A Star Orbiting A Black Hole Shows That Relativity Still Holds

Artist's depiction of the three stars orbiting Sgr A*. ESO/M Parsa/L. Calçada

Astronomers have provided one of the most precise tests of general relativity yet, by monitoring how stars are orbiting the supermassive black hole at the center of the Milky Way. And Einstein’s theory passes the test, once again.

The study, published in the Astrophysical Journal, focused on mapping in detail the orbits of S2, S38, and S55, the three shortest period stars that orbit Sagittarius A*. The team made a comparison between the predictions of Newtonian dynamics and general relativity, showing a clear discrepancy between the observations and the Newton theory of gravity.

The motion of S2 agrees much better with Einstein than Newton. Both the location of the supermassive black hole and the shape of the star's orbit are a bit different than expected if we assume that the Newtonian Theory is correct.

“The Galactic Centre really is the best laboratory to study the motion of stars in a relativistic environment," lead author Marzieh Parsa, from the University of Cologne, said in a statement. "I was amazed how well we could apply the methods we developed with simulated stars to the high-precision data for the innermost high-velocity stars close to the supermassive black hole."

While the results are in agreement with general relativity, it doesn’t mean that Einstein’s theory cannot be questioned. Other groups have looked at these stars to check how relativity holds up in the most taxing testing environment. The observations have large enough uncertainties to allow for corrections to Einstein’s theory, but so far nothing has shown a need for it. Occam razor tells us that if we should bet on a particular theory, general relativity is a solid choice. More work, however, is necessary to determine whether that's the correct choice or not.

"During the course of our analysis we realized that to determine relativistic effects for S2, one definitely needs to know the full orbit to very high precision," commented Andreas Eckart, team leader at the University of Cologne.

The research also gave new estimates on the mass of Sagittarius A* and its distance from us. It has a mass of about 4.7 million times the size of the Sun and is located 27,800 light-years from our planet.

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