A while back, I wrote about why there are no green stars in the sky (you should read that post first; some of the concepts below are explained there). But that's not to say there are no green objects in space. There are!
Low-density gas clouds, like NGC 6826 (which I studied for my Masters degree, incidentally), emit light in a very different way than stars do; they aren't blackbodies at all. They are more like fluorescent lights, giving off lights at very specific colors, and not continuously across the spectrum. If the cosmic gas is hot, and contains oxygen, the nebula can strongly emit light at about 500 nanometers, smack in the green part of the spectrum. They also emit some red and other colors, but they don't emit across the entire spectrum like stars do. These objects do in fact look very green through the telescope, but remember, ...
