Astronomers have found something more interesting at the center of our Milky Way galaxy than just a black hole: a star orbiting that black hole at the record-breaking speed of 3,100 miles per second. It takes less than twelve years to make one full orbit, but its speed isn't its only scientifically interesting aspect.
The star's environment and proximity to the black hole will provide physicists with a unique opportunity to test Einstein's theory of relativity (which states that objects with mass distort the fabric of spacetime).
Using orbiting bodies to confirm Einstein's general relativity isn't a new kind of observation to make. In the 19th century, astronomers noticed Mercury's orbit didn't do what Newton's laws of gravity said it should. But when Einstein's theory of relativity came on the scene, the little planet's irregular motion was explained: the Sun's mass was curving spacetime, and because Mercury orbits so close to the Sun it was affected by this curve. The same phenomenon has been observed around neutron stars, but the effects in both cases aren't very pronounced — neutron stars only weigh about three times as much as our Sun.
The newly discovered star, called S0-102, is one in a class of similar "S-stars" that surrounds the Milky Way's central black hole, and it's the fastest one scientists have found:
Pictured above, from the Keck Observatory: "The orbits of stars within the central arcsecond of our galaxy. In the background, the central portion of a diffraction-limited image taken in 2012 is displayed." (Image Credit: Andrea Ghez and research team at UCLA/data sets obtained with W. M. Keck Telescopes)
The second fastest, and the record holder until recently, is a star called S0-2 that circles the black hole once every 16 years. The presence of two short-period stars is significant, since having a pair will give scientists a chance to look at two objects' change in orientation over time. This will ultimately yield more precise measurements on how much space has been curved by our galaxy's black hole.
Measuring the warping effects of the Milky Way's black hole on spacetime will be much easier than observations done on the Sun or neutron stars, because the black hole is about 4 million times as heavy as our Sun. That means the warping effects will be much greater, and the results that much more interesting. And because both S0-102 and S0-2 look to be quite close to the black hole, the effects should be clearly visible, increasing the likelihood of an accurate measurement. Having two stars will also help astronomers calculate out the effects of other objects in the vicinity. Comparing observed motions should make it clear what is an effect of the black hole and what is an effect of a nearby dead star.
Another way scientists will test relativity will be to measure the stars' redshift. The light traveling from a star through curved space should be shifted towards the longer end of the electromagnetic spectrum compared to light passing through "normal space."
Because the stars travel around the black hole so quickly, it won't take too long for scientists to observe a full orbit. And both stars will make their closest approach in the next 10 years — S0-102 in 2020 and S0-2 in 2018 or 2019. So by the end of the decade we should have some interesting new evidence about general relativity. Right now, scientists don't expect their observations will challenge Einstein's theory of relativity, but there's always a chance that some result will come as a big surprise.