Are there any green stars? If not why? I know that a star's color is based upon its temperature. Stars seem to exist in every other color in the visible spectrum. Why not green?
Your question is a good one! I actually asked one of my astronomy professors about that once, because it is true that the color of a star depends on its temperature, and stars with a wide range of temperatures do exist. The answer is that there are stars that are green, that is, they emit their peak radiation at a wavelength that we define as green. In fact, the sun is a yellow-green star so is close to that temperature.
However, stars emit radiation over a broad range of wavelengths, and the human eye is most sensitive to yellow and green radiation. When a star is green, it is pretty much right in the middle of the visible spectrum. It is radiating strongly at all visible wavelengths, with most of the radiation right in the middle. When we look at the star, then, all these colors are mixed and the result is the color white. So you won't ever see a green-looking star through a telescope.
There are also purple stars, which emit peak radiation in the violet part of the spectrum. But we don't see purple stars either because the human eye is more sensitive to blue light than to purple light. If a star is emitting a lot in the violet, it will also be radiating in the blue, and so these stars look blue to us. This is why the colors that we see for stars are:
with red being the coolest stars and blue the hottest.
This page updated on June 27, 2015