Astronomers using the Keck Observatory have detected some strange objects at the center of the Milky Way galaxy. At a glance they look like ordinary clouds of gas, but they seem to have a more solid core that keeps them together. The truth may lie somewhere in-between. These G-objects, as they're known, seem to be stars hiding under puffy shrouds of dust.

The first G-object was discovered in 2004, and a second was later identified in 2012. These objects, known as G1 and G2 respectively, were initially thought to be just clouds of gas, but that assumption fell apart when the objects didn't. They managed to swing past the supermassive black hole at the center of the galaxy without being torn to shreds, indicating they were no ordinary gas clouds.

"If they were gas clouds, G1 and G2 would not have been able to stay intact," says Mark Morris, a co-principal investigator on the new study. "Our view of the G-objects is that they are bloated stars – stars that have become so large that the tidal forces exerted by the central black hole can pull matter off of their stellar atmospheres when the stars get close enough, but have a stellar core with enough mass to remain intact."

Now it seems that G1 and G2 might have a few friends. A team of researchers studying over 12 years of Keck data has identified three new candidates that could be G-objects. Unsurprisingly, they've been named G3, G4 and G5.

The team started by peering at the Galactic Center using Keck's OH-Suppressing Infrared Imaging Spectrograph (OSIRIS). The astronomers weren't specifically looking for G-objects, but noticed distinct movements and characteristics that indicated their presence. To get a closer look, the team isolated them from the background emission using a Keck-developed tool called OSIRIS-Volume Display (OsrsVol), and analyzed them in three dimensions over a long period of time.

"These compact dusty stellar objects move extremely fast and close to our galaxy's supermassive black hole," says Anna Ciurlo, lead researcher on the study. "It is fascinating to watch them move from year to year. How did they get there? And what will they become? They must have an interesting story to tell."

This story, the researchers believe, starts with binary stars. Over time, the orbits of these pairs are destabilized by the immense gravitational pull of the giant black hole, until they crash into each other. The objects that we're now seeing could be the aftermath of these stellar mergers.

"In the aftermath of such a merger, the resulting single object would be 'puffed up,' or distended, for a rather long period of time, perhaps a million years, before it settles down and appears like a normal-sized star," says Morris.

The ultimate test of whether these things are G-objects or not will come when they pass by the supermassive black hole. If they can shrug off the gravitational forces and retain most of their shape, they'll really have earned the title. Unfortunately, this won't be happening for a while yet.

"We'll have to wait a few decades for this to happen; about 20 years for G3, and decades longer for G4 and G5," says Morris. "In the meantime, we can learn more about these puffballs by following their dynamical evolution using OSIRIS."

The research was presented at the American Astronomical Society Meeting this week.

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