Discovered by an international team using the European Southern Observatory’s Very Large Telescope Interferometer (VLTI) in Chile, S301 is the fastest star ever recorded. Orbiting the supermassive black hole Sagittarius A* at a staggering 25,000 kilometers per second—roughly 8% of the speed of light—this star is set to revolutionize astrophysics.
Deep inside the Galactic Center, hidden behind dense clouds of cosmic dust and gas, sits Sagittarius A*. This behemoth packs 4.3 million solar masses into a diameter of roughly 25 million kilometers. Around it, a dense population of stars whips through complex gravitational trajectories. Until recently, the star S2 held the speed record at 7,650 kilometers per second, completing an elliptical 16-year orbit that famously helped confirm Albert Einstein’s general theory of relativity back in 2018.
S301 has shattered previous records. The newly discovered star cuts through its orbit around Sagittarius A* in just 8.7 years. During its closest approach, known as the pericenter, S301 plunges ten times deeper into the black hole’s gravitational well than S2 ever has, grazing the dark giant at a distance a mere 20% farther than Saturn’s distance from our Sun.
Probing Spacetime Twist and Frame-Dragging
That extreme velocity and proximity do more than just rewrite records. According to Felix Mang of the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, lead author of a study published in Nature, “What has special about this star is that it orbits Sagittarius A* in a very narrow orbit… and approaches the black hole at just 12 times the distance from the Earth to the Sun. Something that has no precedent.”
General relativity dictates that massive rotating objects don’t just sit passively in space; they drag the very fabric of spacetime along with them in a swirling vortex effect known as frame-dragging or the Lense-Thirring effect. Picture a dense bowl of spaghetti being stirred by a fork. S301 is effectively swimming straight through those twisted gravitational threads, causing detectable orbital shifts over time.
As Stefan Gillessen, a co-investigator at the Max Planck Institute for Extraterrestrial Physics, explains, “The proximity means that the star will really feel the spin of the black hole. For the first time, we will really be able to directly measure the spin of a massive black hole, which would be a key test of Einstein’s theory.”
A Cosmic Laboratory for the No-Hair Theorem
Researchers view the Galactic Center as a remote laboratory where stars act as high-precision measuring probes. Felix Mang notes that having such a probe to measure spacetime curvature around a massive black hole “is simply great.” By observing S301 through at least a couple of full orbital cycles, astrophysicists aim to test the black hole “no-hair” theorem, which states that a black hole’s gravitational field can be predicted using solely its mass and its spin.
Juan Osorno, an astronomer at the Paris Observatory, highlights the efficiency of this new celestial tool: “Without this star, we would need to measure the movement of other stars for several decades more to even get close to measuring the spin of the black hole.”
The Hills Mechanism and the Path to 2031
The presence of S301 so close to Sagittarius A* poses a distinct evolutionary puzzle. No star could possibly form intact in such a hostile neighborhood without being shredded by tidal forces. Consequently, S301 had to arrive there after its birth.
Astrophysicists attribute its current trajectory to the Hills Mechanism. S301 likely traveled as part of a binary star system that drifted too close to the supermassive black hole. Sagittarius A* disrupted the pair, capturing S301 into its current orbit while ejecting its sibling toward the depths of the galaxy at high velocity.
Astronomers are now setting their sights on the future. S301 is scheduled to return to its pericenter in late 2031. By that time, instruments like the European Southern Observatory’s Extremely Large Telescope (ELT) are expected to be operating at full capacity, finally unlocking the secrets that the dark lord of the galactic center has been hiding for millions of years.
