Astronomers solve puzzle of gas cloud near centre of Milky Way


Telescopes from Hawaii's Keck Observatory helped astronomers discover that a G2, a gas cloud near the centre of the galaxy, is a pair of stars that have merged together.
Ethan Tweedie

For years astronomers have been observing what they believed was a hydrogen gas cloud, headed towards the black hole at the centre of the Milky Way, which they called G2.

"It was one of the most watched events in astronomy in my career," says Andrea Ghez, professor of physics and astronomy at the University of California, Los Angeles. She has headed a team at Hawaii's Keck Observatory that has determined G2 is most likely two stars that have merged together into an extremely large star, its movements influenced by the black hole's powerful gravitational field. The results have been published in Astrophysical Journal Letters.

Black holes form out of the collapse of matter and have such high density that nothing can escape their gravitational pull, including light. Ghez said G2 appears to be one of an emerging class of stars near the black hole at the centre of the Milky Way that are created because its gravity drives binary stars to merge into one.

"This may be happening more than we thought," says Ghez. "The stars at the centre 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 mergers that are calm now."

She says when two stars merge the new body expands for more than a million years before it settles back down. G2 appears to be at the inflated stage now and is experiencing what she calls "spaghettification", a phenomenon in which large objects near black holes become elongated. At the same time the gas on G2's surface is being heated by the stars around it, creating an enormous cloud of gas and dust.

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