Fastest star in the Milky Way feels the rotation of our central black hole

The supermassive black hole at the center of our galaxy, known as Sagittarius A*, is believed to be spinning. Astronomers have recently discovered the fastest-known star in the Milky Way, designated as S301, which is orbiting this black hole in an extremely close orbit. This discovery could provide new insights into the spin of Sagittarius A*.

Black holes, which are extremely dense concentrations of matter, are thought to cause space-time to rotate around them due to a phenomenon called frame dragging or the Lense-Thirring effect. However, this effect is difficult to measure unless the orbiting star is very close to the black hole.

The research team, led by Felix Mang of the Max Planck Institute for Extraterrestrial Physics, determined the mass of Sagittarius A* to a sub-percent precision. Measuring the spin of a black hole is crucial, as a black hole is characterized by its mass, spin, and electric charge. In the case of Sagittarius A*, the spin can be measured through various methods, including X-ray reflection spectra, thermal X-ray emission, and gravitational-wave signatures from merging black hole binaries.

Sagittarius A* is located approximately 27,000 light-years from Earth, offering a unique opportunity to directly measure its spin.

Several dozen stars are in close orbit around Sagittarius A*, and the ones that come closest to the black hole are most effective at probing its spin as they traverse the deepest parts of the gravitational potential. The newly discovered star S301 is an ideal cosmic probe for studying Sagittarius A*'s spin. Despite being only slightly heavier than the Sun and having a radius slightly larger than the Sun, S301 is not particularly remarkable.

However, its eccentric orbit and very close approach to the supermassive black hole make it a noteworthy discovery.

Initial observations of S301 indicated that it was a foreground or background star, but dedicated follow-up observations in 2024 revealed a significant acceleration in its proper motion, suggesting that it may be gravitationally bound to Sagittarius A*. Continued observations in 2025 confirmed that S301 is indeed a very eccentric and tightly orbiting star.

By combing through archival data, the research team was able to locate traces of S301 in its orbits from 2021 and 2017, providing a strong indication that the star is gravitationally bound to Sagittarius A*. The team determined that S301 has a very short orbital period of just 8.7 years, making it the shortest known orbital period around the supermassive black hole.

This breaks the previous record, which was held by the star S55 with an orbital period of about 12 years. However, what makes S301's path extraordinary is its orbital eccentricity of about 0.98, which brings the star to a pericentre distance of approximately 1.78 billion kilometres – roughly 10 times closer than the pericentre distance of another star, S2, which orbits Sagittarius A*.

At its closest approach, S301 travels at about 25,000 km/s, making it the fastest known star in the Milky Way. Due to its proximity to the supermassive black hole, S301 experiences the effects of general relativity, including a phenomenon called the Schwarzschild precession, which shifts the star's orbit by about 2° per revolution.

Over the course of approximately 1500-1600 years, these slight shifts accumulate into a full 360° rotation of the orbit. The Lense-Thirring precession, a spin-induced effect caused by Sagittarius A* spinning, also affects S301's orbit, making it a powerful probe of the curved space-time around the black hole. By precisely tracking the star's motion, astronomers can further investigate the spin and characteristics of Sagittarius A*.