A star cataloged as S301 is now the fastest object ever clocked orbiting the Milky Way’s central black hole, reaching speeds of 25,000 kilometers per second as it swings through its closest approach to Sagittarius A*. The European Southern Observatory announced the discovery on 19 August 2026, made with the Very Large Telescope Interferometer’s GRAVITY+ instrument, an upgraded system built by a consortium led by the Max Planck Institute for Extraterrestrial Physics.
What makes S301 remarkable is not just its speed but how close it gets to the four-million-solar-mass black hole at the galaxy’s center. On its highly elliptical, roughly 8.7-year orbit, the star’s closest approach brings it to just over twelve astronomical units from Sagittarius A*, about the distance separating the sun from Saturn, a proximity no previously observed star has matched.
A Firefly Beside a Floodlight
Finding S301 at all took four decades of accumulated observing history at the galactic center. The star is roughly two billion times fainter than Betelgeuse, one of the brightest stars in Orion, and the much brighter stars orbiting closer to Earth’s line of sight outshine it by a factor of around 100,000. Researchers at the Max Planck Institute for Extraterrestrial Physics had to develop a new data-analysis method just to pull the faint signal out from underneath its brighter neighbors.
The detection leaned on GRAVITY+, an upgrade to the original GRAVITY instrument that combines light from four 8-meter telescopes at the European Southern Observatory’s Paranal site in Chile into a single, far sharper view, an angular resolution roughly 40 times better than any one of those telescopes could achieve alone. A newly completed laser guide star system, four beams fired into the night sky above Paranal to correct for atmospheric blur, boosted the instrument’s sensitivity to faint targets like S301 by a factor of ten to a hundred, according to the European Southern Observatory’s announcement of the discovery.
Close Enough to Feel Spacetime Being Dragged
S301’s orbit does not trace a clean ellipse the way Newtonian gravity would predict, and that distortion is the whole reason astronomers care about this particular star. Felix Mang, a doctoral student at the Max Planck Institute for Extraterrestrial Physics and corresponding author of the study describing the discovery, said S301 is the first star known to orbit directly inside the region around Sagittarius A* where the black hole’s rotation drags the surrounding spacetime with it, a phenomenon called the Lense-Thirring effect. Mang described the measurement as going beyond simple spacetime curvature, capturing instead the distortion caused specifically by the black hole’s own spin.
A second, larger relativistic effect called Schwarzschild precession, a gradual twisting of the orbit’s closest-approach point, had already been measured in a different, more distant star called S2. S301 probes conditions roughly ten times closer to the black hole than S2 ever reaches, according to Reinhard Genzel, director at the Max Planck Institute for Extraterrestrial Physics and the 2020 Nobel physics laureate for proving Sagittarius A* is a supermassive black hole. Genzel said the team expects to measure the black hole’s rotation directly within the next decade, calling it a milestone for general relativity.
What a Single Star Can Prove That an Image Cannot
Unlike the blurred image of hot gas swirling around a black hole that the Event Horizon Telescope produced for the galaxy M87, tracking S301’s motion lets researchers watch a discrete object move rather than infer conditions from diffuse light. Stefan Gillessen, a senior scientist at the Max Planck Institute for Extraterrestrial Physics and a corresponding author on the study, said the long-term goal is testing the Kerr metric, the mathematical description of spacetime around a spinning black hole, and that S301’s orbit could eventually offer evidence bearing on the no-hair theorem, the idea that a black hole is fully described by just three properties: mass, spin and electric charge.
Frank Eisenhauer, a Max Planck Institute for Extraterrestrial Physics director and principal investigator of the GRAVITY and GRAVITY+ projects, pointed to the upcoming MICADO instrument on the European Southern Observatory’s Extremely Large Telescope, still under development by a consortium the institute leads, as the next tool that will pair with GRAVITY+ to reconstruct S301’s full three-dimensional motion and pin down the black hole’s spin directly.
A Discovery Four Years in the Making
S301 was first picked out of the data in spring 2023, but confirming it as a genuinely new, extraordinarily faint star near Sagittarius A* took years of follow-up observation before the team felt confident enough to publish. The Max Planck Institute for Extraterrestrial Physics’s own account of the discovery describes measurements taken at a resolution of just a few milliarcseconds, roughly equivalent to picking out the size of a car sitting on the surface of the moon.
That level of precision is what let the team distinguish S301’s position from the black hole’s own light and from the much brighter stars crowded around it in every image, a reconstruction the European Southern Observatory has since turned into a time-lapse visualization of S301’s orbit around Sagittarius A*. Researchers now plan to keep tracking the star’s path over the coming years, watching for the small, cumulative deviations that would confirm Sagittarius A* is spinning and, eventually, how fast.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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