New Closest Star to Milky Way’s Black Hole May Reveal Its Spin

Astronomers have discovered a faint star named S301 orbiting Sagittarius A*, the 4.3 million-solar-mass supermassive black hole at the center of the Milky Way, according to research published Aug. 19 in the journal Nature. The star completes a lap every 8.7 years, making it the closest and fastest star ever seen orbiting our galaxy’s central black hole, skimming past it at roughly 55 million mph (90 million km/h)—about 8% the speed of light—on an extremely eccentric orbit that brings it within 12 astronomical units of the object.

Tracking S301 and the Galactic Center Environment

Felix Mang, a doctoral student at the Max Planck Institute for Extraterrestrial Physics in Germany and co-author of the study, told Live Science that the discovery was accomplished using photographs taken by the GRAVITY instrument on the European Southern Observatory’s Very Large Telescope in Chile. The star first appeared in spring 2023 as a faint smudge northwest of the black hole. After establishing a rough orbit, the research team checked archival observations and located the star in data from 2021 and 2017. In total, 19 measured positions spanning eight years trace out a complete, closed ellipse in the sky.

S301 beats the previous record holder, which held a 12-year orbit. Its path is stretched into a needle-thin ellipse that passes about 12 times the distance between Earth and the sun from the black hole, roughly 10 times closer than the bright star S2 ever gets. Because S301 is a standard main-sequence star with a mass of roughly 1.5 solar masses and possesses enough compactness to resist being torn apart by the black hole’s gravitational tides, it survives this intense closeness without harm. Its wildly elongated orbit points toward a violent origin.

Measuring the Spin of Sagittarius A*

For decades, most understanding of Sagittarius A* came from watching stars orbiting the supermassive black hole, which sits approximately 27,000 light-years away. Until now, the primary tool for studying this region was S2, a bright star on a 16-year orbit that astronomers have tracked since 1992 through more than two full laps. Tracking S2 helped reveal two hallmark predictions of Albert Einstein’s theory of general relativity: light from the star losing energy as it climbs out of the gravity well, and the slow rotation of its elliptical orbit known as Schwarzschild precession.

Those relativistic effects depend only on the mass of the black hole, which is nearly 1037 kilograms. However, black holes possess a second fundamental property: rotation. A spinning black hole drags the fabric of space around with it like a spoon stirring honey. Because that dragging effect fades extremely quickly with distance, measuring rotation requires a star that dives much closer than S2 ever reaches. Researchers expect that S301’s tight orbit will allow them to secure measurements of the spin rate of Sagittarius A* within about a decade.

Testing General Relativity and Stellar Binaries

Studying stars near Sagittarius A* offers physicists a unique natural laboratory where gravity is far stronger than anywhere else that can be observed closely. Felix Mang noted that stars orbiting the black hole serve as luminous probes of curved spacetime. Testing these environments also allows physicists to probe alternatives to general relativity, such as theories postulating a fundamental fifth force arising from attempts to unify gravity and quantum physics. So far, according to Mang, only an upper limit has been derived for any deviations from Einstein’s predictions, and tests continue to tighten.

The extreme nature of the galactic center also affects stellar populations. A separate study published in the Astrophysical Journal by researchers at UCLA’s Galactic Center Group and the Keck Observatory analyzed 16 young supermassive S-stars orbiting within a light-month of the black hole. First author Devin Chu reported that all 16 stars were singletons, with no spectroscopic binary companions found. While similar stars in Earth’s solar neighborhood have a binary fraction of around 70%, the upper limit near the Milky Way’s black hole is just 47%, indicating that the intense environment drives nearby stellar binaries to merge or be disrupted.