At the center of the Milky Way lurks Sagittarius A*, a supermassive black hole that is four million times the mass of the Sun. Located 26,000 light-years away, Sagittarius A* and a dense star cluster around it provide a unique laboratory for testing physics in an otherwise unexplored and extreme regime of gravity. One of these stars, S2 (also known as S0-2), sweeps in towards Sagittarius A* to a closest distance less than 20 billion km. At its closest approach to the black hole, S2 is hurtling through space at almost 3% of the speed of light, completing an orbit once every 16 years. New observations made with ESO’s Very Large Telescope (VLT) have revealed that S2 moves just as predicted by Einstein’s general theory of relativity. Its orbit is shaped like a rosette and not like an ellipse as predicted by Newton’s theory of gravity.
This simulation shows the orbits of stars very close to Sagittarius A*, a supermassive black hole at the heart of the Milky Way. One of these stars, S2, orbits every 16 years and is passing very close to the black hole in May 2018. Image credit: ESO / L. Calçada / Spaceengine.org.
Most stars and planets have a non-circular orbit and therefore move closer to and further away from the object they are rotating around.
S2’s orbit precesses, meaning that the location of its closest point to the supermassive black hole changes with each turn, such that the next orbit is rotated with regard to the previous one, creating a rosette shape.
General relativity provides a precise prediction of how much its orbit changes and the latest measurements from this research exactly match the theory.
This effect, known as Schwarzschild precession, had never before been measured for a star around a supermassive black hole.
“After following the star in its orbit for over two and a half decades, our exquisite measurements robustly detect S2’s Schwarzschild precession in its path around Sagittarius A*,” said Dr. Stefan Gillessen, an astronomer in the Max Planck Institute for Extraterrestrial Physics.
“Einstein’s general relativity predicts that bound orbits of one object around another are not closed, as in Newtonian gravity, but precess forwards in the plane of motion,” said Dr. Reinhard Genzel, Director of the Max Planck Institute for Extraterrestrial Physics and a researcher at the University of California, Berkeley.
“This famous effect — first seen in the orbit of the planet Mercury around the Sun — was the first evidence in favor of general relativity.”
“One hundred years later we have now detected the same effect in the motion of a star orbiting the compact radio source Sagittarius A* at the center of the Milky Way.”
“This observational breakthrough strengthens the evidence that Sagittarius A* must be a supermassive black hole of 4 million times the mass of the Sun.”
The study also helped the astronomers learn more about the vicinity of Sagittarius A*.
“Because the S2 measurements follow general relativity so well, we can set stringent limits on how much invisible material, such as distributed dark matter or possible smaller black holes, is present around Sagittarius A*,” said Observatoire de Paris astronomer Dr. Guy Perrin and Dr. Karine Perraut from the Universite Grenoble Alpes.
“This is of great interest for understanding the formation and evolution of supermassive black holes.”
The findings appear in the journal Astronomy & Astrophysics.
R. Abuter et al (GRAVITY Collaboration). 2020. Detection of the Schwarzschild precession in the orbit of the star S2 near the Galactic centre massive black hole. A&A 636, L5; doi: 10.1051/0004-6361/202037813