Now this should bother you, because there is no such thing as a green star! What causes this green glow? So when you see that green glow coming from what looks like a very diffuse star, you'll know what that is. The ultraviolet light that permeates the nebula excites the electrons in the oxygen atoms, and when they transition back down, that green glow we see comes from those oxygen atoms fluorescing. The green color tells you it isn't a star, but the halo of oxygen tells you it used to be!