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 that light them (see cluster + nebula).
At the eyepiece
A UHC or OIII nebula filter makes emission nebulae stand out dramatically, even from the suburbs. Use low power and a large exit pupil.
In AstroPilot
Object pages say which filter suits each nebula.