The mystery of a thin, bizarre object in the center of the Milky Way headed toward our galaxy’s enormous black hole has been solved by University of California, Los Angeles astronomers using the W. M. Keck Observatory’s telescopes atop Mauna Kea.
The scientists studied the object, known as G2, during its closest approach to the black hole this summer, and found the black hole did not dine on it. The research was published Monday in Astrophysical Journal Letters.
While some scientists believed the object was a cloud of hydrogen gas that would be torn apart in a fiery show, the team proved it to be a pair of binary stars that had been orbiting the black hole in tandem and merged together into an extremely large star cloaked in gas and dust and directed by the black hole’s gravitational field.
“G2 survived and continues happily on its orbit; a gas cloud would not do that,” said Andrea Ghez, UCLA professor of physics and astronomy who holds the Lauren B. Leichtman and Arthur E. Levine Chair in Astrophysics, and directs the UCLA Galactic Center Group. “G2 was completely unaffected by the black hole; no fireworks.”
Ghez and her colleagues — who include lead author Gunther Witzel, a UCLA postdoctoral scholar in Ghez’s research group, and Mark Morris, a UCLA professor of physics and astronomy — studied the event with both of the 10-meter telescopes at Keck Observatory. The scientists used adaptive optics to correct the distorting effects of the Earth’s atmosphere and reveal the region around the black hole.
Massive stars in our galaxy, Ghez noted, are primarily found in pairs. When the two stars merge into one, the star expands for more than 10,000 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. It makes an unusual, 300-year elliptical orbit around the black hole and Ghez’s group was in place at Keck Observatory to gather the data this summer.
Black holes, which form out of the collapse of matter, have such high density that nothing can escape their gravitational pull, not even light. They cannot be seen directly, but their influence on nearby stars is visible and provides a signature, said Ghez.