"Long before it's in the papers"
June 03, 2013

RETURN TO THE WORLD SCIENCE HOME PAGE


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 have spot­ted these stel­lar homi­cides be­fore, but re­search­ers say 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,” 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 near­by, she added.

Ge­zari and her team think hy­dro­gen-filled lay­ers ori­gi­nally sur­round­ing the star’s co­re were lifted off long 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 sci­ent­ists sur­mise. Af­ter con­sum­ing most of its hy­dro­gen fu­el, it had likely bal­looned in size, be­com­ing a so-called red gi­ant. The research­ers think the bloat­ed star was loop­ing around the black hole in an elon­gat­ed or­bit, si­m­i­lar to a comet’s path 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 per gal­axy. To find the one event, Ge­za­ri’s team mon­i­tored hun­dreds of thou­sands of ga­lax­ies by their ul­tra­vi­o­let light with the Gal­axy Ev­o­lu­tion Ex­plor­er, a space-based ob­servatory. The team also scanned ga­la­xies by their vis­i­ble light with the Pan-STARRS1 tel­e­scope on Mount Ha­le­a­ka­la, Ha­waii. 

In June 2010, they spot­ted an intrigu­ing burst of light with both tele­scopes. They con­tin­ued to mon­i­tor the flare as it reached peak bright­ness a month lat­er, then slowly faded 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 three mil­lion suns, which equals the weight of our Milky Way’s black hole.

“The glow­ing he­li­um was a trac­er for an ex­tra­or­di­narily hot ac­cre­tion [in­fall] 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 (32 mil­lion km) 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.


* * *

Send us a comment on this story, or send it to a friend









 

[../index-contents.html]

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.