At the center of the Milky Way rests Sagittarius A*, a supermassive black hole weighing over 4.6 million times our Sun. Around it, there are stars and gas. Now, astronomers from UCLA and the W. M. Keck Observatory have found something else: curious objects that form a class of their own.
The first object of this new “G” class was discovered in 2005. A second one, G2, was found in 2012. Now four more have been announced in a new Nature study. The objects are believed to be the end product of a merger between two stars. The resulting large star has a thick envelope of gas, and whenever it gets closer to the black hole, it gets stretched out like an interstellar gas cloud.
“These objects look like gas but behave like stars,” co-author Professor Andrea Ghez, director of the UCLA Galactic Center Group, said in a statement.
“At the time of closest approach, G2 had a really strange signature,” Ghez said. “We had seen it before, but it didn’t look too peculiar until it got close to the black hole and became elongated, and much of its gas was torn apart. It went from being a pretty innocuous object when it was far from the black hole to one that was really stretched out and distorted at its closest approach and lost its outer shell, and now it’s getting more compact again.”
The team suggests that Sagittarius A* might have been playing an important role in facilitating mergers of this type. The team is confident that the G objects are stars because while the gas was stretched during the close passage to the black hole, they detected that the dusty component within the gas was not.
“Something must have kept it compact and enabled it to survive its encounter with the black hole. This is evidence for a stellar object inside G2,” lead author Anna Ciurlo, a UCLA postdoctoral researcher, said.
“The unique dataset that Professor Ghez’s group has gathered during more than 20 years is what allowed us to make this discovery,” she added. “We now have a population of ‘G’ objects, so it is not a matter of explaining a ‘one-time event’ like G2.”
The orbits of the six G objects range between 100 and 1,000 years for a single journey around the supermassive black hole. G2 lost some gas in 2014 during its close approach, and it is possible that the activity seen in 2019 is related to this material finally reaching Sagittarius A*.