If you think about the sheer number of other stars out there, and the range of possible colors our eyes can detect, it seems like there should be green stars, doesn’t it? After all, there’s very clearly a green band in a rainbow. But a rainbow is just a demonstration of all the independent colors that make up the white light we get from our sun, and the sun itself is glowing to produce that light.
Takakkaw Falls, Yoho National Park, British Columbia, Canada. Image credit: Michael Rogers, CC BY-SA 3.0
All stars glow in a particular color depending on how hot they are, but they’re not glowing only in that color in the same way that we see the green light in the rainbow only because it’s been separated from the other colors. Each star glows in a range of colors (corresponding to a range of frequencies of light), but they will produce some frequencies more than others. It turns out that we can predict the exact range of colors produced by a star, along with the frequency it produces the most. This peak frequency is determined entirely by the temperature of the star. We can do this calculation so easily because stars are well described as a black-body -- any object which reflects no light, and glows according to its own internal heat falls into this class of objects.
There are a couple of black-body objects on Earth we might be familiar with, and they’re all things we should avoid touching. Molten iron, for instance (and in fact any forged metal), will glow because of the temperature to which it has been heated in the forge. Both red-hot and white-hot metal will burn you badly, but the brighter white that metal is, the hotter it is. We actually use this property for science on Earth; for example, if you’re studying volcanoes. If you’d like to know how hot the lava is without sticking a thermometer in it by hand (potentially dangerous), you can find out the temperature by checking out the color of the uncooled lava. The closer to yellow-white that lava is, the hotter it is.
Metal, after being heated in a forge, glows bright yellow, cooling to a darker orange-red. Image credit: Alex Lines, CC 2.0 A-SA
But this doesn’t explain the lack of green stars. We can certainly make things burn green - we’ve managed to come up with green fireworks, for instance, but this burning is not the same as heating something until it glows green. Green fireworks are usually that color because certain salts (usually copper chloride or barium chloride) have been mixed in with the gunpowder. Heating those salts makes them glow at certain frequencies, just like a fluorescent light bulb. Critically, this isn’t a heat-based process
Green fireworks explode in the night sky; the green due to the inclusion of barium salts. Image credit: Jerry Daykin, CC-2.0-A-SA
I said earlier that based 100% on the temperature of the star, I can tell you the peak frequency of light that star produces. By that logic, it’s entirely possible for a star to have a most-produced color that is green. And in fact this is true; and technically this could be called a green star. However, if you were to go look at that star, or any other black body object, heat its way to a peak frequency of green, what you would see would be disappointingly un-green. As you heat from the lowest temperatures, you go from a dark red, to orange, to yellow, to white. From white, if you continue heating, you can get to blue, but blue black-body objects tend to be extremely hot, with peak frequencies in the ultraviolet, and what we’re seeing as blue is the cooler tail extending down into the blue end of the visible spectrum.
We’ve totally skipped green! We replaced it with white. Because green is close to the center of the visible range, even if a star is producing a lot of green light, it’s also producing a lot of yellow and blue light, and the mixture appears white to our eyes. If the peak of the light production is off to one side or another, we can spot the shift towards orange or blue, but with green, we see a near perfect white light.