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Old 03-20-2014, 08:58 AM   #1751
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It’s the cosmic event of the year. Right now, telescopes all over the world are turning to our galaxy’s center, where for the first time ever they may have a front-row look at a supermassive black hole consuming a gas cloud.

By observing this galactic snack fest, astronomers should be able to figure out what’s going on in the black hole’s immediate vicinity and potentially even witness some gas disappear into the massive object’s maw. What they see may help scientists solve a decades-old puzzle about why our galaxy’s central black hole is so quiet.

Astronomers are gearing up to watch this show using many different telescopes with different wavelengths of light to capture all the information they can. But they are still unsure what exactly they will see.

“It’s a bit like the moment before a penalty shot in soccer,” said astrophysicist Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics in Germany, one of the leaders of the observation campaign. Everyone knows a shot is about to be taken, but nobody knows outcome will be. “This is the most tense moment when one player is trying to shoot against someone on the other side.”


Though we think of them as cosmic vacuum cleaners, black holes are actually just like any other massive body, such as a star. This means other objects can safely orbit them, until they get within a particular distance and pass what’s known as the event horizon, after which there is no escaping being sucked in.

The gas cloud currently headed for the central black hole could either continue on its current orbit and slingshot around the black hole or it could run into surrounding gas and dust, which will make it lose speed and start sliding down toward the black hole. The first scenario could give scientists insight into the evolution of galaxies and better understand the history of our Milky Way’s own black hole. In the second case, they might get to watch the black hole consume a sizable dinner.

No matter the outcome, “it will be absolutely stunning to see the physics at work,” said Gillessen.

In 2011, Gillessen and his colleagues made a chance discovery of a small cloud of gas and dust near the galactic center. Dubbing it G2, they soon plotted the cloud’s orbit, which showed that it was headed directly for the supermassive black hole at the Milky Way’s center and would reach it in 2013. With further data refinements, they realized their initial prediction was a bit off and the cloud would be swinging near the black hole this month. Now that G2 is finally starting to make its closest approach, it’s feeling pulled by the black hole’s enormous gravity.

“This gas thing has been shredded into an extremely long spaghetti configuration,” said astrophysicist Reinhard Genzel, also of the Max Planck Institute, who helped discover G2.

G2 started as a blob with approximately three times the mass of Earth. It has been falling in almost a perfectly straight line toward the central black hole at speeds exceeding 5 million mph. Because it is a diffuse and elongated object, there is no specific time when G2 is expected to be closest to the black hole (“We can’t say this will happen some Friday afternoon at 5 p.m.,” said Genzel.) You should of course keep in mind that the Milky Way’s center is 26,000 light-years from Earth so all this actually happened 26,000 years ago.


Scene from the computer simulation above showing one possible trajectory for G2 as it passes around the central black hole. Image: ESO/MPE/M. Schartmann/L. Calçada
Some models predict that G2 will slam into an atmosphere of gas and dust that hangs around in a disk around the central black hole, a remnant of previous feeding binges. If so, the cloud could heat up to temperatures greater than 10 million degrees, producing X-rays and other radiation that will be visible to our telescopes. Some of G2’s dust could even start spiraling into the black hole like water circling a drain, which would also heat it up and produce radiation.

But the galactic center is a place of many mysteries and a great deal of weirdness. In addition to the gargantuan black hole – with its mass of four million suns – scientists predict that there could be around 10,000 solar mass black holes in the Milky Way’s central region. These are the leftovers of enormous stars that once blazed in the galactic center but burned through their fuel, exploding as a dramatic supernova and crunching down into black holes. Other dead stellar cores, such as white dwarfs and neutron stars, also litter this area.

“There’s a reasonable chance that one of those might be hit, and that would be absolutely fantastic,” said Gillessen. The event might be detectable by telescopes on Earth and could give astronomers insight into the dynamics of these smaller black holes.

There is also the possibility that the gas and dust surrounding our galactic supermassive black hole is too diffuse for G2 to slam into it. In this case, the gas cloud will drift serenely through this region with little interaction, following a path set by gravity. Its closest approach to the central black hole will bring it only within about 20 light-hours, or about five times the distance between the sun and Neptune, at the edge of our solar system. Thus far, this is what G2 has been doing. Though observations have only begun, astronomers have seen no substantial increase in radiation coming from the central Milky Way region, putting constraints on the amount and density of gas and dust there.

If this is true, and the area around the black hole is relatively empty, it could help explain why our galaxy’s black hole doesn’t produce much radiation. Distant supermassive black holes that we see out in the universe are often spewing copious amounts of energy in the form of radiation jets. These objects, known as quasars, are some of the brightest things ever seen. Scientists don’t yet know how they work but they are thought to be an early stage of galactic evolution, when their central supermassive black hole is consuming huge amounts of material, producing tons of energy. How and why quasars turn off is an ongoing area of research.

Astronomers know that as recently as a few hundred years ago, the Milky Way’s central supermassive black hole was producing much more radiation. The reflected X-ray echoes of this era have been observed bouncing off clouds of gas and dust. Why this energetic period stopped is a mystery. With future data from G2’s trajectory, we might find out that our galactic black hole simply ran out of things to eat.

As far as being able to actually witness material slip beyond the black hole’s event horizon and disappear forever, that will have to wait for another day. Because it’s so far away, no current telescope has the resolution to see that well in the galactic center. Astronomers are trying to coordinate different telescopes right now as part of the Event Horizon Telescope project, which could image the area just around the supermassive black hole. But the earliest that this project will be up and running is 2016. Perhaps some blobs from G2 will get torn off and be hurtling right into the black hole at this time?

“If we’re lucky we could see the effects of special and general relativity,” said Genzel, talking about the Event Horizon Telescope. “But probably not with this cloud.”

Testing relativity requires a reliable clock, he added, and G2 doesn’t emit any sort of periodic radiation that would be useful for this.

Video: Computer simulations showing one possible trajectory for G2 as it passes around the central black hole. ESO/MPE/M. Schartmann/L. Calçada


Adam is a Wired Science staff writer. He lives in Oakland, Ca near a lake and enjoys space, physics, and other sciency things.

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