Question: Why is the sun yellow?

Ian Stephenson answered on 17 Jun 2014:

There’s a bunch of stuff there… from nuclear physics, to human perception, via anatomy!

First off nuclear fusion in the sun gives of electromagnetic radiation at a wide range of frequencies. A lot of it gets blocked by the atmosphere (which is a good thing!). What gets through, we experience as light and heat.

Our eyes have “cones” in them that get triggered by different frequencies of light. There are three types, so any spectrum of light will trigger each type to a different degree. We use the relative amounts of each to determine a colour, and give it a name.

When you have “equal” triggering of all three cone types we call that white. The trouble is that “equal” is pretty fuzzy – how much red is equal to how much blue, or green? It’s subjective. So our brains look around and decide that what we can see must be a mix of colours, that are kind of white on average. If you take a white sheet of paper out in the sun, it will look white. If you take it indoors under fluorescent lights it will look white., but its actually changed colour! If you can find somewhere with a suitable window and put a white sheet of paper outside, then go inside into artificial lighting, let your eyes adjust, and then glance outside at the paper it will look yellow, because the sun has more energy in that part of the spectrum that fluorescent lights do. But if you let your eyes adjust to sunlight, it will look white again…

colour is tricky stuff!