Why are blue stars the hottest when we associate blue with cold temperatures?
In: Physics
Wow these are some vague comments…, there is not a single actual explanation in here. In fact, half the posts are freaking questions. Why do you guys bother posting when you don’t have an answer?
Right, so, the reason we associate blue with coldness has nothing to do with temperature and even less to do with stars. It’s an accident of our circumstances. A lot of different variables conspire to give us that association, but it doesn’t have anything to do with light itself.
If you feel brave, take a look at [Mie scattering](https://en.wikipedia.org/wiki/Mie_scattering) and [Rayleigh scattering](https://en.wikipedia.org/wiki/Rayleigh_scattering). Short version of the story is that, in winter, the sun remains low, which results in more blue light being scattered around, giving everything this wintery, cool look (as well as just a general gray from the lower light levels). Whereas in summer, the atmosphere scatters far less, since the sun is much higher, so everything appears much closer to the color of the sun, which happens to be sort of orange and very much not blue. The story is a good bit more complicated than that, too much so for this comment. There are a lot of reasons why we associate blue with the cold, such as the color of water, ice and the sky, say. But ultimately, it’s just an accident, on a different planet, people may very well associate red with coldness. After all, Mars is pretty red, yet absolutely freezing.
The color of stars is, for the most part, a direct result of something called [Black-body radiation](https://en.wikipedia.org/wiki/Black-body_radiation). In other words, all baryonic matter emits light according to its temperature. Baryonic matter is basically all matter, excluding weird stuff like neutrinos and whatever the hell a black hole is. Among some other things, but, really, don’t worry about that part.
Now, any such matter always emits light, but at low temperatures the emitted light has very long wavelengths, which are invisible to us. Eventually, somewhere around 600°C, some of the emitted light comes out in wavelengths we can see, though most is still in infrared, which is invisible to humans. However, the higher the temperature goes, the more the light shifts towards visible light, so it gets brighter and slowly turns from red towards yellow, then white and finally blue. Note however that these emissions cover a large part of the spectrum, which is where the white light comes from, since “white” isn’t an actual wavelength, but rather what we see when the entire visible spectrum is covered. In other words, when you reach 7000°C or thereabouts, you get emissions of all the colors, which we see as white. As you go higher in temperature, you get fewer and fewer red photons and more and more blue ones instead, so it slowly shifts from white to blue. And that’s how stars get their colors. They are just very hot. And, as you can see, blue stars are the hottest.
And the actual ELI5 version: Hot things glow red, hotter things glow yellow, even hotter things glow white and the really hot stuff glows blue and all of that is true for everything from a kettle (a very molten one in the later cases) to a star. What exact color any given object ends up having involves more factors than just that, however. It’s just the idealized case.
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