A lone star called S2 orbiting a supermassive black hole in the center of our galaxy has demonstrated the prediction of general relativity in the most extreme environment in which we can test it.
Putting together dozens of observations, astronomers have shown that S2 is not an ellipse with a fixed position; rather, the orbit moves like a spirograph pattern — a phenomenon known as the Schwarzschild precession.
This is the first time a Schwarzschild precession has been detected around a supermassive black hole, demonstrating that it persists even as we observe the orbits of stars in the most gravitationally extreme environments.
In addition, general equations of relativity can be used to accurately predict orbital changes – and these calculations match exactly with S2 observations.
“Einstein's general theory of relativity predicts that the bound orbits of one object around another are not closed, as in Newtonian gravity, but precess forward in the plane of motion,” explained astrophysicist Reinhard Hansel of the Max Planck Institute for Alien Physics (MPE) in Germany.
This famous effect – first seen in the orbit of the planet Mercury around the Sun – was the first evidence in favor of general relativity. A hundred years later, we discovered the same effect when a star orbiting the Sagittarius A black hole in the center of the Milky Way moves.
S2 orbits Sagittarius A in a long elliptical orbit every 16 years. On its closest approach, or periastron, it is 17 light-hours from the black hole, or just over four times the distance from the Sun to Neptune.
It may sound far away, but when you're dealing with something as massive as Sagittarius A, it's surprisingly close, and the gravitational impact from the black hole accelerates the star to nearly 3 percent the speed of light as it rotates. It is one of the closest stars to the galactic center.
'Because the S2 measurements follow general relativity so well, we can set strict limits on how much invisible material, such as distributed dark matter or possibly smaller black holes, is present around Sagittarius A,' said astrophysicists Guy Perrin and Karin Perrault from the Paris Observatory – Place de Meudon and the Observatory of Grenoble in France, respectively.
“This is of great interest for understanding the formation and evolution of supermassive black holes.”
The study was published in Astronomy and Astrophysics.
