Star dance around a black hole

After nearly three decades of work, scientists have revealed that a star orbiting the supermassive black hole at the centre of the Milky Way moves as predicted by Einstein’s Theory of General Relativity. 

They say observations made with ESO’s Very Large Telescope (VLT) show that its orbit is shaped like a rosette, not like an ellipse as predicted by Newton’s theory of gravity.

The findings are presented in a paper to appear in the journal Astronomy & Astrophysics. 

“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,” says Reinhard Genzel, Director of Germany’s Max Planck Institute for Extraterrestrial Physics (MPE).

“This famous effect – first seen in the orbit of the planet Mercury around the Sun – was the first evidence in favour 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 centre of the Milky Way. 

“This observational breakthrough strengthens the evidence that Sagittarius A* must be a supermassive black hole of four million times the mass of the Sun.” 

Sagittarius A* and the dense cluster of stars around it are about 26,000 light-years from the Sun. One of these stars is S2.

While 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 processes. 

This means 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.

Because S2 takes years to orbit Sagittarius A*, this had to be a long-term project. 

The result is the culmination of 27 years of observations using, for the best part of this time, a fleet of instruments at ESO’s VLT, located in the Atacama Desert in Chile. 

An international team, with collaborators from France, Portugal, Germany and ESO, made over 330 measurements in total, using the GRAVITY, SINFONI and NACO instruments.

The researchers say their work will also help scientists learn more about the vicinity of Sagittarius A* and the formation and evolution of supermassive black holes,”

Because the S2 measurements follow General Relativity so well, it will be possible to set stringent limits on how much invisible material, such as distributed dark matter or possible smaller black holes, is present around Sagittarius A*.