A star with an 8.7-year orbit turns black hole spin into an observing schedule

A star that laps the Milky Way's central black hole every 8.7 years and reaches roughly 25,000 kilometres per second at closest approach was reported on Aug. 19 in Nature ^[1][2][4]. The consequence is a timetable rather than a spectacle: the orbit passes close enough to register the black hole dragging spacetime around with it, and the team says the spin of Sagittarius A* should be measurable within about ten years ^[9][12].

Live Science renders that peak speed as about 55 million mph, or 90 million km/h ^[3]. Speed is the headline; distance is the physics. S301 comes within about 12 times the Earth-Sun distance of the black hole, roughly 1.7 billion kilometres, a little beyond Saturn's orbit ^[5][6] and around ten times closer than S2 ever gets ^[7].

S2 has been the workhorse: a 16-year orbit, tracked since 1992 through more than two full laps, which yielded two general-relativistic signatures, the star's light losing energy climbing out of the gravity well and the slow turning of its ellipse known as Schwarzschild precession ^[13]. Both depend only on mass ^[14], quoted as about 4 million solar masses, or 4.3 million in Live Science's account ^[27]. Rotation is the other property, and the frame-dragging it produces falls off steeply with distance, so it needs a star that dives much closer than S2 ^[12]. On S301 the Schwarzschild precession is already conspicuous, swinging the ellipse by roughly 2 degrees per orbit ^[15]. According to the study team, it is the first known star usable for a direct measurement of a black hole's rotation ^[8]. Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics calls that a key test of Einstein's theory ^[10]; co-author Juan Osorno of LIRA, Observatoire de Paris-PSL, says that without S301 the alternative was several more decades of tracking other stars ^[11].

That matters because the existing spin measurements are inferences. Spin can be read from X-rays emitted by gas near some black holes and from gravitational waves in mergers, but both require modelling and assumptions that leave the results contested ^[24]. Christopher Reynolds of the University of Maryland calls them good but indirect measures, and says the star offers a much more direct one ^[25]. Spin is also a growth history: fast spin implies steady accretion, slow spin a more chaotic diet, and spin is thought to shape jets and winds ^[26].

The observing burden is real. S301 appears about 2 billion times fainter than Betelgeuse ^[18], and the detection required the GRAVITY instrument, now GRAVITY+, on ESO's Very Large Telescope Interferometer at Paranal, which combines four 8-metre telescopes for 15 times the resolution of one; principal investigator Frank Eisenhauer says no other observatory can do it ^[17]. The star first appeared in spring 2023 as a faint smudge, and archival data from 2021 and 2017 filled in 19 positions across eight years that close a full ellipse ^[19]. It looks like an ordinary main-sequence star of about 1.5 solar masses, compact enough to survive the tides ^[22]; since stars cannot form that close in, the team proposes it was half of a binary torn apart, with the companion flung out as a hypervelocity star ^[23][31].

S301's last perihelion was in early 2023 ^[20], which puts the next one around late 2031 ^[21] - inside the ten-year window. Watch also the fifth-force constraint: co-author Felix Mang says only an upper limit has been derived so far, and it keeps dropping ^[29].