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How do supermassive black holes get so big? A peculiar diet, perhaps.

New research suggests that the supermassive black holes at the cores of some galaxies could grow so large by consuming one star from a binary system and flinging the other into space.

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Seventeen years later, with supermassive black holes already an accepted feature in galaxies, Warren Brown at the Center for Astrophysics reported finding the first so-called hypervelocity star heading out of the Milky Way. Since then, astronomers have cataloged about 20 hypervelocity stars leaving the galaxy.

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Researchers began to work back from these stellar speedsters to derive an estimate of the number of orphans left in the black hole's vicinity.

“If you just do a back-of-the-envelope calculation, you begin to realize that's a lot of mass – about the same, if not more than the mass of the black hole itself,” says Ben Bromley, an astronomer at the University of Utah and the study's lead author. “A good fraction of the black hole's mass likely came from the captured partners” of binaries over the course of the black hole's lifetime so far.

Why would binaries provide ripe pickings for a central black hole? From the black hole's perspective, two stars orbiting a common center of mass represent a significantly larger object than a single star, Dr. Bromley explains, one easier to disrupt at a a longer distance.

With the black hole in the center of the Milky Way, a binary system must approach within about 93 million miles, or one Astronomical Unit (the average distance between the earth and sun), in order to be disrupted. An individual star would need to approach the black hole to within a Mercury-like distance to before the black hole begins to dismantle it, Bromley says.

Over time, the orphaned stars reach a relatively stable number – until a new star is orphaned and dragged into closer proximity to the black hole. At that point, orbital interactions among all the stars in this inner cluster send one hapless star to be torn apart by the black hole itself.

The flare this final dance triggers – a so-called tidal disruption event – has been spotted in other galaxies. Based on these observations researchers estimate the events take place once every 1,000 to 100,000 years. Meanwhile, the galaxy is producing hypervelocity stars at about the same pace range, the team estimates.

The calculations have their limits, Kenyon cautions. For one thing, he says, the team assumes that binary systems at the center of the galaxy have the right-sized stars at the right separation distances to make it easy for the black hole to part them. That may or may not be the case.

In the end, the feedstock adding heft to a supermassive black hole may come from a number of sources. In some cases, however, the team's model for black-hole growth may represent the dominant process. It could explain how black holes in some of the largest elliptical galaxies, with central black holes of several billion solar masses, can bulk up when the galaxies they inhabit have so little gas to feed on.

The team, which included Center for Astrophysics astronomers Margaret Geller and Dr. Brown, notes that additional tests of their proposed feeding mechanism will come with observations of hypervelocity stars and the galactic interior with a new generation of ground and space-based telescopes. These should be able to detect and characterize stars much fainter than current telescopes can capture.  

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