In astronomy, a bulge is a huge, tightly packed group of stars. The term commonly refers to the central group of stars found in most spiral galaxies. A giant Hubble mosaic of the Crab Nebula. ... The Pleiades, an open cluster of stars in the constellation of Taurus. ... A spiral galaxy is a type of galaxy in the Hubble sequence which is characterized by the following physical properties: Spiral Galaxy M74 presents a face-on view of its spiral arms. ...
The bulge in galaxy spirals is usually composed of Population II stars, small, red and old. This is because all stars were born together with the galaxy, thus at least several billion years ago. Only small and reddish stars can live for this long. Stars can be grouped into two general types called Population I and Population II. The criteria for classification include space velocity, location in the galaxy, age, chemical composition, and differences in distribution on the Hertzsprung-Russell diagram. ...
Most bulges are thought to host a supermassive black hole at their center. Such black holes have never been directly observed, but many indirect proofs exist. Top: artists conception of a supermassive black hole drawing material from a nearby star. ...
Some 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.
Bulges have similar properties to those of elliptical galaxies (scaled down to lower mass and luminosity).
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