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Mysterious Objects Seen Lurking Close To Supermassive Black Hole At Milky Way Center

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Researchers have found three strange objects moving alarmingly close to the Sagittarius A*, the supermassive black hole sitting at the center of the Milky Way galaxy.

After 12 years of observations at the W.M. Keck Observatory in Maunakea, Hawaii, the team has concluded that three mysterious objects are currently making their way around the black hole.

No definitive explanation has been made, but the team's hypothesis is that these are part of the G-class of objects.

What Are G-Class Objects?

Back in 2004, a group of astronomers observed a dusty red object, which they called G1, circulating around Sagittarius A*. A similar object called G2 was seen whizzing by in 2012. Scientists initially believed they were clouds of gas.

However, when the objects zoomed close to Sagittarius A*, they surprisingly survived, leading astronomers to revise their gas cloud hypothesis and think that G1 and G2 are bloated stars. They are massive enough to resist being ripped apart by the black hole and are surrounded by a huge cloud of dust and gas.

G3, G4, And G5

Using Keck's OH-Suppressing Infrared Imaging Spectrograph (OSIRIS), the team was able to take measurements of the gas and dust surrounding the black hole, which led them to find three new objects moving super-fast and close to Sagittarius A*.

"It is fascinating to watch them move from year to year," Anna Ciurlo, postdoctoral scholar at the University of California Los Angeles, says. "How did they get there? And what will they become? They must have an interesting story to tell."

G3, G4, and G5 are bright, red, dusty objects that emit hydrogen. They have relatively low temperatures registering at a few hundred Kelvin. In contrast, normal stars have temperatures of 2,000 Kelvin and up. They look puffy, like clouds, but they behave like compact, solid stars with huge mass. From the team's line of sight, the objects are moving a tenth as fast as G1 and G2 and are brighter.

Why Are They There?

Near the center of the Milky Way, about one-tenth light-year away from the black hole, are three dozen massive young stars called B-type stars traveling in elongated orbits around the black hole. Some of them even come as close as the Earth is to the sun.

The problem is the presence of the black hole should make the birth of new stars impossible in this part of the galaxy. Its strong gravitational pull should rip any clumps of hydrogen that is used to form stars apart. So why are the G-objects there?

The researchers think the Kozai-Lidov mechanism could provide an explanation. When two stars sitting close to each other near a black hole, the black hole's massive gravitational pull distorts their orbits and causes them to crash into each other.

The collision causes the stars to merge into each other and generate enormous amounts of energy. This gives birth to a massive new star surrounded by a dusty red cloud that will eventually fade away to become stars that look like the B-type stars located around Sagittarius A*.

"This may provide us with insight into a process which may be responsible for the recently discovered stellar mass black hole mergers that have been detected through gravitational waves," says Andrea Ghez, founder and director of UCLA's Galactic Center Orbits Initiative.

More Mysteries

The Kozai-Lidov mechanism, however, does not totally account for the recent observations. G5 has an elongated orbit that may have been caused by the black hole, but G3's orbit is circular.

Although the team's current hypothesis remains, it is still possible that not all of the G-class objects have the same origins.

Ciurlo and her team will continue observing the path of these objects. In particular, they will be paying attention to when they will make their way closest to the black hole. This will take place in 20 years for G3 and longer for G4 and G5.

The team is interested to know if all three objects will survive being torn apart by Sagittarius A* once they get to their closest point to the black hole.

The team's findings were presented at the American Astronomical Society Meeting in Denver on June 6.

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