Astronomers have discovered the Milky Way’s fastest-known star, S301, zooming around the supermassive black hole Sagittarius A* at up to 25,000 kilometers per second. Detected using the European Southern Observatory’s Very Large Telescope, the extreme star completes an 8.7-year orbit that offers an unprecedented window into general relativity and black-hole spin.
Space is full of extreme extremes, but few objects rival the velocity and perilous path of a newly identified star near the center of our galaxy. Discovered by researchers at the Max Planck Institute for Extraterrestrial Physics in Germany, S301 hits a peak speed of around 15,500 miles per second, or roughly 25,000 kilometers per second. That velocity translates to more than 8 percent of the speed of light.
To put that in perspective, the star accelerates to roughly 100,000 times faster than a commercial airplane as it slingshots around Sagittarius A*, the 4-million-solar-mass black hole situated at the heart of the Milky Way. Researchers first spotted the faint point of infrared light in observations from the spring of 2023, subsequently tracing its path backward through archival data from 2021 and 2017.
Tracking an Unprecedented Orbit With ESO Instruments
Spotting S301 was no small feat. The celestial body is roughly two billion times fainter than Betelgeuse, the tenth brightest star in the sky, and sits in one of the most crowded and obscured regions of the cosmos. Astronomers overcame these observational hurdles by employing a suite of advanced instruments located in the Chilean desert, including the GRAVITY instrument at the European Southern Observatory’s Very Large Telescope Interferometer.
By combining light from four separate telescopes, researchers could isolate the faint flicker of the star amidst the intense glare of the galactic core. Their tracking revealed a highly stretched orbit with a period of just 8.7 years, edging out previous record holders like the star S55 to become the fastest star ever detected in our galaxy.
“What is special about this star is that it’s orbiting Sagittarius A* on a very tight orbit, taking just 8.7 years to complete it, and is approaching the black hole at a mere 12 times the distance of Earth to the sun. That is unprecedented.”
Felix Mang, Max Planck Institute for Extraterrestrial Physics
A Violent Binary Past and Extreme Proximity
Scientists believe S301 did not always lead such a perilous existence. The prevailing theory suggests the star was once part of a binary system before venturing too close to the supermassive black hole. The almighty gravitational pull of Sagittarius A* tore the pair apart, trapping S301 in a tight, bound orbit while its former partner was blasted out with such immense force that it likely escaped the galaxy entirely.
At its closest approach, S301 plunges roughly 10 times closer to the black hole than the famous star S2, reaching a distance comparable to the gap between Saturn and our Sun. This extreme proximity exposes the star to intense gravitational forces that alter how researchers view the environment surrounding Sagittarius A*.
Testing Einstein’s Theory of General Relativity
The true scientific value of S301 goes far beyond setting a speed record. Because it grazes the immediate vicinity of the black hole, the star acts as a natural probe for testing Albert Einstein’s theory of general relativity under extreme physical conditions. Reinhard Genzel, who shared a 2020 physics Nobel Prize for discovering the central black hole, noted the significance of the findings in a public statement.
Einstein’s equations predict that massive, rotating objects drag the very fabric of spacetime along with them—a phenomenon known as the Lense-Thirring effect, or frame-dragging. While ordinary spacetime curvature accounts for the vast majority of shifts in stellar orbits near the black hole, researchers calculate that the rotation of Sagittarius A* could add an extra twist to S301’s path during every single pass.
Measuring the Spin of Sagittarius A*
For decades, astronomers have mapped the mass of Sagittarius A* with high precision, pinning it at approximately 4.3 million times the mass of our Sun. However, determining the rotation rate, or spin, of the black hole has proven significantly more difficult, traditionally relying on indirect inferences from X-rays, jets, or distant mergers.
S301 provides a direct dynamical alternative. By tracking the star as it navigates a region where the frame-dragging effect is extreme, scientists hope to calculate the black hole’s rotation directly from stellar trajectories rather than complex models of infalling matter.
What Comes Next in 2031
Observing the subtle accumulation of frame-dragging shifts requires patience. Researchers are currently relying on data spanning nearly a decade, but the team will receive another prime opportunity when S301 makes its next closest approach to Sagittarius A* in 2031.
Data gathered across two complete orbital cycles should grant the trajectory precision necessary to pin down the elusive spin of our galaxy’s central engine, closing a major gap in modern astrophysics.
