Supermassive Black Holes Grow by Eating Binary Star Partners

Posted on April 2, 2012

A study by University of Utah astrophysicist Ben Bromley and colleagues at the Smithsonian Astrophysical Observatory has found that supermassive black holes grow by swallowing single stars from pairs of stars that wander too close. The researchers say that during the last 10 billion years, the Milky Way's supermassive black hole ate 10 million solar masses.

The process works where the black hole captures one star, while sending the other star hurling out of our galaxy at an extreme velocity. You can read more about these escaping runaway planets here. The above artist's conception shows a supermassive black hole with its tremendous gravity capturing one bluish star from a pair of binary stars, while hurling the second yellow star away at a hypervelocity of more than 1 million mph.

Professor Ben Bromley says, "We found black holes grow enormously as a result of sucking in captured binary star partners. I believe this has got to be the dominant method for growing supermassive black holes. There are two ways to grow a supermassive black hole: with gas clouds and with stars. Sometimes there's gas and sometimes there is not. We know that from observations of other galaxies. But there are always stars.'

Bromley also says, "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. 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."

The researchers 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 work in terms of the rates at which hypervelocity stars are produced, binary partners are captured, the captured stars are bound to the black hole in elongated orbits and then sucked into it. The scientists compared the results with actual observations of supermassive black holes, stars clustering near them and "tidal disruption events" in which black holes in other galaxies are seen to shred stars while pulling them into the hole.

Bromley says, "It fits together, and it works. 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."

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