There’s more physics involved in this tree than you might expect. But hey, did you not notice the name of the project? Did you seriously think our boss let us play with this stuff without it having any relevance to our work? Pfshaw!
If you take a lump of something – like a chunk of iron, maybe, or a big ball of gas like, er, a star – and heat it up, it radiates light. Physicists talk about ‘black body radiation’, which is the distribution of wavelengths – colours – emitted by the theoretically perfect chunk of stuff. It turns out, quite a lot of things (lumps of ion, balls of gas we like to call ‘stars’) fit quite closely to the ideal black body radiation emission spectrum, and we end up using that theoretical basis to describe shades of white.
Ah. I mentioned ‘shades of white’, didn’t I? Well, yes. OK, back up a bit: you know sometimes you take a photograph indoors, and everything looks yellow? Or you take a photograph outdoors and everything looks blue? That’s because your camera has guessed a colour temperature – a colour for ‘white’ – which is way off. It’s assumed the scene is being lit by an ideal black body which is glowing as if heated to a temperature that doesn’t match the light in the room. Your camera has to pick something to represent white, and every so often it simply guesses wrong.
Colour, it turns out, is as much about your perception as it is about the world itself. Your eye and brain are ridiculously good at guessing what colour is neutral – white – at any given, and they interpret other colours by referring to that. This works really well for us, unless we’re trying to decide if a dress is blue or white.
OK, back to ideal black body radiators: by convention, black body colours are described by the temperature of the chunk of stuff, expressed in scientists’ favourite temperature unit, Kelvin. One Kelvin is the same change as one degree Celsius, but with Kelvin you start counting from 0K = -273.15°C. You can’t have negative Kelvin, but that’s another story for another day. For now, roll with it.
The output from our black body radiation function. Temperature along the x axis (Kelvin). Ignore the y axis, it’s just a colour.
Heat a lump of stuff to a mere 1500K (~1225°C) and it’ll glow a nice warming red. Heat it more, to about 6300K (6026°C) and it looks white, in most circumstances. Heat it way way more, to 12000K (11726°C) and it’ll blaze with an icy-looking blue-white heat. So here’s the really weird part: we call the redder-looking colours ‘warm’ and the more blue colours ‘cold,’ when the temperatures which generate them are completely the opposite. ‘Red hot’ is cooler than ‘white hot’ which turns out to be cooler in turn than ‘blue hot.’
In practice, not all that many things stick too closely to the ‘ideal black body’ colour temperature curves. Lumps of iron are pretty close, at least until they start to boil at around 3000K. But boil a big enough lump of iron, wrap it in rather a lot of hydrogen, and you’ve got yourself a star – and it turns out many stars are a reasonable fit to the colour profile above.
Our tree, then, can approximate the colour of a range of stars. Tweet it the surface temperature of a star, and it’ll come pretty close to glowing that colour. So tweet it 5280K, the surface temperature of our Sun, and it glows…
…ah, but then you have to know what colour the Sun is.