Washington, Apr 3 (IANS): Supermassive black holes can grow still bigger by ripping apart double-star systems and swallowing one of the stars, says a new astrophysics study.

"Black holes are very efficient eating machines," said Scott Kenyon of the Harvard-Smithsonian Centre for Astrophysics (CfA). "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."

"I believe this has got to be the dominant method for growing supermassive black holes," added Benjamin Bromley of the University of Utah, who led the study, reports The Astrophysical Journal Letters.

Their work follows up on the 2005 discovery, by a team of CfA astronomers led by Warren Brown, of hypervelocity stars - stars that were flung out of the galactic centre by gravitational forces and are travelling fast enough to escape the Milky Way. Hypervelocity stars can travel tens of thousands of times as fast as a bullet.

They originate from a binary (double) star system that wanders too close to the Milky Way's central blackhole. Tidal forces capture one star and eject the other, according to an Utah and CfA statement.

The star that is captured into orbit around the black hole later becomes fodder for the galactic monster. As many as half of all stars are in binary pairs, so they are plentiful in the Milky Way and other galaxies.

The new study looked at each step in the process of a supermassive black hole eating binary stars, and calculated what would be required for the process to match observations.

Their theory shows that the Milky Way's supermassive black hole has doubled to quadrupled in mass during the past five billion to 10 billion years by eating stars.