Sgr A* is the name we’ve given to the supermassive black hole that sits at the center of the Milky Way. We’ve known about its presence since the 1970s but only managed to image it within the past few years. In the intervening time, most of our understanding of the object was obtained by watching a group of stars that orbit the black hole, helping us get a good estimate of its mass and size. In essence, those stars acted as instruments that let us peer into an environment we couldn’t study any other way.

In Wednesday’s issue of Nature, researchers describe a recently discovered star that is on an extremely eccentric orbit that takes it closer to Sgr A* than anything we’ve previously identified. It gets so close that it may help us get our first measurements of the spin of the black hole.

Reading the spin

There’s an entire population of stars that orbit relatively close to Sgr A. We can estimate their mass based on their brightness and spectral features. Using their masses and a reconstruction of their orbits using several years of data, we can figure out just how supermassive Sgr A is (nearly 1037 kilograms).

Figuring out a black hole’s mass doesn’t seem to tell us much about it. But there isn’t exactly much else to tell. Our understanding of relativity says that, beyond mass, there are only two distinct properties of black holes. One of them is charge, and the other is spin. There’s no way to get information about charge. But spin is a different matter. We know that the materials that form black holes have momentum that has to be conserved by the spin of the resulting body. And our detection of gravitational waves from black hole mergers tells us something about the spin.