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.” 

Related: Images: black holes of the universe

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.)

Such a long observational window was necessary to pick up S2’s precession, for the star takes 16 Earth years to complete one orbit around Sagittarius A*. 

The observed precession matched the predictions of general relativity exactly, which could lead to further discoveries down the road, the researchers said.

“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*,” team members Guy Perrin and Karine Perraut — of the Paris Observatory-PSL and the Grenoble Institute of Planetology and Astrophysics in France, respectively — said in the same statement. 

“This is of great interest for understanding the formation and evolution of supermassive black holes,” they added.

This simulation shows the orbits of stars very close to the supermassive black hole at the heart of the Milky Way. One of these stars, named S2, orbits every 16 years and is passing very close to the black hole in May 2018. (Image credit: ESO/L. Calçada/

The new study, which was published online today (April 16) in the journal Astronomy & Astrophysics, may presage even more exciting black-hole insights to come. For example, coming megascopes such as ESO’s Extremely Large Telescope could allow astronomers to track stars that get even closer to Sagittarius A* than S2 does, the researchers said.

“If we are lucky, we might capture stars close enough that they actually feel the rotation, the spin, of the black hole,” said study team member Andreas Eckart of Cologne University in Germany. “That would be again a completely different level of testing relativity.”

Mike Wall is the author of “Out There” (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook


Liked Liked