This Star Orbits Our Galaxy's Heart at a Record-Breaking Speed—and Could Help Uncover Secrets About the Behemoth Black Hole There

At the center of our Milky Way galaxy, some 27,000 light-years from Earth, sits a supermassive black hole called Sagittarius A*. Astronomers have long suspected that it’s spinning, but they’ve never been able to confirm it. Now, however, they have identified a stellar research assistant: A speedy star that periodically gets up close and personal with Sagittarius A*.

During its closest approach to the supermassive object, the star, dubbed S301, should be affected by the black hole’s rotation, allowing scientists to measure the spin directly for the first time. The findings were published August 19 in the journal Nature. Gillessen and his colleagues detected S301 with the European Southern Observatory’s Very Large Telescope Interferometer in Chile.

It combines light from several telescopes to observe extremely faint astronomical objects. They spied the star near Sagittarius A* in spring 2023 and collected follow-up measurements over the next couple of years. That data was also combined with archival observations dating back to 2017.

Quick fact: How massive is this supermassive object? Sagittarius A* is estimated to be about 4.3 million times as massive as our sun. Analyses revealed that S301 has an elongated, oval-shaped orbit, with the black hole at one far end.

Along that path, the star reaches a top speed of about 56 million miles per hour—roughly 8 percent the speed of light —and is the fastest star ever recorded in the Milky Way. As it approaches Sagittarius A*, “the star comes in in one direction, getting faster and faster and faster, then gets a very sharp turn around the black hole,” Gillessen tells Ari Daniel for NPR’s “All Things Considered.” Then “it flies back out again and decelerates until it’s at its farthest point. Once you know the orbit, it’s pretty much like the Swiss railway system.

Things are there precisely on time.” It takes just 8.7 years to complete a loop around Sagittarius A*, and at its closest point, the two objects are separated by merely 12 times the distance between Earth and the sun. S301’s routine proximity to the supermassive black hole makes it the first known star that could be used to measure Sagittarius A*’s rotation directly. It can do that because Einstein’s general theory of relativity posits that a rotating black hole also twists space-time, which should change the orbits of nearby stars.