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Coupled stars seen as chief diet for hungry black holes

April 4, 2012
Courtesy of the University of Utah
and World Science staff

Gi­ant black holes in the cen­ters of ga­lax­ies may grow to their enor­mous sizes by swal­low­ing sin­gle stars from pairs of stars that wan­der too close, a new study pro­poses.

The study is based on new cal­cula­t­ions and pre­vi­ous ob­serva­t­ions of ga­lax­ies in­clud­ing our Milky Way. “We found black holes grow enor­mously as a re­sult of suck­ing in cap­tured bi­na­ry star part­ners,” said Uni­vers­ity of Utah as­t­ro­phys­i­cist Ben Brom­ley, lead au­thor of the work, pub­lished April 2 on­line in the jour­nal As­t­ro­phys­i­cal Jour­nal Let­ters.

Artist’s con­cep­tion of a su­per­mas­sive black hole (low­er left) with its tre­men­dous grav­i­ty cap­tur­ing one star (blu­ish, cen­ter) from a pair of bi­na­ry stars, while hurl­ing the sec­ond star (yel­low­ish, up­per right) away at more than a mil­lion miles per hour. The gray­ish blobs are oth­er stars cap­tured in a clus­ter near the black hole. They ap­pear dis­tort­ed be­cause the black hole’s grav­i­ty curves space­time and thus bends the star­light. (Cred­it: Ben Brom­ley, U. of Utah)


“I be­lieve this has got to be the dom­i­nant meth­od for grow­ing su­per­mas­sive black holes,” the gi­ant, hyper-dense ob­jects that sit at the hearts of ga­lax­ies, he added. “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.”

Bi­na­ry stars are pairs of stars that or­bit each oth­er.

“Our mech­an­ism is an ef­fi­cient way to br­ing a star to a black hole,” Brom­ley said. “It’s really hard to tar­get a sin­gle star at a black hole. It’s a lot eas­i­er to throw a bi­na­ry at it,” just as it’s more dif­fi­cult to hit a tar­get us­ing a sling­shot, which hurls a sin­gle stone, than with a bo­la, which hurls two weights con­nect­ed by a cord.

Black holes are ob­jects in space so com­pact that their gravity over­whelms every­thing nearby and not even light can es­cape them. Jets of light and en­er­gy can flow from a black hole’s vicin­ity, though, as gas and stars are sucked in­to it.

Small black holes are be­lieved to re­sult from the col­lapse of in­di­vid­ual stars. But the cen­ters of most ga­lax­ies, in­clud­ing our own Milky Way, host “su­per­mas­sive” black holes that are as heavy as one mil­lion to 10 bil­lion Suns put to­geth­er. Sci­en­tists 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.

A bi­na­ry star “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,” Brom­ley said. “The bi­na­ry does­n’t have to get nearly as close for one of the stars to get ripped away and cap­tured.”

To prove the the­o­ry will re­quire more pow­er­ful tele­scopes to find three key signs, Brom­ley added. These are: large num­bers of small stars cap­tured near su­per­mas­sive black holes, more ob­serva­t­ions of stars be­ing “shred­ded” by gra­vity from black holes, and large num­bers of “hy­pe­r­ve­lo­city stars” that are flung from ga­lax­ies at more than a mil­lion miles per hour when their bi­na­ry part­ners are cap­tured.


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Giant black holes in the centers of most galaxies may have grown to their enormous size by swallowing single stars from pairs of stars that wandered too close, a new study proposes. Using new calculations and previous observations of galaxies including our Milky Way, “we found black holes grow enormously as a result of sucking in captured binary star partners,” said University of Utah astrophysicist Ben Bromley, lead author of the study. “I believe this has got to be the dominant method for growing supermassive black holes,” the giant, hyper-dense objects that sit at the hearts of galaxies, he added. “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.” Binary stars are pairs of stars that orbit each other. “Our mechanism is an efficient way to bring a star to a black hole,” Bromley said. “It’s really hard to target a single star at a black hole. It’s a lot easier to throw a binary at it,” just as it’s more difficult to hit a target using a slingshot, which hurls a single stone, than with a bola, which hurls two weights connected by a cord. 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 are believed to result from the collapse of individual stars. But the centers of most galaxies, including our own Milky Way, host “supermassive” black holes that contain stars amounting to the equivalent of one million to 10 billion Suns. Scientists 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 said they grow primarily by capturing and sucking in stars. A binary star “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,” Bromley said. “The binary doesn’t have to get nearly as close for one of the stars to get ripped away and captured.” To prove the theory will require more powerful telescopes to find three key signs, Bromley added. These are: large numbers of small stars captured near supermassive black holes, more observations of stars being “shredded” by gravity from black holes, and large numbers of “hypervelocity stars” that are flung from galaxies at more than a million miles per hour when their binary partners are captured. The study was published April 2 online in the journal Astrophysical Journal Letters.