Is it yellow because from Earth it usually looks yellow to us? Or is it white because the sun gives off all wavelengths of light (white light)? Or is it some other color?
In: Planetary Science
Short answer, white.
To some extent, everything is white. The light we see from our sun is from something called “black body radiation”. It’s a fancy way of saying that when things get hot, they start glowing. Fun fact, this is what we’re talking about when we talk about light “temperature”. A 4000K light has the same spectrum as a piece of matter heated to 4000 Kelvin.
The weird thing is that they glow in the entire electromagnetic spectrum. They start emitting everything from radio waves, to gamma waves, every wavelength between them and every wavelength beyond them. That includes the entire visible spectrum. Lower frequencies have less energy and are emitted more. Higher frequencies have more energy and are emitted less.
The other weird thing is that the amounts of each wavelength have nothing at all to do with the type of element that’s getting hot. It’s entirely a question of how hot it is. The hotter it is, the more EM radiation is released at each wavelength.
We can only see a little bit of that radiation. It needs to be in the visible spectrum and it needs to be bright enough. A cold lump of steel emits so little of any radiation that it looks black (unless you have an other light source). As it gets hotter, the reds are the first wavelengths to get strong enough to see. As it continues to get hotter, you’ll see more and more of the higher wavelengths and that makes it look more orange and then yellow. Eventually you see so many of all the visible wavelengths that it just looks white.
There are variations between whites though. Hotter stars look “bluer” that is, they have more light visible at shorter wavelengths. Cooler stars look “redder”, for the opposite reason.
Then there’s an other weirdness. Some of the gasses in the atmosphere absorb very specific wavelengths and re-emit them at longer wavelengths. If you look at a star with a spectroscope (basically a box with a prism and a very clear screen) you can see black bands of missing light. Those are the wavelengths absorbed by certain gasses. So it’s not “truly” white, since we’re missing a few wavelengths. It’s pretty useful since we can use it to analyze the atmospheres of stars.
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