# Emission nebula

Also called: HII region, H II region, star-forming region.

A cloud of gas made to glow by the ultraviolet light of hot young stars inside it — a stellar nursery.

Young, very hot stars flood their surroundings with ultraviolet light, which strips the electrons from hydrogen and oxygen atoms; when they recombine, the gas glows at particular colours — deep red hydrogen-alpha, blue-green hydrogen-beta and doubly ionised oxygen (OIII). Photographs show red; the eye, insensitive to deep red in the dark, sees grey or a faint green.

The Orion Nebula (M 42), the Lagoon (M 8) and the North America Nebula (NGC 7000) are the great examples. Many contain the young [clusters](https://astropilot.space/glossary/open-cluster) that light them (see [cluster + nebula](https://astropilot.space/glossary/cluster-nebula)).

**At the eyepiece:** A UHC or OIII [nebula filter](https://astropilot.space/glossary/nebula-filter) makes emission nebulae stand out dramatically, even from the suburbs. Use low power and a large [exit pupil](https://astropilot.space/glossary/exit-pupil).

**In AstroPilot:** Object pages say which filter suits each nebula.

**Examples:** [M42](https://astropilot.space/object/M42.md), [M8](https://astropilot.space/object/M8.md), [NGC7000](https://astropilot.space/object/NGC7000.md), [M17](https://astropilot.space/object/M17.md)

**Related:** [Nebula](https://astropilot.space/glossary/nebula.md), [Nebula filter](https://astropilot.space/glossary/nebula-filter.md), [Cluster + nebula](https://astropilot.space/glossary/cluster-nebula.md), [Reflection nebula](https://astropilot.space/glossary/reflection-nebula.md)

From the AstroPilot astronomy glossary: https://astropilot.space/glossary/emission-nebula