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An image from W. M. Keck Observatory near infrared data shows that G2 survived its closest approach to the black hole and continues happily on its orbit. The green circle just to its right depicts the location of the invisible supermassive black hole. Credit: Andrea Ghez, Gunther Witzel/UCLA Galactic Center Group/W. M. Keck Observatory

Mysterious Object “G2” at Galactic Center is Actually Binary Star

4 Nov , 2014

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A mysterious object swinging around the supermassive black hole in the center our galaxy has surprised astronomers by actually surviving what many thought would be a devastating encounter. And with its survival, researchers have finally been able to solve the conundrum of what the object – known as G2 — actually is. Since G2 was discovered in 2011, there was a debate whether it was a huge cloud of hydrogen gas or a star surrounded by gas. Turns out, it was neither … or actually, all of the above, and more.

Astronomers now say that G2 is most likely a pair of binary stars that had been orbiting the black hole in tandem and merged together into an extremely large star, cloaked in gas and dust.

“G2 survived and continued happily on its orbit; a simple gas cloud would not have done that,” said Andrea Ghez from UCLA, who has led the observations of G2. “G2 was basically unaffected by the black hole. There were no fireworks.”

This was one of the “most watched” recent events in astronomy, since it was the first time astronomers have been able to view an encounter with a black hole like this in “real time.” The thought was that watching G2’s demise would not only reveal what this object was, but also provide more information on how matter behaves near black holes and how supermassive black holes “eat” and evolve.

The two Keck 10-meter domes atop Mauna Kea. (Rick Peterson/WMKO)

The two Keck 10-meter domes atop Mauna Kea. (Rick Peterson/WMKO)

Using the Keck Observatory, Ghez and her team have been able to keep an eye on G2’s movements and how the black hole’s powerful gravitational field affected it.

While some researchers initially thought G2 was a gas cloud, others argued that they weren’t seeing the amount of stretching or “spaghettification” that would be expected if this was just a cloud of gas.

As Ghez told Universe Today earlier this year, she thought it was a star. “Its orbit looks so much like the orbits of other stars,” she said. “There’s clearly some phenomenon that is happening, and there is some layer of gas that’s interacting because you see the tidal stretching, but that doesn’t prevent a star being in the center.”

Now, after watching the object the past few months, Ghez said G2 appears to be just one of an emerging class of stars near the black hole that are created because the black hole’s powerful gravity drives binary stars to merge into one. She also noted that, in our galaxy, massive stars primarily come in pairs. She says the star suffered an abrasion to its outer layer but otherwise will be fine.

Ghez explained in a UCLA press release that when two stars near the black hole merge into one, the star expands for more than 1 million years before it settles back down.

“This may be happening more than we thought. The stars at the center of the galaxy are massive and mostly binaries,” she said. “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 and her colleagues also determined that G2 appears to be in that inflated stage now and is still undergoing some spaghettification, where it is being elongated. At the same time, the gas at G2’s surface is being heated by stars around it, creating an enormous cloud of gas and dust that has shrouded most of the massive star.

Usually in astrophysics, timescales of events taking place are very long — not over the course of several months. But it’s important to note that G2 actually made this journey around the galactic center around 25,000 years ago. Because of the amount of time it takes light to travel, we can only now observe this event which happened long ago.

“We are seeing phenomena about black holes that you can’t watch anywhere else in the universe,” Ghez added. “We are starting to understand the physics of black holes in a way that has never been possible before.”

The research has been published in the journal Astrophysical Journal Letters.

Further reading: UCLA, Keck

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Adrian Morgan
Member
November 4, 2014 7:28 PM

What do you mean by “met its demise around 25,000 years ago”? The point of the article is that it *didn’t* meet its demise in this encounter at all; it survived.

Invonnative
Member
Invonnative
November 4, 2014 11:59 PM

It means that although we seemingly are watching a star avoid death, the light we are watching it with is many, many years old. As a result, this event is actually far in the past in relation to this article, which means much has happened since then. The death of the star 25,000 years ago, apparently, is included within that frame.

Adrian Morgan
Member
November 5, 2014 7:23 AM

I don’t think you’ve understood the question. I believe Nancy misused the phrase “met its demise” to mean something like “danced with death”, and the question was my way of pointing out the error as politely as possible.

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