A general term for material, usually gas and dust, surrounding a star. After a star's birth, the surrounding cloud absorbs the visible light, re-emitting it as infrared radiation. Most young stars go through a violent phase, ejecting material at speeds of hundreds of kilometres per second. When this ejected material collides with the surrounding gas, bipolar molecular outflows (see bipolar outflow) and Herbig–Haro objects can result. If the star is hot its ultraviolet radiation ionizes the gas, producing an emission nebula. These combined effects destroy the cloud, so that the star becomes visible. For most of the star's life a tenuous stellar wind blows. At the end of a star's life it ejects mass copiously. Depending on the mass of the star and the stage in its evolution, this may result in a cool circumstellar envelope, a planetary nebula, or even a supernova remnant.
Subjects: Astronomy and Astrophysics.