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How G2 survived the black hole at our Milky Way’s heart

Astronomers watched in fascination as an object called G2 appeared to edge closer to our Milky Way’s central black hole. But G2 survived!

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. Image via Keck Observatory

An image from W. M. Keck Observatory near infrared data shows that G2 survived its closest approach to the black hole in northern summer 2014. The green circle just to its right depicts the location of the invisible supermassive black hole. Image via Keck Observatory

In 2011, astronomers said they’d discovered a cloud of gas – with several times Earth’s mass – accelerating fast towards the supermassive black hole at the center of our Milky Way. The cloud appeared to be undergoing spaghettification – sometimes called the noodle effect – stretching and elongating as it neared the black hole. It was thought at first the cloud – which came to be called G2 – would meet a fiery end as it passed into the Milky Way’s black hole as early as 2013. It did not, but astronomers now say it passed closest to the hole in northern summer of 2014. Unexpectedly, however, our galaxy’s black hole did not swallow it. Instead, G2 survived! This week (November 3, 2014), UCLA astronomers published a new paper in the journal Astrophysical Journal Letters offering a new explanation as to what G2 is and how it survived its black hole passage.

Andrea Ghez, UCLA professor of physics and astronomy, said in a press release:

G2 survived and continues happily on its orbit; a gas cloud would not do that

G2 was completely unaffected by the black hole. There were no fireworks.

Ghez’ team now suggests that G2 is a pair of binary stars that had been orbiting the black hole in tandem and merged together into an extremely large star. This merger may have been cloaked in gas and dust, and choreographed by the object’s proximity to the black hole’s powerful gravitational field. Ghez said:

G2 is not alone. We’re seeing a new class of stars near the black hole, and as a consequence of the black hole.

Ghez noted that massive stars in our galaxy primarily come in pairs. When the two stars merge into one, the star expands for more than one million years, Ghez said …

… 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. It’s possible that many of the stars we’ve been watching and not understanding may be the end product of a merger that are calm now.

G2, on the other hand, would be in its expansive stage now.

It appears to have an unusual, 300-year elliptical orbit around the black hole.

Telescopes from Hawaii’s W.M. Keck Observatory use a powerful technology called adaptive optics, which enabled UCLA astronomers to discover that G2 is a pair of binary stars that merged together, cloaked in gas and dust.  Image via Keck Observatory

Telescopes from Hawaii’s W.M. Keck Observatory use a powerful technology called adaptive optics, which enabled UCLA astronomers to study G2 and suggest it is a pair of binary stars that merged together, cloaked in gas and dust. Image via Keck Observatory

Bottom line: Astronomers watched in fascination as an object called G2 – apparently a gas cloud – appeared to edge closer and closer to our Milky Way’s central black hole. But G2 survived! Now, instead of a gas cloud, astronomers believe G2 is a binary star system, that is, two that had been orbiting the black hole in tandem and merged together into an extremely large star.

Deborah Byrd

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