Milky Way’s dark heart prepares for a 2013 lunch date

At the gravitational heart of the Milky Way, around which all the matter in the galaxy ultimately rotates, is a dark, crushing mass, just a bit bigger than the size our solar system and more than 4 million times more massive than our own Sun. It is the supermassive black hole named Sagittarius A* (pronounced 'Sagittarius A-star').

This has been a very interesting month for news about Sagittarius A* (Sgr A*).

On October 4th, astronomers at the Keck Observatory in Hawaii reported that they had finished tracking a star — named S0-102 — as it made an astoundingly-fast 11.5-year orbit around Sagittarius A*. For reference, if Earth orbited our Sun at the speed that S0-102 orbits Sgr A*, our year would only be 6 days long. Before then, astronomers knew of another star — S0-2 — with a 16-year orbit around the black hole, but having the orbit of two stars gives them so much more information.

"I'm extremely pleased to find two stars that orbit our galaxy's supermassive black hole in much less than a human lifetime," said Andrea Ghez, professor of physics and astronomy at UCLA, who is leader of the discovery team. "It is the tango of S0-102 and S0-2 that will reveal the true geometry of space and time near a black hole for the first time. This measurement cannot be done with one star alone."

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The Keck astronomers were able to find S0-102, which is 16 times dimmer than S0-2, after they began their observation sessions by firing a laser into the sky, causing sodium atoms 90 km above the surface to glow, in effect creating an artificial 'star'. By analyzing this 'star', they figured out exactly how much distortion the atmosphere was causing, and they were able to adjust the telescope's collection mirror to compensate. Stars that had previously been blurry came into sharp focus.

"The fact that we can find stars that are so close to the black hole is phenomenal," said Ghez. "Now it's a whole new ballgame, in terms of the kinds of experiments we can do to understand how black holes grow over time, the role supermassive black holes play in the center of galaxies, and whether Einstein's theory of general relativity is valid near a black hole, where this theory has never been tested before. It's exciting to now have a means to open up this window."

Things had been quiet with Sgr A* since, but on Monday, a group of physicists announced that they had developed computer simulations involving Sgr A*, that shows how the supermassive black hole is going to consume most of a large cloud of gas and dust — called G2 — in September of 2013. The lunch date will end with most of the cloud of gas spiraling into the black hole, some ending up in Sgr A*'s accretion disk and the rest being torn apart and flung out of Sgr A*'s gravity well.

Discovered in 2002, it is still unknown exactly what G2 is made of or where it came from.

Astronomers do know that the dust in the cloud has a temperature of about 550 degrees Kelvin, and the temperature of the gas — mostly hydrogen — is around 10,000 degrees Kelvin.

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As for its origin, "speculation ranges from it having been an old star that had kind of a burp and lost some of its outer atmosphere, to something that was trying to be a planet and couldn't quite manage it because the environment was too hot." said astrophysicist Stephen Murray, one of the developers of the simulations.

As it is eaten by Sgr A*, the cloud will heat up to spectacular temperatures, which will cause the black hole to light up when viewed by radio telescopes, the orbiting Chandra X-ray observatory, and NASA's new Nuclear Spectroscopic Telescope Array (NuSTAR). The entire interaction between Sgr A* and G2 will take decades, so astronomers will have a long time to study the event.

Eleven months is a long time to wait for such an event to start, but last night, it appears as though we were given a sneak preview of G2's date next September, when NuSTAR saw Sagittarius A* suddenly flare up!

"We got lucky to have captured an outburst from the black hole during our observing campaign," said Fiona Harrison, NuSTAR principal investigator at Caltech. "These data will help us better understand the gentle giant at the heart of our galaxy and why it sometimes flares up for a few hours and then returns to slumber."

There's no word yet on what Sgr A* ate that caused the flare-up, but every time this happens, astronomers get more information on what happens when black holes consume objects and how they grow larger in the process.

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