Relativity observed near Milky Way's black hole

The stars in orbit around the Galactic Centre have given astronomers new insight into the monster black hole that lies at its heart.

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Although largely invisible, researchers have located the position of the super massive black hole at the centre of our Galaxy by observing the motion of stars that orbit close to it. Its position is shown here by the blue halo.
Credit: ESO/M. Parsa/L. Calçada

Words: Elizabeth Pearson

 

The subtle effects of relativity on the stars orbiting the supermassive black hole at the heart of the Milky Way have been seen for the first time.

The observations have allowed theorists to test the theory of relativity in one of its most extreme environments – near a black hole.

Weighing in at over four million solar masses, Sagittarius A* (Sag A*) is a monster of a black hole and resides in the Galactic Centre 26,000 lightyears away. It is surrounded by a family of stars locked in a tight orbit around it, known as S-stars.

Astronomers used ESO’s Very Large Telescope to measure the orbit of one of these, S2, very precisely and found its path had pulled slightly away from the shape that would be expected if classical mechanics alone were at play.

“The Galactic Centre really is the best laboratory to study stars in a relativistic environment,” says Marzieh Parsa, lead author of the study. “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.”

Attention has turned to S2 in recent years because in 2018, S2’s 15-year orbit will make its closest approach to Sag A*. In preparation, the team have been mapping the star’s orbit to a high precision using the VLT’s near-infrared adaptive optics instruments.

The most important part, however, has proven to be the distant part of S2’s orbit, as this is what helped to determine 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 a very high precision,” comments Andreas Eckart, from the University of Cologne, who led the team.

By studying the stars around Sag A*, the observers hope to learn more about the black hole keeping them in check. These measurements have helped to increase the accuracy of our distance and mass measurements of the black hole.

The watch over S2 will continue for several more years as it makes its closest approach. The researchers will not just be searching for relativity but any deviations from it, as this could be a sign of previously unknown physics that can only occur on the edge of a black hole. 


 

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