Scientists have identified a star with an eccentric orbit that ventures closer to the supermassive black hole, Sgr A*, than any other known object. This proximity could offer humanity’s first direct measurement of the black hole's spin.
Theorised since the 1970s, Sgr A* was only imaged recently, thanks to a group of stars that orbit it. These stellar sentinels help us infer Sgr A*'s mass and size, but this new star takes our understanding closer than ever before.
By analysing the masses of these nearby stars based on their brightness and spectral features, researchers can estimate how massive Sgr A* is—nearly 1037 kilograms. But knowing a black hole’s mass tells us little about its nature.
The two unique properties of a black hole are charge and spin. Charge isn’t observable, but spin is crucial. The momentum from the materials forming the black hole must be conserved by this spin, and gravitational wave detections hint at this property too.
