The Galaxy’s Fastest Star Could Reveal the Secrets of a Supermassive Black Hole

S301, the fastest star in the galaxy, is known to reach a blistering 25,000 kilometers per second at its peak speed. This makes it more than 100 times faster than our Sun, which orbits the center of the Milky Way at roughly 230 km/s. At its closest approach to Sagittarius A*, the supermassive black hole at the heart of our galaxy, S301 moves at about 8 percent of the speed of light.

The key to S301's extreme speed lies in the intense gravitational pull of Sagittarius A*, which accelerates the star as it gets closer. However, the star does not get swallowed by the black hole; instead, it follows an elongated orbit around it. The shape of this orbit explains the star's varying speed, as it moves faster when it is closest to the black hole and slower when it is farther away.

This behavior is similar to what we observe in some comets, like Halley's Comet. While S301 is the fastest star currently known in the Milky Way and the closest to Sagittarius A*, it likely did not form in its current location. Instead, it might have been part of a binary star system that was torn apart by the tidal forces of Sagittarius A*, with S301 being captured by the black hole and its companion star being ejected with tremendous velocity, possibly leading it out of the galaxy.

Scientists believe that within the next decade, they will be able to measure the spin of Sagittarius A* using S301 as a reference point. This black hole, like all others, has two fundamental properties that scientists use to describe it: mass and spin. While there are indications that Sagittarius A* is spinning, determining its exact spin rate and direction has been challenging.

However, observing the small deviations in S301's orbit caused by the black hole's gravity could provide the answer. Observations of S301 began in 2023 using the GRAVITY instrument on the European Southern Observatory's Very Large Telescope Interferometer (VLTI) in Chile. By tracking its movements and combining data from observations since 2017, scientists have been able to reconstruct its orbit with greater precision.

This has allowed them to determine that S301 passed its closest approach to Sagittarius A* in early 2023, and it will do so again in 2031. While other stars, such as S2, have helped astronomers determine the mass of Sagittarius A* and test general relativity, S301's proximity to the black hole offers a unique opportunity to directly measure its spin.

This could be achieved within the next decade, which is a significant improvement over the several more decades required by observing other stars.