What is an Emission Nebula?

Emission nebula are clouds of high temperature ionized gas.  Atoms are excited by high energy light from a nearby star and emit photons as they fall back to lower energy states.   Most emission nebula are red because Hydrogen is the most common gas. They emit light at the hydrogen alpha wavelength.  These types of nebula are often the brightest of all nebulas and can be active star formation regions.

In these cases massive gas clouds collapse under their own weight, and then due to the conservation of angular momentum, begin to spin, flatten, and contract. Over a few million years, the temperature at the core with rise until it’s sufficient for nuclear fusion to occur and a star is born.

Good examples of emission nebula include the Orion nebula (M42), the Rosette nebula (Caldwell 49), and the Lagoon nebula (M8).