The two W. M. Keck Telescopes on Mauna Kea, Hawaii, observing the galactic… (Ethan Tweedie Photography )
Squinting into the dark heart of the Milky Way, astronomers have discovered the closest star yet to the galaxy’s supermassive black hole.
The relatively dim star, S0-102, takes just 11.5 years to circle the black hole. The previous record-holder, S0-2, took 16 years to make its way around.
A black hole is a star whose mass has collapsed to a point, a singularity. Its intense gravity distorts space-time so much that not even light can escape. The one at the center of the Milky Way contains the mass of 4 million suns.
Finding stars so close to the galaxy’s black hole would allow scientists to test Einstein’s general theory of relativity, which describes how gravity behaves, to see if its predictions still hold up under such extreme conditions.
“We’re a hundred times closer to the singularity than anyone’s ever tested the theory of relativity before,” said UCLA astrophysicist Andrea Ghez, coauthor of the study released Thursday by the journal Science.
But pinpointing and tracking such distant stars is no easy feat. Astronomers using ground telescopes have to contend with the atmosphere’s interference – those swirling ebbs and flows of air that blur telescopes’ sight and cause the constant light from stars to flicker, giving them their “twinkle.”
To see clearly, scientists need to take that twinkle out. And in recent years, they’ve developed ways to do so, called “adaptive optics.”
The astronomers who found S0-102 did so with Hawaii’s Keck Observatory, which sports a fancy system with a laser "guidestar" and a deformable mirror. That mirror can change shape, because it’s made up of many small, movable mirrors. The “guidestar” helps them track the pattern of distortion caused by the turbulent air.
In response, the deformable mirror essentially arranges its many tiny mirrors into the reverse pattern, shifting moment to moment with the atmosphere, erasing the distortion.
These improving optics are what helped the team pick this rather dim star out of the crowd that inevitably forms closer into a spiral galaxy. Now with two stars to compare, they'll be able to properly test Einstein's general theory of relativity.
To test general relativity, the scientists look for a particular pattern of motion around the black hole. Rather than make a complete ellipse around the singularity, Einstein’s theory predicts that the star’s orbit will shift each time it loops around, essentially tracing an unbroken circle of overlapping “petals” around the black hole.
The scientists will be looking to see if any unforeseen anomalies pop up – though ultimately, they’re not expecting any major surprises. But the theory must be probed, Ghez said, because there are known limits to Einstein’s theory: It breaks down on the very small scale – which is where quantum mechanics steps in.
“Einstein’s theory is one of the four fundamental forces, and it’s one of the least tested,” Ghez said. “So you’d basically like to know it’s correct.”
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