A nebula containing a very hot star can be excited to self luminosity, resulting in what is termed an emission nebula. A nebulous region which is excited to luminosity in this way is also called an H-II region since hydrogen (H) is the most abundant element. Emission nebulae are huge masses of gas that absorb ultraviolet radiation from nearby hot stars and reradiate it as bright-line emission. The most famous example of an emission nebula is Messier Object 42 (M.42), the great nebula in Orion. Another is the Eta Carinae Nebula in the southern sky. The larger emission nebulae are most often associated with the very hot 'O' and 'BO 'stars and may contain dense groups of these most luminous stars. The hot central stars in the emission nebulae often appear to have cleared away the dust from their immediate surrounding, creating a hole or dust-free bubble inside an otherwise dusty region of space. (See the section on Solar Wind)

Nebulae can also become luminous when a nearby bright star causes them to shine by reflected light. The Pleiades was the first reflection nebula to be observed. The reflection nebulae reflect the light of stars embedded within them. It is not known whether reflection nebulae are only dark clouds that happen to be near a bright star or whether some actual physical relationship may exist between the reflection nebulae and the stars that illuminate them. It has been noted that stars of 'B1' or later (see section on spectral Type) produce reflection nebulae, while stars of 'BO' or earlier produce emission nebulae.

In some nebulae, the star producing the illumination is not hot enough to make the nebulosity shine by its own light and the result is a reflection nebula. Perhaps the most famous reflection nebula is the one in the Pleiades star cluster.

Copyright (c) 1997-99 Michael Erlewine