Supermassive Black Holes Grow Larger By Dining On Suns

Ever wondered how a supermassive black hole got to be, well, supermassive? According to a new study led by an astrophysicist from the University of Utah, black holes get to be supermassive by feasting on surrounding stars.

Supermassive black holes are now known to rest at the center of mature galaxies, but since their discovery, scientists have been puzzled as to what the black holes were feasting on to grow to such enormous sizes.

Ben Bromley, lead author of the study, says that it isn’t just the result of black holes feasting on stars that causes ordinary black holes to grow into supermassive monsters–they were also feasting on binary star systems (a pair of stars that orbit each other).

“Black holes are very efficient eating machines. 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,” said Scott Kenyon, one of the study’s researchers. “I believe this has got to be the dominant method for growing supermassive black holes,” Bromley added.

The study follows up on the discovery that some stars in the galactic core were being flung out of the Milky Way by the supermassive black hole resting in the galaxy’s core. This happens when binary star systems are pulled too close to the supermassive black hole, and one is then flung away at unimaginably high speeds–what scientists then call “hypervelocity” stars. The other, perhaps less fortunate star, is captured in the black hole’s orbit, where it will slowly be digested by the supermassive black hole.

“We put the numbers together for observed hypervelocity stars and other evidence, and found that the rate of binary encounters [with our galaxy’s supermassive black hole] would mean most of the mass of the galaxy’s black hole came from binary stars,” Bromley explains. “We estimated these interactions for supermassive black holes in other galaxies and found that they too can grow to billions of solar masses in this way.”