the star as ageing (as well as being 'petulant' and 'injured').² NASA employs scientifically qualified people, but its public websites are intended for a broad, general audience, perhaps explaining the anthropomorphic references.

Thus, we might understand references to stars as alive as being a metaphorical device used in communicating astronomical ideas to the general public. Yet, an examination of the scientific literature might instead suggest instead that astronomers DO consider stars to be alive. But, that is a topic for another piece.

Work cited:

Crane, L. (2021). Giant gas burp made Betelgeuse go dim. New Scientist, 250(3340), 22. doi:10.1016/S0262-4079(21)01094-0
Hamacher, D. W., & Frew, D. J. (2010). An aboriginal Australian record of the great eruption of Eta Carinae. Journal of Astronomical History and Heritage, 13(3), 220-234.
Montargès, M., Cannon, E., Lagadec, E., de Koter, A., Kervella, P., Sanchez-Bermudez, J., . . . Danchi, W. (2021). A dusty veil shading Betelgeuse during its Great Dimming. Nature, 594(7863), 365-368. doi:10.1038/s41586-021-03546-8
Motion, A. 2022, Space for more science. Astrophysics and Aboriginal astronomy on TikTok, Chemistry World, December 2022, p.15 (https://www.chemistryworld.com/opinion/space-for-more-science/4016585.article)

Notes

¹ William Edward Stanbridge (1816-1894) was an Englishman who moved to Australia in 1841. He asked Boorong informants about their astronomy, and recorded their accounts. He presented a report to the Philosophical Institute of Victoria in 1857 and published two papers (Hamacher & Frew, 2010). The website Australian Indigenous Astronomy explains that

"The larger star of [of the binary system] Eta Car is unstable and undergoes occasional violent outbursts, where it sheds material from its outer shells, making it exceptionally bright. During the 1840s, Eta Car went through such an outburst where it shed 20 solar masses of its outer shell and became the second brightest star in the night sky, after Sirius, before fading from view a few years later. This event, commonly called a "supernova-impostor" event, has been deemed the "Great Eruption of Eta Carinae". The remnant of this explosion is evident by the Homunculus Nebulae [see figure above – nebulae are anything that appears cloud-like to astronomical observation]. This identification shows that the Boorong had noted the sudden brightness of this star and incorporated it into their oral traditions."
Duane Hamacher

A paper in the Journal of Astronomical History and Heritage concludes that

"the Boorong people observed 𝜂 Carinae in the nineteenth century, which we identify using Stanbridge's description of its position in Robur Carolinum, its colour and brightness, its designation (966 Lac, implying it is associated with the Carina Nebula), and the relationship between stellar brightness and positions of characters in Boorong oral traditions. In other words, the nineteenth century outburst of 𝜂 Carinae was recognised by the Boorong and incorporated into their oral traditions"
Hamacher & Frew 2010, p.231

² The images reproduced here are presented on a NASA website under the heading 'Hubble Sees Red Supergiant Star Betelgeuse Slowly Recovering After Blowing Its Top'. This is apparently not a metaphor as the site informs readers that"Betelgeuse quite literally blew its top in 2019". Betelgeuse is described as a "monster star", and its activity as "surprisingly petulant behaviour" and a "titanic convulsion in an ageing star", such that "Betelgeuse is now struggling to recover from this injury."

This seems rather anthropomorphic – petulance and struggle are surely concepts that refer to sentient deliberate actors in the world, not massive hot balls of gas. However, anthropomorphic narratives are often used to make scientific ideas accessible.

Read about anthropomorphism

The recovery (from 'injury') is described in terms of two similes,

"The star's interior convection cells, which drive the regular pulsation may be sloshing around like an imbalanced washing machine tub, Dupree suggests. … spectra imply that the outer layers may be back to normal, but the surface is still bouncing like a plate of gelatin dessert [jelly] as the photosphere rebuilds itself."
NASA Website

Read about science similes

Counting both the bright and the very dim

What is 1% of a very large, unknown, number?

Keith S. Taber

1, skip 99; 2, skip 99; 3, skip 99; 4,… skip 99, 1 000 000 000!
(Image by FelixMittermeier from Pixabay)

How can we count the number of stars in the galaxy?

On the BBC radio programme 'More or Less' it was mooted that there might be one hundred billion (100 000 000 000) stars in our own Milky Way Galaxy (and that this might be a considerable underestimate).

The estimate was suggested by Prof. Catherine Heymans who is
the Astronomer Royal for Scotland and Professor of Astrophysics at the University of Edinburgh.

Programme presenter Tim Harford was tackling a question sent in by a young listener (who is very almost four years of age) about whether there are more bees in the world than stars in the galaxy? (Spoiler alert: Prof. Catherine Heymans confessed to knowing less about bees than stars.)

