Fastest Known Star in the Galaxy Could Help Reveal a Black Hole's Spin

Astronomers have detected the fastest star yet known in the Milky Way. The star, named S301, orbits Sagittarius A*, the supermassive black hole at the galaxy's center, which has a mass roughly four million times that of the Sun.

S301 completes one orbit around Sagittarius A* every 8.7 years, reaching a maximum speed of 25,000 kilometers per second — about 8 percent of the speed of light and more than 100 times faster than the Sun's orbital speed around the galactic center. According to the European Southern Observatory (ESO), at its closest approach the star comes about as close to the black hole as Saturn is to the Sun.

The black hole's immense gravity accelerates S301 as it nears its closest point, but instead of falling in, the star follows a highly elongated orbit — similar to how some comets, such as Halley's Comet, move through the solar system. This shape also explains why its speed varies so dramatically, moving fastest near the black hole and slower further away.

A possible torn-apart binary

ESO explains that S301's orbital characteristics, combined with the fact that stars cannot form so close to a massive black hole, suggest it was once part of a binary system torn apart by Sagittarius A*'s tidal forces. In that process, S301 was captured by the black hole's gravity while its companion star was likely flung out at high speed, possibly leaving the galaxy entirely.

Measuring the black hole's spin

Astronomers hope that within a decade they will be able to determine Sagittarius A*'s spin, one of the two fundamental properties, along with mass, used to describe a black hole. General relativity predicts that a rotating black hole drags spacetime around it, subtly altering the orbit of nearby objects. If researchers can measure such deviations in S301's path over the coming years, they could determine the direction and magnitude of the black hole's spin.

S301 was first observed in 2023 using the GRAVITY instrument on ESO's Very Large Telescope Interferometer in Chile, and scientists later traced its movement back through data collected since 2017. This allowed them to determine that the star's closest approach occurred in early 2023, with the next expected in 2031. Other stars, such as S2, have previously helped confirm the black hole's mass and test general relativity, but S301's proximity makes it a particularly promising target for measuring spin directly.