Kwok, Sun Faculty of Science, University of Hong Kong, Pokfulam, Hong Kong.
Last reviewed:January 2020
- Discovery and catalogs
- Spectral characteristics
- Observing techniques
- Internal dynamics
- Importance of studying nebulae
- Related Primary Literature
- Additional Reading
A concentration of interstellar matter that is largely gaseous but can contain almost all states of matter, including solid particles, and that is a constituent of the Milky Way Galaxy or another galaxy. The term “nebula” was originally used to refer to any fixed, extended, and usually fuzzy luminous celestial object. With increased angular resolution of telescopes, astronomers learned that nebulae can be separated into two classes: those that are stellar systems made up of individual stars, and those that are gaseous in nature and diffuse in appearance. Examples of stellar systems include galaxies (which contain billions of stars and are located outside our own Milky Way Galaxy) and star clusters such as open clusters and globular clusters (which contain thousands of stars and are within the Milky Way Galaxy). This article is restricted to the modern definition of nebulae, which are gaseous objects usually located within the Milky Way Galaxy, although with increasingly powerful telescopes gaseous nebulae can now be observed in external galaxies. See also: Galaxy, external; Milky Way Galaxy; Star
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