This artists impression shows the orbits of three of the stars very close to the supermassive black hole at the centre of the Milky Way. Analysis of data from ESOs Very Large Telescope and other telescopes suggests that the orbits of these stars may show the subtle effects predicted by Einsteins general theory of relativity. There are hints that the orbit of the star called S2 is deviating slightly from the path calculated using classical physics. The position of the supermassive black hole is marked with a white circle with a blue halo. Credit: ESO/M. Parsa/L. Calada

A new analysis of data from ESOs Very Large Telescope and other telescopes suggests that the orbits of stars around the supermassive black hole at the centre of the Milky Way may show the subtle effects predicted by Einsteins general theory of relativity. There are hints that the orbit of the star S2 is deviating slightly from the path calculated using classical physics. This tantalising result is a prelude to much more precise measurements and tests of relativity that will be made using the GRAVITY instrument as star S2 passes very close to the black hole in 2018.

At the centre of the Milky Way, 26 000 light-years from Earth, lies the closestsupermassive black hole, which has a mass four million times that of the Sun. This monster is surrounded by a small group of stars orbiting at high speed in the black holes very strong gravitational field. It is a perfect environment in which to test gravitational physics, and particularly Einsteins generaltheory of relativity.

A team of German and Czech astronomers have now applied new analysis techniques to existing observations of the stars orbiting the black hole, accumulated usingESOs Very Large Telescope(VLT) in Chile and others over the last twenty years. They compare the measured star orbits to predictions made using classicalNewtonian gravityas well as predictions from general relativity.

The team found suggestions of a small change in the motion of one of the stars, known as S2, that is consistent with the predictions of general relativity. The change due to relativistic effects amounts to only a few percent in the shape of the orbit, as well as only about one sixth of a degree in the orientation of the orbit. If confirmed, this would be the first time that a measurement of the strength of the general relativistic effects has been achieved for stars orbiting a supermassive black hole.

Marzieh Parsa, PhD student at the University of Cologne, Germany and lead author of the paper, is delighted: The Galactic Centre really is the best laboratory to study the motion of stars in a relativistic environment. 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.

The high accuracy of the positional measurements, made possible by the VLTs near-infrared adaptive optics instruments, was essential for the study. These were vital not only during the stars close approach to the black hole, but particularly during the time when S2 was further away from the black hole. The latter data allowed an accurate determination of the shape of the orbit.

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

As well as more precise information about the orbit of the star S2, the new analysis also gives the mass of the black hole and its distance from Earth to a higher degree of accuracy.

Co-author Vladimir Karas from the Academy of Sciences in Prague, the Czech Republic, is excited about the future:This opens up an avenue for more theory and experiments in this sector of science.

This analysis is a prelude to an exciting period for observations of the Galactic Centre by astronomers around the world. During 2018 the star S2 will make a very close approach to the supermassive black hole. This time theGRAVITY instrument, developed by a large international consortium led by the Max-Planck-Institut fr extraterrestrische Physik in Garching, Germany, and installed on the VLT Interferometer, will be available to help measure the orbit much more precisely than is currently possible. Not only is GRAVITY, which is already making high-precision measurements of the Galactic Centre, expected to reveal the general relativistic effects very clearly, but also it will allow astronomers to look for deviations from general relativity that might reveal new physics.

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Hint of relativity effects in stars orbiting centre of galaxy - Astronomy Now Online