Emission nebula

The most common source of ionization is high-energy ultraviolet photons emitted from a nearby hot star.

[1][2] Usually, a young star will ionize part of the same cloud from which it was born, although only massive, hot stars can release sufficient energy to ionize a significant part of a cloud.

In many emission nebulae, an entire cluster of young stars is contributing energy.

Most emission nebulae are about 90% hydrogen, with the remaining helium, oxygen, nitrogen, and other elements.

Emission nebulae often have dark areas in them which result from clouds of dust which block the light.