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Black Holes Encyclopedia


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Alternate Names

Mayall II


Intermediate mass


near the Andromeda galaxy (M31)

Finder Chart


2.3 million light-years (0.76 megaparsecs)


14,000 to 23,000 times the mass of the Sun


52,000-86,000 miles (84,000-138,000 km), roughly equivalent to the diameter of Jupiter

Discovery Methods



G1 is a cluster of about 12 million stars that orbits the center of M31, the Andromeda galaxy. Its center appears to contain a black hole about 18,000 times the mass of the Sun. The black hole is one of the best candidates for the class known as intermediate-mass black holes -- black holes that are more massive than stars, but much less massive than the black holes that inhabit the cores of many galaxies.

There is an intense debate about whether G1 contains a central black hole. Different teams, using similar data but different analysis techniques, argue opposite sides of the debate. However, the most recent results, based on observations with Hubble Space Telescope and the Keck Observatory in Hawaii, side with the black hole interpretation. Because they combine the light-gathering power of one of the world's largest telescopes with the clarity of HST, these observations represent the best possible set of data that can be obtained for black hole studies.

Astronomers have long classified G1 as a globular cluster -- a tightly packed ball-shaped cluster of old stars. Globulars form along with their parent galaxies, and are among the galaxies' oldest members.

Recent observations and analysis, however, suggest that G1 may not be a classic globular cluster. Instead, it may be the merged cores of several small galaxies. The cluster is brighter than most globulars, its stars show a more varied chemical composition, and the dispersed remains of a galaxy appear to linger near the cluster.

The possible black hole lends credence to the idea that G1 is a galactic core. Observations show that there is a relationship between the mass of the stars in the "bulges" found in the centers of many galaxies, and the supermassive black holes found in the centers of the bulges. The mass of G1's black hole, which is inferred from the speeds of stars that orbit the cluster's core (measured with Hubble Space Telescope), seems to follow this same relationship.

There is intense interest as to whether globular clusters contain black holes and, even more basic, whether intermediate-mass black holes even exist at all. Astronomers have little knowledge of how supermassive black holes grow, and a natural idea is to have them start as much smaller black holes which later grow by "accreting" mass from stars and clouds of gas and dust.

The intermediate mass black hole in G1 strengthens the idea that supermassive black holes form from "seed" black holes that grow along with the bulges of stars in their cores.



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This document was last modified: November 8, 2011.


Artist's Rendering

Ground-Based Photo

Space-Based Photo


Animation takes you into the heart of G1 to view its central black hole.