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The mysterious G2 (center) snapped by the Keck telescopes shortly after its close encounter with a supermassive black hole.

The mysterious G2 (center) snapped by the Keck telescopes shortly after its close encounter with a supermassive black hole.

Andrea Ghez, Gunther Witzel/UCLA Galactic Center Group/W. M. Keck Observatory

Astronomers identify mysterious object at heart of Milky Way

A mysterious astronomical object known simply as G2 has intrigued and confounded researchers ever since it was found to be on a near-collision course with the supermassive black hole at the center of our galaxy. When it failed to produce the predicted celestial fireworks over the past few months, astronomers were left scratching their heads. Now, one team thinks it has figured out why nothing happened: G2 is not a gas cloud but a strange, more stable object formed from a pair of recently merged stars.

Clouds of gas and dust near the center of our Milky Way galaxy make it extremely difficult to observe things there. When astronomers calculated G2’s trajectory in 2012, they thought it was a cloud of gas in an orbit that would take it very close to the Milky Way’s central black hole, known as Sagittarius A* (Sgr A*). Researchers predicted that, as G2 neared the center, the black hole’s intense gravity would tear the object apart and drag it toward the hole’s surface, producing a shower of radiation that could reveal much about the environment close to Sgr A*. But as G2 passed by this summer, the galactic center was conspicuously quiet.

A team led by Andrea Ghez of the University of California, Los Angeles, used the twin 10-meter Keck telescopes in Hawaii to observe G2 between March and August—spanning its expected closest approach and the months that followed. Viewing at the infrared wavelength of 3 micrometers, which can penetrate the fog around Sgr A*, the team found that G2 “continues to survive in orbit—it has not changed,” Ghez says. A gas cloud alone could not have survived a pass so close to the black hole, she says; she concludes that G2 must have a star at its heart, surrounded by a shell of dust. “It’s an unusual star, large, calculated from its brightness,” she says. The star is twice the mass of our sun but 100 times its size, the team reports this week in The Astrophysical Journal Letters.

Ghez thinks her group has caught a pair of binary stars in the act of merging. Binary systems are common throughout the galaxy, she says; near the center, the black hole’s gravity encourages them to spiral together. The result is a new star so hot that it swells up like a freshly ignited young protostar. Studying how such objects form might help scientists solve mysteries surrounding Sgr A*’s close neighborhood, such as why it contains many more young-looking stars than expected but almost no older giant stars.

Not all researchers agree on the nature of G2. A team led by Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, which originally identified G2’s trajectory in 2012, observed it in spring 2014 using the European Southern Observatory’s Very Large Telescope in Chile and published the results in July. They said their observations showed that G2 was behaving just as a cloud of gas should: stretching out before the close approach and pulling together again after it.

The lack of fireworks was no problem, they said, because any gas stripped off G2 during the flyby might take months or years to fall onto the surface. “So far there are no fireworks, but the gas is doing what was expected,” Gillessen told Science in August. “It’s fun to see how it is unfolding in front of our eyes.” His team noticed that another apparent gas cloud, dubbed G1, had followed a similar path 13 years ago and suggested that both clouds could be individual “knots” of denser material in a continuous gas streamer in orbit around Sgr A*.

Theorist Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, is amused by the differing interpretations, because he published two theoretical papers providing possible explanations for the source of G2: one, that it surrounded a star that was shedding gas; and the other, that a giant star situated elsewhere shed its outer layer producing a stream of gas with clumps in it. Both are plausible, he says, “but I don’t think we have enough data to decide.” He doesn’t think it will take long to settle it: If it’s the latter scenario then the giant star should be easy to find.