Speedy Star Orbits the Milky Way’s Black Hole

At the very center of the Milky Way, astronomers found a star moving at an incredible speed. S301 circles the supermassive black hole Sagittarius A* in just 8.7 years, on an extremely stretched-out orbit.

At its closest, S301 is about 1.7 billion kilometers from the black hole—about the distance between Saturn and the Sun—and speeds up to about 25,000 km/s. That is more than 8 percent of the speed of light and the fastest speed measured so far for a star in the Milky Way.

S301 may also become a special tool for studying the black hole itself. According to general relativity, a spinning black hole drags spacetime along with it, slightly changing the orbits of nearby objects. This is called the Lense–Thirring effect.

S301 passes so close to Sagittarius A* that this effect on its orbit should be detectable. Future observations using the VLTI interferometer and the future Extremely Large Telescope may let astronomers directly measure, for the first time, how fast the black hole at the center of our Galaxy spins.