How Do Supermassive Black Holes Get So Big?

PHOTO: Sagittarius A black hole
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For years, astronomers have puzzled over the diet needed to bulk up supermassive black holes – powerful gravitational traps that lurk in the centers of galaxies.

Now, scientists have added a new ingredient that they say could account for a substantial portion of a supermassive black hole's heft: stars wrested from binary star systems that wandered too close for their own good.

One star in a binary paring gets ejected from the galaxy at extraordinary speeds. The star left behind orbits even closer to the black hole, joining other stars the black hole has orphaned.

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At some point, the zone in which the orphans orbit reaches its capacity. After that, gravitational interactions between all the orphans virtually guarantee that when a new star enters the zone, another star must vanish into the black hole itself, signaling its end with a brief flash of high-energy radiation.

Since about half of the stars in the galaxy appear as binary pairs, they should be abundant enough to feed the process, the team holds.

The calculations that led to this scenario represent "a proof of concept," notes Scott Kenyon, a researcher at the Harvard-Smithsonian Center for Astrophysics, based in Cambridge, Mass., and a member of the team formally presenting the scenario today in the journal Astrophysical Journal Letters.

Processes operating at the heart of the Milky Way, and by extension other galaxies, aren't well understood, Dr. Kenyon explains. The team hopes its work will help open a window on those processes – in particular, how supermassive black holes grow.

Black holes are objects with gravity so strong that not even light can escape. So-called stellar mass black holes form from the remnants of a massive star that ends its life in an enormous explosion known as a supernova.

But the path from a stellar-mass black hole to a supermassive black hole with the mass of millions or billions of stars is far less clear. A range of possible explanations exists for starting the process – from the direct collapse of a gas cloud to the slow accumulation and merger of stellar-mass black holes at a galaxy's center.

Unfortunately, "they all have problems," Kenyon says.

Enter binary stars, which became candidates as black-hole fodder through a somewhat circuitous route.

In 1988, when the study of supermassive black holes in galaxies was in its infancy, Jack Hills, an astrophysicist at the Los Alamos National Laboratory, proposed that the smoking gun for a supermassive black hole at the center a galaxy would come in the form of stars vaulting from the galactic center at speeds of more than 1 million miles an hour – essentially fast enough to escape from the galaxy. He calculated that the gravitational interaction of a supermassive black hole with a close-in binary star would eject one of the two stars and draw the remaining orphan ever closer.

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.

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.

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