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Astronomers identify mystery object at center of galaxy

Nov. 4, 2014
Courtesy of UCLA
and World Science staff

As­tro­no­mers say they have solved a mys­tery about a thin, bi­zarre ob­ject head­ed to­ward the mon­ster black hole at the cen­ter of our gal­axy.

The sci­en­tists stud­ied the ob­ject, known as G2, dur­ing its clos­est ap­proach to the black hole this sum­mer, and found the black hole did­n’t eat it. Black holes are ob­jects so com­pact that noth­ing can es­cape their gravita­t­ional pull, not even light. They can’t be seen di­rect­ly, but their char­act­er­istic in­flu­ence on near­by ob­jects can.

An image from Keck Ob­serv­a­to­ry tak­en in near-infrared light shows that G2 sur­vived its clos­est ap­proach to the black hole and con­tin­ues hap­pi­ly on its or­bit, as­tro­no­mers say. The green cir­cle just to its right de­picts the lo­ca­tion of the in­vis­i­ble su­per­mas­sive black hole. (Cred­it: An­drea Ghez, Gun­ther Witzel/U­CLA Ga­lac­tic Cen­ter Group/W. M. Keck Ob­serv­a­to­ry)


While some sci­en­tists be­lieved G2 was a cloud of hy­dro­gen gas that the black hole would tear apart in a fiery show, the re­search­ers in the new study found it was more in­ter­est­ing. 

“G2 sur­vived and con­tin­ues happily on its or­bit; a gas cloud would not do that,” said An­drea Ghez, Uni­vers­ity of Cal­i­for­nia Los An­ge­les as­tron­o­mer who di­rects the uni­vers­ity’s Ga­lac­tic Cen­ter Group. It “was com­pletely un­af­fect­ed by the black hole; no fire­works.”

In­stead, the team found it was a pair of bi­na­ry stars, or stars that cir­cle each oth­er. The pair had al­so been or­biting the black hole to­geth­er, they said, and merged to­geth­er in­to an ex­tremely large star, cloaked in gas and dust. The black hole’s pow­er­ful gravita­t­ional field chore­ographed the event.

“G2 is not alone,” said Ghez, who uses W. M. Keck Ob­serv­a­to­ry in Ha­waii to study thou­sands of stars in the neigh­bor­hood of the so-called su­per­mas­sive black hole. “We’re see­ing a new class of stars near the black hole, and as a con­se­quence of the black hole.”

The ob­serv­a­to­ry uses a tech­nol­o­gy called adap­tive op­tics to cor­rect the dis­tort­ing ef­fects of the Earth’s at­mos­phere in real time and see more clear­ly. 

“We are see­ing phe­nom­e­na about black holes that you can’t watch an­y­where else in the uni­verse,” Ghez added. “We are start­ing to un­der­stand the phys­ics of black holes in a way that has nev­er been pos­si­ble be­fore, and is pos­si­ble only at the cen­ter of the gal­axy,” where the super­massive black hole lurks.

Mas­sive stars in our gal­axy, she added, mainly come in pairs. When the two merge in­to one, the star ex­pands for more than one mil­lion years “be­fore it set­tles back down,” Ghez said. “This may be hap­pen­ing more than we thought; the stars at the cen­ter of the gal­axy are mas­sive and mostly bi­na­ries. It’s pos­si­ble that many of the stars we’ve been watch­ing and not un­der­standing may be the end prod­uct of a merg­er that are calm now.”

G2, in that ex­plo­sive stage now, has been an ob­ject of fascina­t­ion. “Its clos­est ap­proach to the black hole was one of the most watched events in as­tron­o­my in my ca­reer,” Ghez said.

G2 makes an un­usu­al, 300-year el­lip­ti­cal or­bit around the black hole and Ghez’s group cal­cu­lat­ed its clos­est ap­proach oc­curred this sum­mer—later than oth­er as­tron­o­mers be­lieved. The re­search was pub­lished Nov. 3 in the jour­nal As­t­ro­phys­i­cal Jour­nal Let­ters.


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Astronomers say they have solved a mystery about a thin, bizarre object headed toward the monster black hole at the center of our galaxy. The scientists studied the object, known as G2, during its closest approach to the black hole this summer, and found the black hole didn’t eat it. Black holes are objects so compact that nothing can escape their gravitational pull, not even light. They can’t be seen directly, but their influence on nearby material is visible. While some scientists believed G2 was a cloud of hydrogen gas that the black hole would tear apart in a fiery show, the researchers in the new study found it was more interesting. “G2 survived and continues happily on its orbit; a gas cloud would not do that,” said Andrea Ghez, University of California Los Angeles astronomer who directs the university’s Galactic Center Group. It “was completely unaffected by the black hole; no fireworks.” Instead, the team found it was a pair of binary stars, or stars that circle each other. The pair had also been orbiting the black hole together, they said, and merged together into an extremely large star, cloaked in gas and dust. The black hole’s powerful gravitational field choreographed the event. “G2 is not alone,” said Ghez, who uses W. M. Keck Observatory in Hawaii to study thousands of stars in the neighborhood of the so-called supermassive black hole. “We’re seeing a new class of stars near the black hole, and as a consequence of the black hole.” The observatory uses a technology called adaptive optics to correct the distorting effects of the Earth’s atmosphere in real time and see more clearly. “We are seeing phenomena about black holes that you can’t watch anywhere else in the universe,” Ghez added. “We are starting to understand the physics of black holes in a way that has never been possible before, and is possible only at the center of the galaxy.” Massive stars in our galaxy, she added, mainly come in pairs. When the two merge into one, the star expands for more than one million years “before it settles back down,” Ghez said. “This may be happening more than we thought; the stars at the center of the galaxy are massive and mostly binaries. It’s possible that many of the stars we’ve been watching and not understanding may be the end product of a merger that are calm now.” G2, in that explosive stage now, has been an object of fascination. “Its closest approach to the black hole was one of the most watched events in astronomy in my career,” Ghez said. G2 makes an unusual, 300-year elliptical orbit around the black hole and Ghez’s group calculated its closest approach occurred this summer — later than other astronomers believed —and they were in place at Keck Observatory to gather the data. The research was published Nov. 3 in the journal Astrophysical Journal Letters.