Bulge (astronomy)

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For other uses, see Bulge (disambiguation).

In astronomy, a bulge is a tightly packed group of stars. The term almost exclusively refers to the central group of stars found in most spiral galaxies. These bulges are composed primarily of stars that are older Population II, and hence smaller and redder (see stellar evolution). They are also in orbits that are essentially random compared to the plane of the galaxy, from whence the bulge shape arises. Furthermore, they have very little dust and gas compared to the disk portion of the galaxy, explaining why there are so few young stars (that is, there is little material left from which to form stars).

Bulges have similar properties to those of elliptical galaxies, and many astronomers now view them as essentially the same phenomenon. That is, elliptical galaxies are spiral galaxies that have lost their disk (most likely from a galactic merger with the right set of conditions), and all that is left behind is the bulge. Lenticular galaxies, meanwhile, are somewhere in between.

Most bulges are thought to host a supermassive black hole at their center. Such black holes by definition can not be observed (light cannot escape them), but various pieces of evidence strongly suggest their existence, both in the bulges of spiral galaxies and in the centers of ellipticals.

A few galaxies have bulges with Population I blue, young stars, or a mix of the two populations. While far from clearly understood, this is usually taken as evidence of interaction with another galaxy (such as galaxy merging), that sends new gas to the center and promotes star formation.

[edit] See also

• Galaxy formation and evolution
• Galactic coordinate system
• Disc galaxy
  • Spiral arm
• Galactic halo
• Galactic spheroid
• Galactic corona