A cloud of gas and dust in space. The term was originally applied to any object with a fuzzy telescopic appearance, but with the advent of larger instruments many ‘nebulae’ were found to consist of faint stars. In 1864 W. Huggins discovered that true nebulae could be distinguished from those composed of stars on the basis of their spectra. Nowadays the term ‘nebula’ means a gaseous nebula. The term extragalactic nebula, originally used for galaxies, is now obsolete. There are three broad types of gaseous nebula: emission nebulae, which shine by their own light; reflection nebulae, which reflect light from nearby bright sources such as stars; and dark nebulae (or absorption nebulae) which appear dark against a brighter background. This broad classification scheme has been carried over to other wavelengths, giving rise to terms such as infrared reflection nebula and infrared dark cloud. Emission nebulae include the diffuse nebulae or H II regions around young stars, planetary nebulae around old stars, and supernova remnants.
Subjects: Astronomy and Astrophysics.