Supermassive Black holes eat stars to grow bigger

Supermassive black holes repeatedly capture and swallow single stars from pairs of stars that wander too close, says a new study led by a University of Utah astrophysicist.

Using new calculations and previous observations of our own Milky Way and other galaxies, “we found black holes grow enormously as a result of sucking in captured binary star partners,” says physics and astronomy Professor Ben Bromley, lead author of the study, which is set for online publication April 2 in Astrophysical Journal Letters.

NASA handout illustration of a growing supermassive black hole: Reuters

A binary pair of stars orbiting each other “is essentially a single object much bigger than the size of the individual stars, so it is going to interact with the black hole more efficiently,” he explains. “The binary doesn’t have to get nearly as close for one of the stars to get ripped away and captured.”

Black holes are objects in space so dense that not even light can escape their gravity, although powerful jets of light and energy can be emitted from a black hole’s vicinity as gas and stars are sucked into it.

Small black holes result from the collapse of individual stars. But the centers of most galaxies, including our own Milky Way, are occupied by what are popularly known as “supermassive” black holes that contain mass ranging from 1 million to 10 billion stars the size of our sun.

Astrophysicists long have debated how supermassive black holes grew during the 14 billion years since the universe began in a great expansion of matter and energy named the Big Bang. One side believes black holes grow larger mainly by sucking in vast amounts of gas; the other side says they grow primarily by capturing and sucking in stars.

Just last month, other researchers published a theory that a black hole sucks in “food” by tipping its “plates” – two tilted gas disks colliding as they orbit the black hole – in a way that makes the speeding gas slow down so the black hole can swallow it.

The new theory about binary stars – a pair of stars that orbit each other – arose from Bromley’s earlier research to explain hypervelocity stars, which have been observed leaving our Milky Way galaxy at speeds ranging from 1.1 million to 1.8 million mph, compared with the roughly 350,000 mph speed of most stars.

The hypervelocity stars we see come from binary stars that stray close to the galaxy’s massive black hole,” he says. “The hole peels off one binary partner, while the other partner – the hypervelocity star – gets flung out in a gravitational slingshot.”

As many as half of all stars are in binary pairs, so they are plentiful in the Milky Way and other galaxies, he adds. But the study assumed conservatively that only 10 percent of stars exist in binary pairs.

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 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.


Published Date: Apr 02, 2012 05:26 pm | Updated Date: Apr 02, 2012 05:26 pm

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