Why is the Sun Green?

Question posed by Jennie.

I know, I know: the Sun isn’t green, but in my why is the sky purple? post I boasted that I could offer a fairly convincing argument that it is, and Jennie has called me on that. So here goes:

What is colour?

Well, that’s easy, isn’t it? You look at stuff, and you can easily tell what colour it is, right? Look at the picture to the right, for example, and most of us will see an image that our brains tell us contains a lot of pink.

Unfortunately, when we’re doing science, it’s not always good enough to just look at something and say “that’s sulphur springs yellow 5, that is.” This is because we can get it wrong; our brains can be fooled fairly easily.

If you don’t believe me, take a look at the image to the left. This is a fairly well known illusion published by Edward H. Adelson in 1995. If you don’t know what the illusion’s about, then take a look at the squares marked A and B. They are exactly the same colour*.

Don’t believe me? You could try saving the file to your hard drive, opening it up in an image editor (even MS Paint will do) and using the eyedropper tool. If you couldn’t be bothered doing that, here’s an alternative demonstration that I found on youtube…

* O.k, for the pedants, they are exactly the same shade of grey.
** A blackbody is an ideal rather than an actual thing. It’s a model of a perfect absorber and emitter of radiation that can be adjusted for bodies with different temperatures. The Sun is not a perfect blackbody (if it was, the edge of the yellow squiggly bit of the graph would match up perfectly with the black curve), but as can be seen from the graph, it is a pretty close approximation and helps us to simplify things.
*** “nm” stands for “nanometres”. There are 1,000,000,000 (1 billion) nanometres in one metre.

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