An episode of 'More or Less' asks: Are there more bees in the world or stars in the galaxy?

Hatford asked how the 100 billion stars figure was arrived at:

"have we counted them, or got a computer to count them, or is it more a case of, well, you take a photograph of a section of sky and you sort of say well the rest is probably a bit like that?"

The last suggestion here is of course the basis for many surveys. As long as there is good reason to think a sample is representative of the wider population it is drawn from we can collect data from the sample and make inferences about the population at large.

Read about sampling a population

So, if we counted all the detectable stars in a typical 1% of the sky and then multiplied the count by 100 we would get an approximation to the total number of detectable stars in the whole sky. That would be a reasonable method to find approximately how many stars there are in the galaxy, as long as we thought all the detected stars were in our galaxy and that all the stars in our galaxy were detectable.

Prof. Heymans replied

"So, we have the European Space Agency Gaia mission up at the moment, it was launched in 2013, and that's currently mapping out 1% of all the stars in our Milky Way galaxy, creating a three dimensional map. So, that's looking at 1 billion of the stars, and then to get an idea of how many others are there we look at how bright all the stars are, and we use our sort of models of how different types of stars live [sic] in our Milky Way galaxy to give us that estimate of how many stars are there."
Prof. Catherine Heymans interviewed on 'More or Less'

A tautology?

This seemed to beg a question: how can we know we are mapping 1% of stars, before we know how many stars there are?

This has the appearance of a tautology – a circular argument.

Read about tautology

To count the number of stars in the galaxy,

(i) count 1% of them, and then
(ii) multiply by 100.

So,

If we assume there are one hundred billion, then we need to
count one billion, and then
multiply by 100 to give…
one hundred billion.

Clearly that did not seem right. I am fairly sure that was not what Prof. Haymans meant. As this was a radio programme, the interview was presumably edited to fit within the limited time allocated for this item, so a listener can never be sure that a question and (apparently immediately direct) response that makes the edit fully reflects the original conversation.

Counting the bright ones

According to the website of the Gaia mission, "Gaia will achieve its goals by repeatedly measuring the positions of all objects down to magnitude 20 (about 400 000 times fainter than can be seen with the naked eye)." Hartman's suggestion that "you take a photograph of a section of sky and you sort of say well the rest is probably a bit like that?" seems very reasonable, until you realise that even with a powerful telescope sent outside of the earth's atmosphere, many of the stars in the galaxy may simply not be detectable. So, what we see cannot be considered to be fully representative of what is out there.

It is not then that the scientists have deliberately sampled 1%, but rather they are investigating EVERY star with an apparent brightness above a certain critical cut off. Whether a star makes the cut, depends on such factors as how bright it is (in absolute terms – which we might imagine we would measure from a standard distance ¹) and how close it is, as well as whether the line of sight involves the starlight passing through interstellar dust that absorbs some (or all) of the radiation.

Of course, these are all strictly, largely, unknowns. Astrophysics relies a good on boot-strapping, where our best, but still developing, understanding of one feature is used to build models of other features. In such circumstances, observational tests of predictions from theory are often as much testing the underlying foundations upon which a model used to generate a prediction is built as that specific focal model itself. Knowledge moves on incrementally as adjustments are made to different aspects of interacting models.

Observations are theory-dependent

So, this is, in a sense, a circular process, but it is a virtuous circle rather than just a tautology as there are opportunities for correcting and improving the theoretical framework.

In a sense, what I have described here is true of science more generally, and so when an experiment fails to produce a result predicted by a new theory, it is generally possible to seek to 'save' the theory by suggesting the problem was (if not a human error) not in the actual theory being tested, but in some other part of the more extended theoretical network – such as the theory underpinning the apparatus used to collect data or the the theory behind the analysis used to treat data.

In most mature fields, however, these more foundational features are generally considered to be sound and unlikely to need modifying – so, a scientist who explains that their experiment did not produce the expected answer because electron microscopes or mass spectrometers or Fourier transform analyses do not work they way everyone has for decades thought they did would need to offer a very persuasive case.

However, compared to many other fields, astrophysics has much less direct access to the phenomena it studies (which are often vast in terms of absolute size, distance and duration), and largely relies on observing without being able to manipulate the phenomena, so understandably faces special challenges.

