We already know that black holes swallow stars — and entire solar systems — but what effect does a stellar diet have on black holes? A new study suggests that eating stars is what turns baby black holes into supermassive black holes.
University of Utah physics and astronomy Professor Ben Bromley led a study to answer a question that has been much debated among astronomers: do black holes grow into supermassive black holes as a result of consuming gas or stars? The study, published in the Astrophysical Journal Letters, examines binary stars, pairs of stars the orbit around one another. When a binary star is captured and swallowed by a black hole, its partner is flung from its galaxy at a speed of more than one million mph. When studying the rates at which hypervelocity stars are produced and observations of supermassive black holes, Bromley found the rates of hypervelocity stars were consistent with the rate of tidal disruption events, in which stars are shredded and pulled into supermassive black holes in other galaxies. These findings, Bromley says, would explain how the Milky Way's supermassive black hole has doubled or quadrupled in mass in the last 5-10 billion years:
"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 says. "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."
He also gives us a rather fantastic visual analogy for what's going on with these hungry, hungry black holes:
He refers to the process of a supermassive black hole capturing stars from binary pairs as "filling the bathtub." Once the tub – the area near the black hole – is occupied by a cluster of captured stars, they go "down the drain" into the black hole over millions of years. His study shows the "tub" fills at about the same rate it drains, meaning stars captured by a supermassive black hole eventually are swallowed.
Image from the Jet Propulsion Lab.