Observations (AstronomyCast) Nebulae

A nebula is an interstellar cloud of dust composed mainly of hydrogen and helium with some other ionized gases. Originally nebula was the name for any extended astronomical object, including galaxies beyond the Milky Way. Many nebulae form from the gravitational collapse of gas in the interstellar medium. As the material collapses under its own weight, massive stars may form in the center, and their ultraviolet radiation ionizes the surrounding gas making it visible at optical wavelengths. There are four main types of nebulae: diffuse, planetary, protoplanetary, and supernova. Diffuse nebulae are extended and contain no well defined boundaries. Planetary nebulae form from the gaseous shells that are ejected from low-mass asymptotic giant branch stars when they transform into white dwarfs. Protoplanetary nebula are astronomical objects which are at the short-lived episode during a star's rapid stellar evolution between the late asymptotic giant branch phase and the following planetary nebula phase. A supernova nebula occurs when a high-mass star reaches the end of its life.