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At edge of black hole, a star Albert Einstein would have loved

Scientists have found a star orbiting very close to the supermassive black hole at the center of the Milky Way. It could help scientists give Einstein's brilliance one of its sternest tests yet.

By Staff writer / October 4, 2012

A NASA image made from data provided by the Chandra X-ray Observatory shows the supermassive black hole at the center of the Milky Way Galaxy.

NASA/AP/File

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Astronomers have found a star whipping around an enormous black hole at the center of the Milky Way once every 11.5 years, potentially providing them with a cosmic laboratory to test Albert Einstein's most famous theory and to learn more about how such huge black holes evolve.

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No known star orbits a supermassive black hole faster. Though it is among a group of stars discovered at the galactic center, it is only the second with an orbital period of less than 20 years. The other orbits once every 16 years.

By looking at precision measurements of these two stars when they are nearest the black hole, scientists hope to see if Einstein's general theory of relativity still holds true under the most intense gravitational field the galaxy can deliver.

Moreover, the stellar pair also are expected to help solve puzzles surrounding  black holes in galactic centers and how the black holes affect a galaxy's evolution, notes Andrea Ghez, an astronomer at the University of California at Los Angeles and the leader of the team reporting the discovery in Friday's issue of the journal Science.

"It is the tango" of these two stars "that will reveal the true geometry of space and time near a black hole for the first time," she said in a prepared statement.

According to general relativity, gravity results from mass. That mass – a planet, star, or black hole – warps space and time in its immediate vicinity, a bit like the depression a medicine ball can form when it's placed on a trampoline. This is called a gravitational "well."

A black hole is an object with so much mass – it has warped space and time around it so much – that not even light has enough energy to escape its gravitational well. Stellar-mass black holes, which form from the collapse and explosion of stars at least 10 times more massive than the sun, tip the cosmic scales at between three solar masses and tens of solar masses. The supermassive black hole at the heart of the Milky Way, by contrast, boasts 4 million times the sun's mass.

If a sun-like star were to approach within about 93 million miles of the Milky Way's central black hole, it would in essence become a spaghetti-like streamer of gas destined to vanish into the black hole, says Avi Loeb, chairman of the astronomy department at Harvard University.

The stars that zip around the Milky Way's black hole can reach speeds of up to 22 million miles an hour – a pace that would cover a trip from New York to Los Angeles in less than a second.

The short-period stars among them represent a bonanza for several reasons. First, one complete orbit occurs within a span of time that falls well within the length of a career in astronomy.

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