On a clear night, you can point a telescope to the heavens and find stars of varying colors. The super-giant Antares glows a humble orange-red nestled in the middle of Scorpius. Rigel is an ostentatiously bright star in Orion that shines a brilliant blue. There are also red stars, and yellow ones too. One color you will find missing is green. Some sci-fi movies and video games have taken the liberty of making such emerald celestial objects, but you’ll never find one in real life, and here’s why.
The key is the way radiation is emitted by objects as they heat up. A star emits light because it is very, very hot. So make yourself a miniature star as a visual aid. When you heat up a bit of metal with a heat source, what happens? It starts off emitting virtually no light, then slowly moves into a deep red. After a moment, it reaches a muted amber color, followed by orange. Heat it just a little longer, and you end up with a luminous blue-white glow before your metal learning aid melts. At no point, will you see green.
This happens because of what is called black-body radiation. When you plot the visible light colors a heated object can produce, you get a lopsided plot called the black-body curve that covers a wide range of wavelengths with a peak depending on temperature. Visible light is just the narrowest part of the electromagnetic spectrum. Colder objects may still emit radiation in the form of infrared, and very hot ones can spit out ultraviolet or x-rays. The important thing here, is that an object, like a star, doesn’t emit light in a single wavelength.
Let’s say that a star is a little on the cool side, with a brightness of roughly 4500K. The peak of that black-body curve is in the orange part of the spectrum. This is fine — you can see orange stars just fine. Increase to a light temperature of 6000K, and the peak of the curve is in the blue-green section of the EM spectrum. “But wait,” you might be saying, “shouldn’t that appear green, then?.” Nope.
The problem isn’t with the light emitted by the star, but with our eyes. Our sun actually emits most of its photons in the blue-green part of the spectrum. Our eyes are seeing a wide range of wavelengths, though (remember that black-body curve). All this is actually combined and processed as a single color, in this case white (that’s why outdoor light is best for pictures). That wide spectrum black-body radiation is hitting all three types of cones in the human eye; red, blue, and green sensing. Even if a star like the sun is emitting a lot of green wavelength photons, it is emitting plenty of red and blue too.
Make the star a little hotter, and the color shifts to the blue end. A little cooler, and you get more red. The only way your eye could see green in a star is if it was emitting a very narrow spectrum of green light to hit the middle wavelength green cones. That just doesn’t happen. The wide spectrum of visible light coming out of a star will always be mixed together by our limited eyes into the established stellar colors, but they still look pretty neat.
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