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Astronomers are preparing to watch a star swing by our supermassive black hole

There’s a supermassive black hole in the center of our galaxy called Sagittarius A*, or Sgr A* for short. This massive but invisible object lies about 26,000 light-years away and weighs in at 4 million times the mass of our Sun. Although it’s challenging to observe the area around Sgr A*, it’s not impossible — and now, Sgr A* is helping astronomers once again prove that Einstein was right.

The mounting evidence supporting Einstein’s theory of general relativity has been recently published August 9 in the Astrophysical Journal. That evidence consists of precise measurements of the motion of a star, dubbed S2, which orbits Sgr A* closely, and seems to be deviating from the orbit expected if it wasn’t affected by general relativity. This deviation indicates that Einstein’s theory is indeed in play.

S2 is one of many stars known to orbit Sgr A*, but specifically it is the star that comes closest to the supermassive black hole. That orbit brings it particularly close to the black hole every 16 years — an event that will take place in the middle of next year. As it passes through that region, S2’s orbit is expected to change as a result of the warping of space-time due to Sgr A*’s huge mass.

As 2018 approaches, S2 is already nearing that closest point. Astronomer Andreas Eckart of the University of Cologne in Germany is leading a team that has painstakingly compiled observations of S2 taken over more than 20 years to trace out its orbital motions. Although this data includes another close pass by the black hole 16 years ago, those observations were taken with instruments that don’t have the precision available today. That’s why the upcoming pass is so important — now, astronomers will finally have the resolution necessary to measure more definitively whether S2 is following a Newtonian orbit, which doesn’t take into account general relativity, or an orbit predicted by Einstein’s famous theory.



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