Nebula
Nebula, in astronomy, a localized conglomerate of the gaseous and finely divided dust particles that are spread throughout interstellar space. Before the invention of the telescope, the term nebula (Latin, “cloud”) was applied to all celestial objects of a diffuse appearance. As a result, many objects now known to be star clusters or galaxies were called nebulas. Nebulas exist within other galaxies as well as in our own Milky Way galaxy. They are classified as planetary nebulas, supernova remnants, and diffuse nebulas, including reflecting, emission, and dark nebulas. Small, very bright nebulas known as Herbig-Haro objects are found in dense interstellar clouds, and are probably the products of gas jets expelled by new stars in the process of formation. Diffuse nebulas are extremely large structures, often many light-years wide, that have no definite outline and a tenuous, cloudlike appearance. They are either luminous or dark. The former shine as a result of the light of neighboring ...