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Strange objects found near the Milky Way’s supermassive black hole

The center of our galaxy, the Milky Way, can be a strange place. It’s notoriously hard to see, obscured from our viewpoint by clouds of gas and dust in one of the galaxy’s spiral arms.

But 13 years worth of near-infrared wavelength data from the W. M. Keck Observatory in Hawaii has captured the bizarre reality behind the haze.

The supermassive black hole at the center of our galaxy, Sagittarius A*, is behaving strangely and eating voraciously. And now, astronomers have spotted a weird new class of objects not far from the black hole, according to a study published Wednesday in the journal Nature.

“These objects look like gas and behave like stars,” said Andrea Ghez, study author, director of the UCLA Galactic Center Group and the university’s Lauren B. Leichtman and Arthur E. Levine Professor of Astrophysics.

Normally, these objects appear compact. But they stretch out when they near the black hole during their orbits, which can last from 100 to 1,000 years.

This is the latest detection in a mystery that’s been unfolding since 2005, when Ghez’s research team spotted an object in the center of our galaxy they dubbed G1. In 2012, a second object called G2 was found, and it came close to the black hole in 2014.

The astronomers believe G2 was once really two stars that orbited the black hole together, but merged into one massive star and became obscured by a thick gas and dust cloud.

“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.”

Now, Ghez’s research group has discovered four more objects and determined their orbits: G3, G4. G5 and G6. They’re incredibly different orbits from G1 and G2, which were similar to one another.

“One of the things that has gotten everyone excited about the G objects is that the stuff that gets pulled off of them by tidal forces as they sweep by the central black hole must inevitably fall into the black hole,” said Mark Morris, study co-author and UCLA professor of physics and astronomy. “When that happens, it might be able to produce an impressive fireworks show since the material eaten by the black hole will heat up and emit copious radiation before it disappears across the event horizon.”

The researchers think that all six objects are the result of binary stars — or pairs of stars that orbit one another — that were forced together due to the strong gravity of Sagittarius A*.

Ghez said that mergers of stars may be happening in the universe more often than we thought and likely are quite common.

“Black holes may be driving binary stars to merge. 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, ” Ghez said. “We are learning how galaxies and black holes evolve. The way binary stars interact with each other and with the black hole is very different from how single stars interact with other single stars and with the black hole.”

G2 differs from the rest of the objects because it didn’t undergo as much stretching as the others.

“Something must have kept it compact and enabled it to survive its encounter with the black hole,” said Anna Ciurlo, study author and postdoctoral researcher at UCLA. “This is evidence for a stellar object inside G2.”

Now, the astronomers have enough evidence to show a small group of G objects — rather than a couple of instances — meaning they can continue analyzing them while searching for others.

The fact that these objects have been found near Sagittarius A*, which likely swallowed up the gas it ripped off the stars, means they could actually be feeding the constantly voracious black hole.

It’s further proof that chaos is constantly unfolding at the center of not just the Milky Way but many galaxies in the universe.

“The Earth is in the suburbs compared to the center of the galaxy, which is some 26,000 light-years away,” Ghez said. “The center of our galaxy has a density of stars 1 billion times higher than our part of the galaxy. The gravitational pull is so much stronger. The magnetic fields are more extreme. The center of the galaxy is where extreme astrophysics occurs — the X-sports of astrophysics.”

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