Unless Einstein is wrong, a black hole is defined by three properties: mass, spin, and electric charge.

The spin of a black hole is much more difficult to study.

A black hole’s spin is basically its rotation.

Black hole spin, like mass, is a spacetime property.

To measure the spin of a black hole, you need to study how matter behaves near it.

The spin of some supermassive black holes has been measured.

But we don’t know the spin of the closest supermassive black hole, the one in our own galaxy.

The spin of a black hole creates the same kind of frame-dragging, and by measuring it, we can determine the black hole’s spin.

We can’t put a probe in orbit around the black hole the way we did with Earth, but we can use the next best thing.

Hundreds of stars orbit the black hole at the center of our galaxy.

If we can measure these shifts, we can measure the spin—the greater the spin, the greater the orbit shift.

Given how well we know the orbits of these stars, we know the black hole at the center of our galaxy must be rotating slowly.

The team determined that its spin can be no more than 0.1 on a scale from 0 to 1, meaning it is rotating less than 10% of the maximum possible spin for a black hole.

By contrast, the spin of M87’s black hole is at least 0.4.