S301 — The Star That Lets Astronomers Directly "Weigh" a Black Hole's Spin for the First Time
On August 19, *Nature* published online a paper from 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; one of the corresponding authors is Reinhard Genzel, who shared the 2020 Nobel Prize in Physics with American scientist Andrea Ghez for the discovery of Sagittarius A* (Sgr A*).
The star at the heart of the paper is named S301. It orbits the Milky Way's central supermassive black hole, Sgr A*, at high speed, reaching peak velocities of about 25,000 km/s — more than 8% of the speed of light, or 100,000 times faster than a commercial airliner. One full elliptical orbit takes just 8.7 years. At pericenter (closest approach), it comes within about 12 astronomical units of the black hole, roughly the distance from Saturn to the Sun — making it the closest star to Sgr A* ever observed.
S301 is about two billion times fainter than Sirius. Thanks to joint observations with ESO's Very Large Telescope Interferometer (VLTI) and the GRAVITY+ instrument, its light was first captured in spring 2023; the team then traced its orbital data back to 2017.
Why This Star Was Worth a 10-Year Wait
Sagittarius A* lies about 27,000 light-years from the Solar System and is more than 4 million times the mass of the Sun. Even in its dormant state, its gravity is strong enough to swallow any star that strays too close. For a long time, astronomers could only infer its existence indirectly — by studying the orbits of surrounding stars to deduce the central object's mass. 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 the surrounding spacetime, warping it and affecting the orbits of nearby objects. The problem: this effect only becomes significant when "close distance" and "fast spin" coincide — and stars tracked so far were either too far away or too slow.
S301 satisfies both conditions. Gillessen put it plainly: "S301 is the first known star that can be used to directly measure the spin of a massive black hole."
How a Star's Orbit Reveals a Black Hole's Spin
The principle is not complicated. If Sgr A* does not 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 — on its recent passes through pericenter, the orbit's orientation undergoes a small but measurable rotation.
Felix Mang, PhD student at MPE and first author of the paper: "S301 gives us the opportunity to measure this black hole's spin within the next 10 years."
Genzel added: "Each time S301 passes close to Sagittarius A*, its orbit is slightly altered by the black hole's influence. Observing these orbital changes for a full 10 more years should let us directly determine the black hole's spin. Such a measurement would be unprecedented."
S301 is expected to reach its next closest approach to Sgr A* in 2031. The team will then combine GRAVITY+ observations with data from a next-generation instrument — MICADO on ESO's Extremely Large Telescope (ELT) — to precisely lock down the trajectory across two complete orbital periods.
It Was Originally a "Shredded Partner"
How can a star exist so close to a black hole? Stars cannot form naturally near a black hole. 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 force tore them apart — one star was flung out of the galaxy (possibly fast enough to escape the Milky Way entirely), while the other was captured by the black hole's gravity and trapped on its current elliptical orbit: 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 nearly every test over the past century — gravitational redshift, light bending, gravitational waves, kilonovae from binary neutron star mergers — but the frame-dragging effect from 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 offers something different: Sgr A* is a genuine supermassive black hole, and S301's orbital behavior near pericenter is almost entirely dominated by the black hole's spin — making it the highest-precision "laboratory" currently available.
If the team succeeds in measuring Sgr A*'s spin a decade from now, that number directly tests GR: agreement with general relativity would add another vote of confidence from the universe's most extreme gravitational environment; disagreement would mean 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 continue to accumulate. 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 begin operations around 2030, and once MICADO is online, interferometric precision will take another leap forward.
Broader progress is unfolding on two parallel tracks:
- JWST's "little red dots." 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, powered by a 100,000-solar-mass supermassive black hole 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 S301's measurement of a "mature" black hole's spin.
- **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 next-generation EHT (ngEHT), aiming for resolution capable of discerning frame-dragging. S301's dynamical measurements and ngEHT's imaging will be two independent datasets for testing GR in the 2030s.
Sources: Nature paper DOI 10.1038/s41586-026-10894-w, ESO press release eso2612, MPE press release, Nature news coverage, Xinhua, The Guardian, Space.com.