Star S301 Breaks Orbital Speed Record at the Center of the Milky Way

Astronomers have discovered the fastest star ever documented in the Milky Way, designated as S301, orbiting the galaxy’s central supermassive black hole, Sagittarius A*. Reaching speeds of 25,000 km/s—equivalent to more than 8% of the speed of light—the object completes its orbital trajectory in just 8.7 years. This extreme proximity creates the ideal physical conditions for the first direct measurement of the black hole’s rotation, marking a new frontier for astrophysical observation.

The Spacetime Drag Effect

According to Albert Einstein’s General Theory of Relativity, a spinning black hole drags and distorts the spacetime around it, a continuous phenomenon that alters the orbital dynamics of objects that approach too closely. Until now, mapping the rotation of Sagittarius A*—a stellar body with four million solar masses—would have required decades of uninterrupted observations of the motion of slower and more distant neighboring stars. The orbit of star S301 changes this operational scenario because it is extremely tight: during its periastron, it approaches the black hole at a distance of only 1.78 billion kilometers, about 12 times the distance from the Earth to the Sun. Based on these unusual orbital properties and the fact that stellar systems cannot form under such violent gravitational influence, physical models suggest that S301 operated as part of a binary system. The system was torn apart by the tidal forces of Sagittarius A*, resulting in the gravitational capture of S301 and the likely ejection of its companion out of the galaxy.

High-Resolution Interferometry with the VLTI

The astronomical detection of S301 represented a severe technological challenge, since the celestial body appears in the sky with a brightness two billion times fainter than the star Betelgeuse. Signal isolation was executed through the cutting-edge infrastructure of the Very Large Telescope Interferometer (VLTI), located at ESO’s Paranal Observatory in the Chilean desert. By triggering the modules of the GRAVITY instrument and its upgraded GRAVITY+ version, the technological array actively combines the light collected from four 8-meter telescopes, acting in unison to form a single expanded virtual telescope. This optical method gives the array a spatial resolution 15 times greater than the isolated capability of a conventional 8-meter telescope.

“Worldwide, Paranal is the only place where you can do this type of observations because no other observatory in the world has four 8-metre telescopes that can act together as an interferometer,” states Frank Eisenhauer, GRAVITY+ Principal Investigator and Director at the Max Planck Institute for Extraterrestrial Physics.

Next Steps Toward Measuring the Spin

Retrospective data tracking allowed the team of scientists to recreate the orbital history of S301 since 2017, confirming that its last maximum approach occurred in the early months of 2023. The next close stellar passage is mathematically predicted for the year 2031. To ensure a definitive measurement level, uninterrupted follow-up observations conducted by GRAVITY+ and with the addition of the MICADO instrument on the upcoming Extremely Large Telescope (ELT) will be vital. By rigorously and precisely tracing the trajectory over at least two complete orbits in the next decade, the team will have sensitive enough data to isolate the metric component and directly determine the spin of Sagittarius A* for the first time.