Star's motion around Milky Way's monster black hole proves Einstein right yet again

Einstein's theory of general relativity just passed a dramatic black-hole test with flying colors.

The motion of a star orbiting Sagittarius A*, the supermassive black hole at the heart of our Milky Way galaxy, precisely matches that predicted by general relativity, a new study reports.

"Einstein's general relativity predicts that bound orbits of one object around another are not closed, as in Newtonian gravity, but precess forward 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," study co-author Reinhard Genzel, director of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, said in a statement. 

"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," Genzel added. "This observational breakthrough strengthens the evidence that Sagittarius A* must be a supermassive black hole of 4 million times the mass of the sun." 

Observations made with the European Southern Observatory’s Very Large Telescope in Chile have revealed for the first time that a star orbiting the supermassive black hole at the center of the Milky Way moves just as predicted by Einstein's theory of general relativity. Its orbit is shaped like a rosette and not like an ellipse as predicted by Newton's theory of gravity. This effect, known as Schwarzschild precession, had never before been measured for a star around a supermassive black hole. This artist's impression illustrates the precession of the star's orbit, with the effect exaggerated for easier visualization. (Image credit: ESO/L. Calçada)

The motion Genzel mentioned, called Schwarzschild precession, describes a sort of rotation in an object's elliptical orbit. The location of the object's closest-approach point changes with each lap, so the overall orbit is shaped like a rosette rather than a simple, static ellipse.

Astronomers had never measured Schwarzschild precession in a star zooming around a supermassive black hole — until now.

The research team used the European Southern Observatory's (ESO) Very Large Telescope (VLT) in Chile to track a star called S2 as it looped around Sagittarius A*, which lies about 26,000 light-years from Earth. Over the course of 27 years, the astronomers made more than 330 measurements of S2's position and velocity using multiple VLT instruments. (One of those instruments is called GRAVITY, which gives the research team its name: the GRAVITY collaboration.)

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