in the constellation Ursa Major
7 million times the mass of the Sun
Diameter roughly 30 million miles (45 million km), about 30 times the diameter of the Sun.
M81 is a bright spiral galaxy with an unusually bright, hot "bulge" of stars at its center. Astronomers have offered two interpretations for this bulge.
One says that the bulge is giving birth to many new stars as the result of an encounter with a neighboring galaxy several million years ago. As the galaxies passed close to each other, their gravity compressed big clouds of interstellar gas and dust, causing them to collapse and form hot, massive stars.
A second interpretation says that the nucleus of M81 is a "mini-quasar" -- a disk of hot gas surrounding a supermassive black hole.
A quasar is one of the brightest objects in the universe. It is powered by a large disk of hot gas spiraling into a supermassive black hole at its center. As the gas gets closer to the black hole it is heated to millions of degrees, so it emits enormous amounts of energy. The cores of many galaxies may once have housed quasars. As the black holes gobbled up the gas around them, however, the quasars shut down, leaving a dark, quiet black hole.
M81's black hole, if it has one, may be in the process of shutting down, so it is fainter than a quasar but brighter than the nucleus of a "normal" spiral galaxy.
Some of the evidence that supports the black-hole interpretation comes from radio telescopes, which have determined that the nucleus of M81 is small and densely packed. Radio telescopes also have detected jets of hot gas flowing away from the nucleus at high speed. Such jets likely are powered by strong magnetic fields in the accretion disk around the black hole.
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This document was last modified: November 19, 2009.