A luminous cloud of gas and dust in space which shines with its own light. The light can be generated in several ways. Usually the gas glows because it is exposed to a source of ultraviolet radiation; examples are H II regions and planetary nebulae, which are ionized by central stars. Gas may also glow because it has become ionized in a violent collision with another gas cloud, as in Herbig–Haro objects. Finally, some of the light from supernova remnants such as the Crab Nebula is produced by the process of synchrotron radiation, in which charged particles spiral around the interstellar magnetic field.
Subjects: Astronomy and Astrophysics.