A star has been spotted dancing around the supermassive black hole at the center of the Milky Way, and its orbit has been revealed to be shaped not like an ellipse, but a rosette – just as Einstein's theory of relativity predicted. 

The star S2 orbits the supermassive black hole at the center of our galaxy, Sagittarius A*, every 16 years, 26,000 light-years from the Sun. It’s the second-closest star to the colossal black hole and its location provides an excellent laboratory to test Einstein’s theory of general relativity.

Several predictions of Einstein's theory have already been tested using this star, all passing with flying colors. The latest test, reported in Astronomy & Astrophysics, used 27 years of observations, a large fraction conducted with the European Southern Observatory’s (ESO) Very Large Telescope, and matches perfectly with predictions from the theory.

“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. This famous effect — first seen in the orbit of the planet Mercury around the Sun — was the first evidence in favor of General Relativity," said Reinhard Genzel, Director at the Max Planck Institute for Extraterrestrial Physics (MPE) and architect of the 30-year-long program that led to this finding. "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 centre of the Milky Way."

The researchers were interested in measuring the precession of the star’s orbit. As the star goes around the black hole, its orbit shifts, and its closest point to the black hole changes with each turn. This is the so-called Schwarzschild precession. Observing it over many orbits revealed the star's orbit appears to draw a rosette shape. General relativity predicts exactly how much the orbit changes.

“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*,” Stefan Gillessen, also of the MPE, who led the analysis of the measurements, said in a statement.

This is not the first precession of an orbit predicted and explained by general relativity. According to Newton's theory of gravity, the planets orbit the Sun in an elliptical orbit. But Mercury famously has a weird egg-shaped orbit. Einstein's theory of general relativity states that the planets' ellipses are not immobile, but instead move slowly, performing a so-called precession. But precession has never been measured for a star orbiting a black hole before. Excitingly, this is not just a check for general relativity, it also provides key information about the distribution of matter and massive bodies in 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*," explained Guy Perrin and Karine Perraut, the French lead scientists of the project. "This is of great interest for understanding the formation and evolution of supermassive black holes.”

Earlier this year astronomers discovered S62, a star orbiting even closer to Sagittarius A* than S2. S2 gets as close as less than 20 billion kilometers (12.4 billion miles) from the black hole, and there might be stars closer still. ESO's upcoming Extremely Large Telescope (the "world's biggest eye on the sky") might spot fainter closer stars, expanding on what we can measure.

“If we are lucky, we might capture stars close enough that they actually feel the rotation, the spin, of the black hole,” says Andreas Eckart from Cologne University, another of the lead scientists of the project “That would be again a completely different level of testing relativity."