What could make a star green?

We can never perceive a green star with our eyes alone without some sort of substrate between the two blocking the red and blue light emissions. This is because green falls smack dab in the middle of the visible light spectrum. When a star a appears blue, most of what is being emitted by the star is photons with a frequency that appears blue, and photons with a frequency of ultraviolet or higher. Same goes for red stars, except instead of ultraviolet, it emits frequencies in infrared and lower, resulting again in light that our eyes cannot pick up. We only see the red photons that our eyes are able to pick up.

If we could see further into the ultraviolet and infrared spectrum, "blue" and "red" stars would most likely disappear in favor of whatever colors blue+ultraviolet and red+infrared look like. This is what we see when we look at a white star. It's a star emitting most of its light in the green spectrum, but also emitting reds and blues, thus appearing white.

(Fun side note: Our sun is actually what a "green" star would look like if we could actually see them. It's white when viewed from space. It only changes colors and appears primarily yellow due several factors affecting the photons in our atmosphere. Absorbption of certain wavelengths, Rayleigh scattering, ect.)

Completely unrelated, but the green stars reminded me of the chapter from Surely You're Joking Mr Feynman where he's judging math books [1]:

> Finally I come to a book that says, "Mathematics is used in science in many ways. We will give you an example from astronomy, which is the science of stars." I turn the page, and it says, "Red stars have a temperature of four thousand degrees, yellow stars have a temperature of five thousand degrees . . ." -- so far, so good. It continues: "Green stars have a temperature of seven thousand degrees, blue stars have a temperature of ten thousand degrees, and violet stars have a temperature of . . . (some big number)." There are no green or violet stars, but the figures for the others are roughly correct. It's vaguely right -- but already, trouble! That's the way everything was: Everything was written by somebody who didn't know what the hell he was talking about, so it was a little bit wrong, always! And how we are going to teach well by using books written by people who don't quite understand what they're talking about, I cannot understand. I don't know why, but the books are lousy; UNIVERSALLY LOUSY!

[1] http://www.textbookleague.org/103feyn.htm

As an aside: there is a rather splendid explanation for stars going green in Ken MacLeod's SF novel Learning the World:

https://en.wikipedia.org/wiki/Learning_the_World

Argh I'm halfway through Absolution Gap now, and there's a quote from the bloody epilogue halfway down the page!

Managed to avoid reading most of it fortunately.

Would dumping enough copper into the star work or would that still glow white?

If there's a redshift, there might be a greenshift?

greenfly

envy?

> You cannot [...]

Says who? This StackExchange is for people writing fiction. They can do whatever the hell they want.

It's fine to advise against certain things that might detract from credibility, but telling people they "cannot" do things comes across awfully self-important.

EDIT: Maybe I'm getting downvoted because of the self-importance of telling people they cannot tell people that they cannot do things? :)