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']
