S301: The Star That Lets Astronomers Directly Measure a Black Hole's Spin
On August 19, *Nature* published online a paper by the GRAVITY+ collaboration, "Discovery of a star sensitive to the spin of Sgr A*" (DOI: 10.1038/s41586-026-10894-w). First author Felix Mang is a PhD student at the Max Planck Institute for Extraterrestrial Physics (MPE) in Germany; core researcher Stefan Gillessen is also at MPE; and one of the corresponding authors is Reinhard Genzel, who shared the 2020 Nobel Prize in Physics for the discovery of Sagittarius A* (Sgr A*) with American scientist Andrea Ghez.
The star at the heart of the paper is named S301. It orbits the Milky Way's central supermassive black hole Sgr A* at blistering speed—peaking around 25,000 km/s, more than 8% of the speed of light and roughly 100,000 times the speed of a commercial airliner. One elliptical orbit takes only 8.7 years. At pericenter, it comes within about 12 astronomical units of the black hole—roughly Saturn's distance from the Sun—making it the closest star to Sgr A* ever observed.
S301 is about two billion times fainter than Sirius. It was first detected in spring 2023 through combined observations with ESO's Very Large Telescope Interferometer (VLTI) and the GRAVITY+ instrument; the team then traced its orbital data back to 2017.
Why astronomers waited 10 years for this star
Sgr A* lies about 27,000 light-years from the Solar System and is more than four million times the mass of the Sun. Even in its dormant state, its gravity is strong enough to swallow any star venturing too close. For a long time, astronomers could only infer its existence indirectly—deducing the central object's mass from the orbits of surrounding stars. That work earned Genzel and Ghez the 2020 Nobel Prize in Physics.
But mass is only the black hole's "first parameter." According to Einstein's general relativity, a spinning black hole drags and distorts the surrounding spacetime, affecting the orbits of nearby objects. The problem: this effect becomes significant only when "close distance" and "fast spin" are satisfied simultaneously, and the stars previously trackable by humans were either too far away or too slow.
S301 satisfies both conditions. Gillessen's verdict is blunt: "S301 is the first known star that can be used to directly measure the spin of a massive black hole."
How to read a black hole's spin from a star's orbit
The principle is actually simple. If Sgr A* has no spin, S301's orbit is a clean ellipse. If Sgr A* spins rapidly, the black hole drags spacetime around with it—this "frame-dragging" causes S301's orbital plane to precess. Over its most recent pericenter passages, the orbit's orientation should rotate by a small but measurable amount.
In the words of Felix Mang, MPE PhD student and first author: "S301 gives us the opportunity to measure this black hole's spin within the next 10 years."
Genzel's comment carries more warmth: "Every time S301 passes close to Sagittarius A*, its orbit changes slightly due to the black hole's influence. Observing these orbital changes for another full 10 years, we should be able to directly determine this black hole's spin. Such a measurement would be unprecedented."
S301 is expected to reach its closest point to Sgr A* again in 2031. The team will then combine GRAVITY+ observations with those from a next-generation instrument—MICADO on ESO's Extremely Large Telescope (ELT)—to precisely pin down trajectories over two full orbital periods.
It was originally a "torn-apart partner"
Why can a star be found so close to the black hole? Stars cannot form naturally in its vicinity. The team's inference: S301 was originally part of a binary. When the pair strayed too close to Sgr A*, the black hole's tidal forces ripped them apart—one star was flung out of the galaxy (possibly fast enough to escape it entirely), while the other was captured by the black hole's gravity and trapped on its current elliptical orbit. That captured star is today's S301.
Juan Osorno (LIRA, Observatoire de Paris–PSL) put it vividly: "Without S301, we would need to observe other stars for decades more before we could even approach the goal of measuring the black hole's spin."
The real stakes: testing general relativity
Einstein's general relativity, proposed in 1915, has passed almost every test over the past century—gravitational redshift, light bending, gravitational waves, kilonovae from binary neutron star mergers—but the frame-dragging effect produced by black hole spin has long lacked sufficiently clean experimental data.
The closest previous attempt was Gravity Probe B in 2004, which measured frame-dragging near Earth with limited precision and some controversy. S301's opportunity is different: Sgr A* is a true supermassive black hole, and S301's orbital behavior at pericenter is almost entirely dominated by the black hole's spin—the highest-precision "laboratory" currently available.
If, ten years from now, the team successfully measures Sgr A*'s spin rate, that number itself directly tests GR: if the measured value matches general relativity's prediction, we gain another vote of confidence for GR from the most extreme gravitational environment in the universe; if it doesn't, either GR needs revision or unknown physics exists near black holes.
What to expect in the next 12 months
In the short term, S301's orbital data will keep accumulating. The MPE team, together with LIRA (Paris Observatory), IPAG, FEUP, CENTRA and other institutions, is conducting routine tracking with GRAVITY+. The ELT is expected to come online around 2030, and once MICADO is operational, interferometric precision will take another step up.
Broader progress is advancing on two parallel tracks:
- JWST's "little red dots" question. On August 12, MIT's Rohan Naidu published research in *Nature* on MoM-BH*-1—a new "black hole star" candidate, 100 billion times brighter than an ordinary star, with a 100,000-solar-mass supermassive black hole at its center wrapped in a dense cloud of hydrogen the size of the Solar System. If such objects existed just 660 million years after the Big Bang, the seed-formation mechanism of supermassive black holes would need rewriting—forming an end-to-end loop with the "mature black hole spin" S301 will measure.
- **Sgr A*'s "image."** The Event Horizon Telescope (EHT) released the first image of the Milky Way's central black hole in 2022 and is now upgrading to the ngEHT, aiming to push image resolution to the point of resolving frame-dragging effects. S301's dynamical measurements and the ngEHT's imaging will be two independent datasets testing GR in the 2030s.
Sources: Nature paper DOI 10.1038/s41586-026-10894-w, ESO press release eso2612, MPE press release, simultaneous *Nature* news coverage, Xinhua, The Guardian, Space.com.