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Black hole caught red-handed in stellar homicide?

May 2, 2012
Courtesy of NASA
and World Science staff

As­tro­no­mers say they have gath­ered the most di­rect ev­i­dence yet of a gi­ant black hole shred­ding a star that got too close.

Black holes are ob­jects so dense and heavy that their gra­vity be­comes over­whelm­ing and  drags in an­y­thing that strays too close, in­clud­ing light rays. The larg­est black holes—known as su­per­mas­sive ones—weigh mil­lions to bil­lions times more than the Sun and lurk in the cen­ters of most ga­lax­ies. They lie qui­etly un­til an un­sus­pect­ing vic­tim, such as a star, wan­ders close enough to get drawn in, and is torn apart in the pro­cess.

Im­ages tak­en with NA­SA's Gal­axy Ev­o­lu­tion Ex­plor­er and the Pan-STARRS1 tel­e­scope in Ha­waii show a bright­en­ing in­side a gal­axy caused by a flare from its nu­cle­us. (Im­age cred­it: NA­SA/JPL-Caltech/JHU/STScI/Har­vard-Smith­son­ian CfA)

Astronomers say they have gathered the most direct evidence yet of a giant black hole shredding a star that got too close. Black holes are objects so dense and heavy that their gravity becomes overwhelming, dragging in anything that strays too close, including light rays. The largest black holes—known as supermassive ones—weigh millions to billions times more than the Sun and lurk in the centers of most galaxies. They lie quietly until an unsuspecting victim, such as a star, wanders close enough to get drawn in, and is torn apart in the process. Astronomers have spotted these stellar homicides before, but researchers said this is the first time they can describe the victim. Using a bevy of telescopes, a team of astronomers led by Suvi Gezari of The Johns Hopkins University in Baltimore, Md. has identified that victim as a star rich in helium gas. The star resided in a galaxy 2.7 billion light-years away—a light year being the distance light travels in a year—according to the team’s results, to be published in the May 3 online edition of the journal Nature. “When the star is ripped apart by the gravitational forces of the black hole, some part of the star’s remains falls into the black hole, while the rest is ejected at high speeds. We are seeing the glow from the stellar gas falling into the black hole over time,” said Gezari. “We’re also witnessing the spectral signature of the ejected gas, which we find to be mostly helium.” A spectral signature is a detailed breakdown by color of the light given off by some process; this can reveal which substances were present to begin with. It’s like “gathering evidence from a crime scene,” Gezari said. The observation also yields insights about the harsh environment around black holes and the types of stars swirling around them, she added. Gezari and her team think the hydrogen-filled layers surrounding the star’s core was lifted off a long time ago by the same black hole, explaining why there is only helium left. The star may have been near the end of its life, the astronomers surmise. After consuming most of its hydrogen fuel, it had likely ballooned in size, becoming a so-called red giant. The astronomers think the bloated star was looping around the black hole in an elongated orbit, similar to a comet’s around the sun. On one of its close approaches, the star was stripped of its puffed-up atmosphere by the black hole’s powerful gravity. The stellar remains continued its journey around the center, until it ventured even closer to the black hole to face its ultimate demise. Astronomers have predicted that stripped stars circle the central black hole of our Milky Way galaxy, Gezari pointed out. These close encounters are rare, occurring roughly every 100,000 years. To find this one event, Gezari’s team monitored hundreds of thousands of galaxies in ultraviolet light with the Galaxy Evolution Explorer, a space-based observatory, and in visible light with the Pan-STARRS1 telescope on Mount Haleakala, Hawaii. In June 2010, they spotted a candidate event with both telescopes. They continued to monitor the flare as it reached peak brightness a month later, then slowly fadef over the next year. “The longer the event lasted, the more excited we got, since we realized that this is either a very unusual supernova [stellar explosion] or an entirely different type of event, such as a star being ripped apart by a black hole,” said team member Armin Rest of the Space Telescope Science Institute in Baltimore. By measuring the increase in brightness, the astronomers calculated the black hole’s weight at roughly 3 million suns, which equals the weight of our Milky Way’s black hole. “The glowing helium was a tracer for an extraordinarily hot accretion [infalling] event,” Gezari said. “So that set off an alarm for us. And the fact that no hydrogen was found set off a big alarm that this was not typical gas. You can’t find gas like that lying around near the center of a galaxy. It’s processed gas that has to have come from a stellar core. There’s nothing about this event that could be easily explained by any other phenomenon.” Measurements indicated the gas was moving at more than 20 million miles an hour (over 32 million kilometers an hour). “The place we also see these kinds of velocities are in supernova explosions,” Rest said. “But the fact that it is still shining in ultraviolet light is incompatible with any supernova we know.” “These observations also give us clues on what evidence to look for in the future to find this type of event,” Gezari said.