A star has been discovered speeding closely around the massive black hole at the center of the Milky Way, a new study says. The finding offers the tantalizing possibility of testing Einstein’s general theory of relativity on the grandest of scales.

The faint star, called S0-102, orbits the black hole in 11.2 years, making it the closest large object known in the vicinity of our galaxy’s superdense center. The star travels at speeds of up to 6,600 miles (10,600 kilometers) per second and is in a stable, if changeable, orbit.

S0-102 is only the second star identified as being in short orbit around the Milky Way’s black hole—the other, S0-2, takes about 16 years.

“The fact that we are finding stars this close to the supermassive black hole—a hundred times closer to its event horizon than ever identified before—shows just how fast this field is developing,” said study co-author Andrea Ghez, an astrophysicist at the University of California, Los Angeles. An event horizon is a boundary beyond which nothing, including light, can escape from a black hole.

“Our first goal has been to make the discoveries. But the next layer of science is the fundamental physics because this is an unparalleled laboratory for testing the general theory of relativity.”

The black hole, which has four million times the mass of our sun but is only ten times larger, is named Sagittarius A after the constellation in which it’s located, some 26,000 light-years away from Earth.

Einstein’s theory says that mass can warp space and time, and that has been proven many times over. But the theory has never been confirmed around an immense black hole—where traditional physics is known to break down—or on the scale provided by Sagittarius A and the stars around it.

Now, however, Ghez and colleagues can begin to test what happens to the orbits of stars relatively close to a gigantic black hole. If Einstein was correct, then the orbits will shift slightly with each rotation, never returning to the same spot and gradually producing a daisy-petal pattern.

To best determine the effects of the black hole, researchers have to observe a complete circuit, and especially to know what happens when the star is closest to the black hole—known as the periapse.

The shortness of the orbits is so important because it allows for observations and assessments impossible with stars that might complete an orbit every 60 years or more, as most stars around the central black hole do.


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