M17 - The Omaga Nebula
Gaseous Nebula are clouds of interstellar gas and dust that emit, reflect, or absorb light. The energy source for emission nebula (like M17 shown above) is ultraviolet light produced by very hot stars located inside the nebula. The UV ionizes the interstellar gas, causing it to glow much like a neon sign, except that hydrogen (the most abundant element in the universe) is the main gas that glows in a nebula.
The largest emission nebula contain active stellar nurseries, where hundreds of stars are in the process of being born. The Orion Nebula is the most active star-forming region within a few thousand light-years of Earth.
More examples of Emission Nebula
When massive stars reach the end of their life cycle, they become unstable and eject most of their gas and dust into space, forming a "Planetary Nebula" surrounding the stellar corpse. There are several such nebula visible in small telescopes, the Ring Nebula (M57), the Dumbbell Nebula (M27) and the Crab Nebula (M1), which is the remains of supernova explosion that took place in 1054 AD.
Examples of Planetary Nebula
Images and Text: Copyright 1996, Christopher Galfo.