Utah astrophysicist helps determine how black holes grow

For the past few years, scientists have wondered about mysterious stars at the fringes of the Milky Way, sailing away from the galaxy's center at ultra-fast speeds.

Now a paper co-authored by a University of Utah astrophysicist suggests these "hypervelocity" wayfarers are proof that a massive black hole has grown to gargantuan proportions over billions of years by eating stars, millions of them.

The black hole captures binary star systems, eventually consuming one of the stars and flinging its partner into the galactic boonies, according to a new study Ben Bromley published with colleagues at the Smithsonian Astrophysical Observatory in Cambridge, Mass.

"Black holes are very efficient eating machines," said co-author Scott Kenyon. "They can double their mass in less than a billion years. That may seem long by human standards, but over the history of the galaxy it's pretty fast."

Some scientists have discounted this notion because a moving star would cruise past a black hole, which are puny relative to their massive weight. But half the galaxy's solar systems are binary — two stars orbiting each other — which would more likely be drawn into a black hole, according Bromley, a professor in the U. department of physics and astronomy.

Recently published research by British and Australian scientists concludes that gas, not stars, is the main course on black holes' menu. In other galaxies, black holes are often thousands of times larger than the one in middle of the Milky Way, according to University of Leicester astronomer Andrew King.

"These hugely massive black holes were already full grown when the universe was very young, less than a tenth of its present age," King said in a news release about his research. His explanation is that discs of gas orbit the black hole, then collide and collapse into it. Picture two guys riding motorbikes on a Wall of Death at different angles.

"They collide, they lose the centrifugal force holding them to the walls and fall," King said.

The star-eating hypothesis had its genesis in the mid-1980s at Los Alamos National Laboratory, where astrophysicist Jack Hills theorized how to prove the existence of an unobservable black hole at the center of the Milky Way, the collection of 100 billion stellar systems that sprawls across our night sky. Hills postulated that a black hole's gravitational pull would "disrupt" two-star binary systems, capturing one and ejecting the other.

It took nearly two decades to finally observe one of these ejected stars.

Bromley and Kenyon's co-author Warren Brown detected — with the help of the Sloan Digital Sky Survey in 2005 — a blue star traveling at more than 1 million miles per hour, or about five times faster than typical. Brown's Smithsonian team has since documented about 30 such stars.

"We targeted blue stars at least twice the size of our sun," Kenyon said. "They shouldn't be out at these distances, 150,000 to 200,000 light years from the center. They couldn't have formed there and they're going so fast they can't have been there long. These are a long way out in the boondocks where stars aren't forming."

The scientists believe these "hypervelocity" stars are the remains of binary systems that strayed close to the massive black hole. The one at the Milky Way's center has the weight of 4 million suns, yet its diameter is no bigger than the orbit of Mercury, the planet closest to the sun.

"That's like the Earth fitting inside the Salt Lake Valley," Bromley said.

His team theorized that the black hole captures a star about once every 1,000 to 10,000 years.

"When we look at observations of how stars are accumulating in our galactic center, it's clear that much of the mass of the black hole likely came from binary stars that were torn apart," Bromley said.

Another Smithsonian co-author is Kenyon's wife, Margaret Geller. The scientists' home institutions funded the study, which was published online Monday in the Astrophysical Journal Letters.

bmaffly@sltrib.com —

Black holes: Star eaters or gas guzzlers?

Small black holes form when a star collapses on itself, becoming so dense that no light can escape its gravitational pull. At the centers of most galaxies, however, are "supermassive" black holes ranging in weight from 1 million to 10 billion stars. There is no consensus among astrophysicists about how these holes get so big. Some say they vacuum in vast amounts of gas, while others suspect they eat stars. A new study out of the University of Utah proposes a theory backing the star-eating scenario.