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

S301 completes one orbit of Sagittarius A* every 8.7 years.

At its closest passage S301 travels at around 25,000 kilometres per second, more than 8 percent of the speed of light, making it the fastest known star in the Milky Way.

Live Science describes S301's perihelion speed as roughly 55 million mph (90 million km/h), about 8 percent of the speed of light.

S301's closest approach is around 10 times closer to the black hole than S2 ever gets.

Because S301 comes so close to Sagittarius A*, it is the first known star that could be used to directly measure the rotation of a black hole.

A spinning black hole drags and twists spacetime around it, affecting the orbits of surrounding stars; the effect fades extremely quickly with distance, so measuring rotation requires a star that dives much closer than S2.

The team used the GRAVITY instrument, now GRAVITY+ after an infrastructure upgrade, on ESO's Very Large Telescope Interferometer at Paranal Observatory in Chile, which combines light from four 8-metre telescopes to create a virtual telescope with 15 times the spatial resolution of a single 8-metre telescope; GRAVITY+ principal investigator Frank Eisenhauer said Paranal is the only place worldwide where this observation can be done.

S301 first showed up in spring 2023 as a faint smudge northwest of the black hole; the team then found it in archival observations from 2021 and 2017, for a total of 19 measured positions spanning eight years that trace a complete, closed ellipse.

Sagittarius A*'s mass is about 4 million times that of the Sun according to Science News; Live Science gives 4.3 million solar masses.

The team proposed that S301 was probably once half of a tight double star that strayed too close and was ripped in two, with its partner flung out of the galaxy as a hypervelocity star.

Stars cannot form so close to a massive black hole, which alongside S301's orbital properties indicates the star was likely part of a binary pair torn apart by tidal forces, with the companion kicked out fast enough to leave the galaxy.

S2 is a bright star on a 16-year orbit that astronomers have followed since 1992 through more than two full laps; the tracking revealed light losing energy climbing out of the black hole's gravity well and the slow rotation of its elliptical orbit, known as Schwarzschild precession.

Those two relativistic effects seen in S2's orbit depend only on the black hole's mass.

S301 appears 2 billion times fainter than Betelgeuse.

S301 last made its closest approach to Sagittarius A* in early 2023.

S301 appears to be an ordinary main-sequence star of about 1.5 solar masses, compact enough that the black hole's tides cannot tear it apart.

Black hole spin can also be measured indirectly from X-rays emitted by gas near some black holes and from gravitational waves created in black hole collisions, but these methods require modelling and assumptions that leave some debate over whether the spin is correct.

Christopher Reynolds, an astronomer at the University of Maryland who was not involved in the study, said the indirect methods are still good measures but are indirect, and that the star gives the opportunity to make a much more direct measurement.

Felix Mang said that so far only an upper limit could be derived on deviations from Einstein's predictions, and that it gets progressively lower; one leading alternative to general relativity postulates a fundamental fifth force arising from unifying gravity and quantum physics.

Juan Osorno, an astronomer at LIRA, Observatoire de Paris-PSL, said that without this star researchers would need to measure the motion of other stars for several more decades to get anywhere close to measuring the black hole's spin.

Schwarzschild precession is very apparent in S301's orbit, with the ellipse swinging around by about 2 degrees every lap.

S301's next perihelion falls around late 2031.

Astrophysicist Laura Brenneman of the Harvard-Smithsonian Center for Astrophysics, who was not involved with the study, said a fast spin suggests the black hole has consumed material in a regular manner while a slower spin indicates more chaotic feeding; spin is also thought to influence a black hole's jets and winds.

Felix Mang, a PhD student at the Max Planck Institute for Extraterrestrial Physics and author of the study, says the team hopes to measure the spin of the black hole within the next 10 years.

MPE researcher Stefan Gillessen said that for the first time researchers would be able to measure very directly the spin of a massive black hole, which would be a key test of Einstein's theory.