Happy St. Patrick's Day! In the United States, this is the day where we celebrate the rich Irish culture of green things and vast amounts of booze. Although, usually the harder celebration takes place over the weekend. Everybody is Irish for a day! It also works well because this is when spring comes around again and things start getting green again.
With green in mind, lets have some green space science:
First, we have this awesome green space blob, known as Hanny's Voorwerp (Dutch for Hanny's Object). It was discovered in 2007 by a Dutch school teacher through the Galaxy Zoo project. Note, the green is false color and represents oxygen emission lines. The object is now known to be about the size of the Milky Way Galaxy, and if the afterglow from an ionized dust cloud from a dead quasar (the galaxy seen above the object).
Besides looking very Cthulhu like, it serves to illustrate a point. Most of the astronomy images you see are false color. I think there tends to be an idea that the universe is filled with the brilliant array of a full spectrum of colors. Mostly these colors are assigned to different emission lines from a spectrum, and the actual colors of the Universe are rather dull. But the colors are useful for astronomers, because it helps us identify the building blocks of our Universe. Green actually turns out to be a pretty rare color.
But that isn't to say there aren't green objects in space. Take a look at NGC 6826, a planetary nebula taken from the Hubble Space Telescope. This is a pretty close to true color image, and when seen through a telescope, it does appear green. It also happens to be oxygen rich, which emits color right in the middle of green on the spectrum.
One last object to show you that is green. That is the planet Uranus, whose upper atmosphere is mostly methane. Methane absorbs red light, giving the planet a blue-green hue.
You might also notice that none of these objects are stars. That's because stars don't appear green. It is a pretty tricky thing, and really isn't an issue with stars themselves, the issue (mostly) lies in the human eye. Our Sun emits white light. All the colors in the visual spectrum are emitting from the Sun, and as they are all mixed together, it appears white. Our eyes evolved to suit this environment, so that the light from our Sun appears to be white and other colors are easily discernible.
The problem arises in that the average star like our Sun peaks its emission in the green part of the spectrum. But because our eyes need to see stuff on Earth, it cannot pick out just the green in stars, because red and blue light are also effecting our eyes receptors. (The eyes have receptors, called cones, for red, blue, and green.) In order for their to be a green star, you would need a star that emits only green light, and no other light on the spectrum.
Stars that emit towards the red side of the spectrum, and are therefore cooler, tend to look red; while blue stars are cooler. You can see an example of this in the heating an iron bar, it will go red, then orange, then blue-white, and then melt. You won't see green. And stars won't just emit green light, because they have a lot of elements in them the pull in red and blue light. So there are no green stars, but there are plenty of other colored stars that you probably won't miss it. Happy St. Patrick's Day!