At the center of our Milky Way Galaxy lies the 4-million-solar-mass black hole named Sagittarius A*. This gravitational monster is surrounded by a group of stars orbiting around it at high speed. This extreme environment makes it the perfect place to explore gravitational physics, and particularly to test Einstein’s relativity theory. New observations from a trio of sensitive instruments on ESO’s Very Large Telescope have now allowed astronomers to follow one of these stars, called S2, as it passed very close to Sagittarius A* in May 2018. At the closest point S2 was at a distance of around 12 billion miles (20 billion km) from the black hole and moving at a speed in excess of 15.5 million mph (25 million km per hour) — almost 2.5% of the speed of light.

“This is the second time that we have observed the close passage of S2 around the black hole in the Galactic center,” said Dr. Reinhard Genzel, from the Max Planck Institute for Extraterrestrial Physics in Germany.

“But this time, because of much improved instrumentation, we were able to observe the star with unprecedented resolution.”

Dr. Genzel and colleagues used ESO’s SINFONI (Spectrograph for INtegral Field Observations in the Near Infrared) instrument to measure the velocity of S2 towards and away from Earth and the GRAVITY instrument to make extraordinarily precise measurements of the star’s changing position in order to define the shape of its orbit.

They then compared their measurements, along with previous observations of S2 using other instruments, with the predictions of Newtonian gravity, general relativity and other theories of gravity.

The new results are inconsistent with Newtonian predictions and in excellent agreement with the predictions of general relativity.

“The new measurements clearly reveal an effect called gravitational redshift,” the astronomers said.

“Light from S2 is stretched to longer wavelengths by the very strong gravitational field of the black hole.”

“And the change in the wavelength of light from S2 agrees precisely with that predicted by Einstein’s theory of general relativity.”

“This is the first time that this deviation from the predictions of the simpler Newtonian theory of gravity has been observed in the motion of a star around a supermassive black hole.”

“Our first observations of S2 with GRAVITY, about two years ago, already showed that we would have the ideal black hole laboratory,” said GRAVITY/SINFONI principal investigator Dr. Frank Eisenhauer, also from the Max Planck Institute for Extraterrestrial Physics.

“During the close passage, we could even detect the faint glow around the black hole on most of the images, which allowed us to precisely follow the star on its orbit, ultimately leading to the detection of the gravitational redshift in the spectrum of S2.”

“More than one hundred years after he published his paper setting out the equations of general relativity, Albert Einstein has been proved right once more — in a much more extreme laboratory than he could have possibly imagined.”

“Here in the Solar System we can only test the laws of physics now and under certain circumstances,” said Dr. Françoise Delplancke, head of the System Engineering Department at ESO.

“So it’s very important in astronomy to also check that those laws are still valid where the gravitational fields are very much stronger.”

The results appear in the journal Astronomy & Astrophysics.

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R. Abuter et al (GRAVITY Collaboration). 2018. Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole. A&A 615, L15; doi: 10.1051/0004-6361/201833718