A nebula is an interstellar cloud of gas or dust. There are many types of nebulae.

Diffuse nebulae are illuminated nebulae, which can be divided to:

  • Emission nebulae – these contain clouds of ionized gas, excited by ultraviolet light coming from even distant stars. There are two most common types of emission nebulae – HII regions and planetary nebulae.
  • Reflection nebulae – these are illuminated by the reflection of light of nearby stars. The most know reflection nebula is the Pleiades.

Dark nebulae are not illuminated, because they contain cold dust, which neither emits nor reflect the light. The most famous dark nebula is Horse Head Nebula in the constellation Orion.