Astronomers using the European Southern Observatory’s Very Large Telescope Interferometer in Chile have discovered S301, the fastest known star in the Milky Way. Orbiting the supermassive black hole Sagittarius A* at speeds reaching 25,000 kilometers per second, the star provides a natural laboratory to test Albert Einstein’s theory of general relativity.
Detected from the high-altitude Paranal Observatory in northern Chile and published in the journal Nature, this high-velocity wanderer clocks in at roughly 8 percent of the speed of light. That is about 8% of the speed of light or an immense speed compared to a commercial airliner.
Engineering the Virtual Telescope at Paranal
The discovery was secured using the Very Large Telescope Interferometer (VLTI) and its GRAVITY+ instrument, hosted at the European Southern Observatory’s Paranal site in the Atacama Desert. By combining light collected from four distinct 8-meter primary mirrors, the system constructs a massive virtual telescope. This configuration yields a spatial resolution fifteen times sharper than any single telescope could achieve on its own.
“Of all the places in the world, Paranal is the unique spot where you can make these observations because no other observatory has four telescopes that can act together as an interferometer,” noted Frank Eisenhauer, director of GRAVITY+ at the Max Planck Institute for Extraterrestrial Physics (MPE), in technical communications released by the European Southern Observatory.
This technical precision allowed researchers to reconstruct archival telemetry, tracing the star’s orbital history back to 2017 before confirming its closest approach to Sagittarius A* in early 2023.
Orbital Dynamics in an Extreme Gravitational Well
At periapsis, its point of closest approach, S301 shrinks its distance to the black hole down to roughly 12 times the distance separating Earth from the Sun. That proximity places it roughly as close as Saturn sits from our own Sun.
“What is special about this star is that it orbits Sagitario A* in a very tight orbit, taking only 8.7 years to complete it, and approaches the black hole at barely 12 times the distance from the Earth to the Sun. That is something unprecedented,” stated Felix Mang, a doctoral student at the Max Planck Institute for Extraterrestrial Physics and co-author of the study.
Because stars cannot actually form within the violent tidal forces of a supermassive black hole, astrophysicists conclude that S301 had a more turbulent origin. The structural data points to a former binary system. As the pair passed too close to Sagittarius A*, the black hole’s tidal forces sheared the binary apart. S301 was captured into a tight gravitational lease, while its stellar companion was ejected at hypervelocity, likely escaping the Milky Way entirely.
Putting Einstein’s General Relativity to the Ultimate Test
The discovery is more than just a speed record; it provides a direct mechanism to probe the fabric of spacetime. Astrophysicists widely accept that Sagittarius A* is spinning on its axis. General relativity dictates that a rotating massive body drags and twists the surrounding spacetime, creating an effect known as frame-dragging that warps the predictable paths of nearby orbiting bodies.
Because S301 dives so deep into this extreme gravitational environment and travels at such high fractions of light-speed, it serves as an ideal probe. Researchers estimate that tracking S301 through its next periapsis passage in 2031 will yield enough positional data across two complete orbits to calculate the spin rate of Sagittarius A* directly.
“Without this star, we would have to measure the movement of other stars for several decades more to get close to measuring the spin of the black hole,” explained Juan Osorno, an astronomer at the LIRA laboratory of the Paris-PSL Observatory in France.
As the international team prepares for the 2031 flyby, they intend to pair the existing power of GRAVITY+ with the upcoming Extremely Large Telescope (ELT) currently under construction on Cerro Armazones. Capturing consecutive full orbits will transition black hole physics from theoretical modeling to empirical measurement.
