Squinting into the dark heart of the Milky Way, astronomers have discovered a star that orbits closer to the supermassive black hole at the center of the galaxy than any other star yet observed.
The relatively dim star, known as S0-102, is so close that it takes just 11.5 years to circle the black hole at speeds as high as 5,000 kilometers per second — or 1.7% as fast as the speed of light. 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 called a singularity. Its intense gravity distorts space-time so much that not even light can escape. The black hole at the center of the Milky Way contains the mass of about 4 million suns.
Finding stars in such close proximity to a black hole will allow scientists to test Albert Einstein's general theory of relativity, to see if its predictions about how gravity behaves hold up under extreme conditions.
"We're 100 times closer to the singularity than anyone's ever tested the theory of relativity before," said UCLA astrophysicist Andrea Ghez, coauthor of the study published in Friday's edition of the journal Science. "Einstein's theory is one of the four fundamental forces, and it's one of the least tested."
Ghez and her colleagues have been examining a group of 3,000 or so stars in the region closest to the black hole since 1995. By measuring the motion of the objects from Hawaii's Keck Observatory, they deduced that a black hole must be directing their orbits.
Since then, they've made some surprising discoveries. Among them: The stars close to the center of the galaxy tend to be young rather than old.
"What we've learned about the center of the galaxy is to expect the unexpected," Ghez said. "Almost everything we've predicted about what we should see has actually been quite different."
Pinpointing and tracking such distant, dim stars is no easy feat. Astronomers using ground telescopes have to contend with the atmosphere's interference — the 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 erase that twinkle using technology called adaptive optics.
Keck Observatory has been outfitted to shoot a laser into the sky, creating an artificial "guide star" that helps astronomers track the pattern of distortion caused by turbulent air. The telescope's array of many small, movable mirrors changes shape to compensate for the atmospheric distortion, thus canceling out the visual "noise."
Adaptive optics helped Ghez's international team — which included researchers from Spain and Canada as well as UC Irvine and Caltech — pick this rather dim star out of the crowd that inevitably forms near the center of a spiral galaxy.
Now, with two stars to observe at the same time, scientists will be able to properly test Einstein's famous theory.
For instance, general relativity predicts that a star orbiting close to a singularity will not make a complete ellipse but will shift each time it loops around, essentially tracing an unbroken rosette of overlapping "petals" around the black hole.
But there's a lot of matter hanging around near the center of the galaxy, potentially tugging on the star's orbit. Astronomers need to factor that out in order to measure relativity's effects.
That's where S0-102 comes in. Having a second nearby star to work with will help researchers decipher the degree to which their orbits are due to the black hole's gravity and not to other mass in the area, Ghez said.
"If you want to do this test, you really need two stars," Ghez explained. "It's the tango of the two stars that tells you that you really understand what general relativity is doing to the motion, as opposed to any other effect."
And as astronomers continue to stare into the eye of the Milky Way, they may find dim stars even closer to the black hole, said Harvard theoretical astrophysicist Avi Loeb, who was not involved in the study. A star the size of our sun would be small enough to orbit "100 times closer in," he said.