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The two W. M. Keck telescopes on Mauna Kea, Hawaii.

Hawaii's W. M. Keck Observatory (pictured surveying the Milky Way) helped astronomers identify S0-102.

Photograph courtesy Ethan Tweedie Photography

Marc Kaufman

for National Geographic News

Published October 4, 2012

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."

(Related: "Einstein Theories Confirmed by NASA Gravity Probe.")

Stars' Short Orbits Allow Closer Look at 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. (See black hole pictures.)

Star Tango Revealing

"This is such an important discovery, because for stars located closer to the black hole, the gravitational field to study gets stronger and the effects more pronounced," said Avi Loeb, a Harvard theoretical astrophysicist not involved with the new finding.

That S0-102 was found despite the fact it's 16 times fainter than S0-2 suggests not only that the technology is advancing quickly, but also that there may be many more stars orbiting closer to Sagittarius A than imagined, Loeb said.

(See "Black Hole in Milky Way Seen Snacking on Asteroids?")

But the likely presence of other bodies in the vicinity makes it difficult to test the workings of Einstein's space time warping with one star alone.

That's because the orbit of one star is subject to gravitational pulls from bodies other than Sagittarius A, and so a second star in the region is needed to account for those other forces.

"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," study co-author Ghez said in a statement.

"This measurement cannot be done with one star alone," said Ghez, whose study appears today in the journal Science.

While additional stars even closer to Sagittarius A may be found, there's a limit to how near to the black hole they can be.

Because of its intense gravitational pull, the black hole will tear apart and consume stars that come too close. This well-known phenomenon convinced Ghez's team that stars close to Sagittarius A would be old and tightly formed stars, rather than younger ones with less cohesion.

Yet to their surprise, S0-2 turned out to be a young star—and S0-102 may be as well.

Author of the National Geographic e-book Mars Landing 2012, Marc Kaufman has been a journalist for more than 35 years, including the past 12 as a science and space writer, foreign correspondent, and editor for the Washington Post. He is also author of First Contact: Scientific Breakthroughs in the Hunt for Life Beyond Earth,published in 2011, and has spoken extensively to crowds across the United States and abroad about astrobiology. He lives outside Washington, D.C., with his wife, Lynn Litterine.

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