Free Astronomy Magazine May-June 2020

4 MAY-JUNE 2020 SPACE CHRONICLES The star S2 moves according to Einstein’s Relativity O bservations made with ESO’s Very Large Telescope (VLT) have revealed for the first time that a star orbiting the super- massive black hole at the centre of the Milky Way moves just as pre- dicted 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 long-sought-after re- sult was made possible by increas- ingly precise measurements over nearly 30 years, which have enabled scientists to unlock the mysteries of the behemoth lurking at the heart of our galaxy. “Einstein’s General Relativity pre- dicts that bound orbits of one object around another are not closed, as in Newtonian Gravity, but precess for- by ESO T his artist’s impression illustrates the precession of the star’s orbit, with the effect exaggerated for eas- ier visualisation. [ESO/L. Calçada] wards 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 favour of General Relativity. One hundred years later we have now detected the same effect in the mo- tion of a star orbiting the compact radio source Sagittarius A* at the centre of the Milky Way. This obser- vational breakthrough strengthens the evidence that Sagittarius A* must be a supermassive black hole of 4 million times the mass of the Sun,” says Reinhard Genzel, Direc- tor at the Max Planck Institute for