We know there are red stars and blue stars, and yellow/white stars like our own Sun, but are there green stars? What would it take to get a star be green?
As you probably know, the color of a star depends on the temperature of its surface. The coolest stars are red, and have a surface temperature of less than 3,500 Kelvin. The hottest stars are blue, and have temperatures above 12,000 Kelvin. Our own Sun gives off an almost purely white light, and it measures 6,800 Kelvin.
Stars can be give off light from every point of the spectrum: infrared, red, orange, yellow, green, blue, indigo, violet, and ultraviolet. Astronomers measure the light curve of the photons coming off a star. In other words, that’s the ratio of photons streaming from the star in every part of the spectrum. The hottest stars have their peak in the blue part of the spectrum, and the coolest stars peak in the red. An average star like our Sun actually peaks in the green part of the spectrum. There are more photons coming from our Sun in the green part of the spectrum, and yet it looks white.
The problem is that stars like our Sun cast off photons in so many colors that it all looks white from our perspective. In order to get a green star, you would need to have a light curve that peaks right at green, but doesn’t give off light in many other colors. And there aren’t any stars that can do that. If you make the star hotter, it just gets bluer. And if you make a star cooler, it just becomes orange and then redder. There’s no way to have a light curve that makes a star look green.
So no, there are no green stars.
There are, however, other objects in space that do look green. These give off enough photons in the green spectrum to overwhelm the other colors. But there aren’t many objects out there.