Why we need a theoretical model to finish the count

Researchers can use our best current theories to build a picture of how what we see relates to what is 'out there' given our best interpretations of existing observations. This is why the modelling that Prof. Heymans refers to is so important. Our current best theories tell us that the absolute brightness of stars (which is a key factor in deciding whether they will be detected in a sky survey) depends on their mass, and the stage of their 'evolution'.²

So, completing the count needs a model which allows data for detectable stars to be extrapolated, bearing in mind our best current understanding about the variations in frequencies of different kinds (age, size) of star, how stellar 'densities' vary in different regions of a spiral galaxy like ours, the distribution of dust clouds, and so forth.

…keep in mind we are off-centre, and then allow for the thinning out near the edges, remember there might be a supermassive black hole blocking our view through the centre, take into account dust, acknowledge dwarf stars tend to be missed, take into account that the most massive stars will have long ceased shining, then take away the number you first thought of, and add a bit for luck… (Image by WikiImages from Pixabay)

I have taken the liberty of offering an edited exchange

Hartford: "have we counted [the hundred billion stars], or got a computer to count them, or is it more a case of, well, you take a photograph of a section of sky and you sort of say well the rest is probably a bit like that?"
Heymans "So, we have the European Space Agency Gaia mission up at the moment, it was launched in 2013, and that's currently mapping out…all the stars in our Milky Way galaxy [that are at least magnitude 20 in brightness], creating a three dimensional map. So, that's looking at 1 billion of the [brightest] stars [as seen from our solar system], and then to get an idea of how many others are there we look at how bright all the stars are, and we use our models of how different types of stars [change over time ²] in our Milky Way galaxy to give us that estimate of how many stars are there."

No more tautology. But some very clever and challenging science.

(And are there more bees in the world or stars in the galaxy? The programme is available at https://www.bbc.co.uk/sounds/play/m00187wq.)

Note:

¹ This issue of what we mean by the brightness of a star also arose in a recent post: Baking fresh electrons for the science doughnut

² Stars are not alive, but it is common to talk about their 'life-cycles' and 'births' and 'deaths' as stars can change considerably (in brightness, colour, size) as the nuclear reactions at their core change over time once the hydrogen has all been reacted in fusion reactions.

Some stars are closer than the planets

Stars look so little because they are a long way away, but some stars are closer than the planets

Keith S. Taber

Sophia was a participant in the Understanding Science Project. When I interviewed her in her first year of secondary school (Y7 in the English school system). I asked her about what she remembered about the science she had studied in primary school. She told me about she had studied the topic of space, and had learnt about the nine planets. When I asked her if she could name the planets she produced a list of planets including both the moon and sun: "Pluto, Jupiter, Venus, Uranus, Earth, the Sun, the Moon".

[Read 'The sun is the closest of the eleven planets']

As Sophia thought the sun might be a planet, I asked her what a planet was:

Do you know what a planet is?
Erm, it's like – a round – a sphere, in space, kind of. Though we don't know if people live, animals live there or not.
…If I say someone was going through space, in a spaceship, and they are a long, long way away from earth, they've gone a long way across space, and they came across something in space…And er one of the crew said 'oh that's a planet'. And another one of the crew said 'no, that's not a planet'. And you were in charge, you were the captain. How would you decide who was right, whether that was a planet or not in space?
Er
(pause, c.5s)
I'd look if it was all the things that you thought a planet was.
Good, and what would that be?
If it was round, if it was a bit lumpy, a bit – if it was quite big, not like a little star, well there's no stars that little…

It seemed that Sophia (reasonably) thought stars would be larger than planets, which invited an obvious question, that I assumed would have an almost-as-obvious answer.

Why do they [the stars] look so little?
Because they are a long way away.
Oh, I see. So they are big really?
Yeah.
Okay. What's the difference between a star and a planet then?
A star's made up of different things, but planets – can't – cause you don't really see a planet, so you just see stars quite lot.
That's true, there is lots and lots of stars up there, isn't there? So how can you see the stars and not the planets, do you think?
I think the stars, some stars are closer, maybe, than planets.

There seemed to be something of a contradiction here. Sophia thought that

stars were not as 'little' as planets
but they seemed little because they were a long way away.
but the stars were easier to see than planets
so they might be closer to us than the planets.

Both these arguments are logical enough suggestions (things seem smaller, and may be harder to see, if they are a long way off), but there was a lack of integration of ideas as her two explanations relied on seemingly inconsistent premises (that the stars are "are a long way away" but could be "closer, maybe, than planets").

It seemed that Sophia was not aware, or was not bringing to mind, that stars were self-luminous whereas planets were only seen by reflected light. Lacking (or not considering) that particular piece of information acted as a 'deficiency learning impediment' and led to her explaining why the planets could be more difficult to see by suggesting they might not be as close as some stars.

Not considering luminosity as a criterion also seemed to explain why she was not clear that the (self-luminous) sun was not a planet.

[Read 'The sun is the closest of the eleven planets']