Astronomers have observed a fascinating cosmic event: a distant star that seems to be dancing around a supermassive black hole. The observations, as it turns out, proves something Einstein predicted was completely right.

Observations made with the Very Large Telescope (VLT) have revealed, for the first time, that the star orbiting the supermassive black hole at the center of the Milky Way galaxy moves as predicted by the general theory of relativity of Einstein. Its orbit is shaped like a rosette (and not an ellipse, as predicted by Newton’s theory of gravity).

This highly sought-after result was made possible by increasingly precise measurements carried out over almost 30 years, which has enabled scientists to unlock the mysteries of the lurking giant black hole at the heart of the Milky Way. Einstein’s General Relativity predicts that the bound orbits of one object around another are not closed, as in Newtonian Gravity, but rather have a forward motion of precession in the plane of motion. 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,” revealed Reinhard Genzel, Director at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany.

Located 26,000 light-years from the Sun, Sagittarius A * and the dense cluster of stars around it provide a unique laboratory for testing physics in an extreme and unexplored regime of gravity. One of these stars, S2, rushes toward the supermassive black hole from a distance of fewer than 20 billion km (120 times the distance from the Sun to Earth), making it one of the closest stars to be found orbiting a black hole.

In its closest approach to the black hole, S2 traverses space at almost 3% of the speed of light, completing an orbit once every 16 years.

“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*,” says Stefan Gillessen of the MPE, who led the analysis of the measurements published today in the journal Astronomy & Astrophysics.

Most stars and planets have a non-circular orbit and therefore move closer and further away from the object around which they revolve. S2’s orbit has a precessive motion, which means that the location of its closest point to the supermassive black hole changes with each turn, so that the next orbit rotates relative to the previous one, creating a rosette shape.

General Relativity provides an accurate prediction of how much its orbit changes, and the latest measurements from this research exactly match the theory. This effect, known as the Schwarzschild precession, has never been measured before in a star around a supermassive black hole. The study with the VLT also helps scientists learn more about the surroundings of the supermassive black hole at the center of the Milky Way.

“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*. This is of great interest for understanding the formation and evolution of supermassive black holes,” explain Guy Perrin and Karine Perraut, the French lead scientists of the project.