Why is the sun green?

What colour is the sun? Most people will probably say yellow if countless childhood drawings are anything to go by. If you actually look (don’t! You’ll hurt your eyes) you’ll see that it looks white. But is it actually?

The sun emits something called black body radiation. Everything that has a temperature puts out light, or rather electromagnetic radiation. What colour this radiation is depends upon the temperature of the object. Let’s look at some curves.

Black body radiation curve at different temperatures.

Black body radiation curve at different temperatures.

These are black body radiation curve. They show how bright the light at a certain colour, or wavelength, is for objects at different temperatures (given in Kelvin, which is the temperature in Celcius + 273 degrees). The rainbow shows the wavelengths that are visible to the human eye.

Notice that the peak shifts sideways depending on what temperature the object is. At 3000-4000K the curve only has it’s tail in the visible? You can still see this, but it’s not hugely bright and will probably look a little on the red side. At 5000K you’ll really see it growing, and it will look whiter as the curve goes across the whole of the rainbow, and if you add all the colours of the rainbow together you get white. By 6000K we have something that really looks white without a hint of red, the peak landing slap bang in the middle of the spectrum. If you put even hotter curves on here then you’d see that they were almost entirely in the blue, so these would look blue. Keep going at it will get soooo hot that most of it’s radiation is in the UV and will look quite dim indeed.

The sun is at about 5578K, which means it’s peak is at a wavelength of 502nm, which means that the sun is, infact, green! Of course, this is only the peak of its emission. It’s still glowing all the way across the spectrum which is why it looks white but maybe next time your adding a sun to your drawing you’ll reach for the green and not the yellow paint.

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