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Bulge (astronomy)
In astronomy, a bulge is a tightly packed group of stars within a larger formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies (see galactic spheroid). Bulges were historically thought to be elliptical galaxies that happened to have a disk of stars around them, but high-resolution images using the Hubble Space Telescope have revealed that many bulges lie at the heart of a spiral galaxy. It is now thought that there are at least two types of bulges: bulges that are like ellipticals and bulges that are like spiral galaxies.
== Classical bulges ==
Bulges that have properties similar to those of elliptical galaxies are often called ''classical bulges'' due to their similarity to the historic view of bulges.〔Sandage, Allan "The Hubble Atlas of Galaxies" Washington: Carnegie Institution, 1961〕 These bulges are composed primarily of stars that are older, Population II stars, and hence have a reddish hue (see stellar evolution).〔 These stars are also in orbits that are essentially random compared to the plane of the galaxy, giving the bulge a distinct spherical form.〔 Due to the lack of dust and gasses, bulges tend to have almost no star formation. The distribution of light is described by de Vaucouleurs' law.
Classical bulges are thought to be the result of collisions of smaller structures. These disrupt the paths of stars, resulting in the randomness of bulge orbits. Following such a merger, gas clouds are more likely to be converted into stars, due to the shocks from the merger (see star formation).
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