S301 is a record-breaking star; it now provides a unique laboratory for testing general relativity and paves the way for the first measurement of the rotation of Sagittarius A*.
GRAVITY+ pushes the boundaries and reveals a record-breaking star
For many years, a systematic observation programme – in which the Paris Observatory – PSL participates through the Laboratory for Instrumentation and Research in Astrophysics (LIRA) – has aimed to map, with ever-increasing precision, the surroundings of Sagittarius A* (Sgr A*), the supermassive black hole with a mass of 4.3 million solar masses located at the centre of our Galaxy.
LIRA has notably contributed to the design of GRAVITY,, which combines the light from the four giant telescopes of ESO’s Very Large Telescope (VLT) in Chile. The instrument has made it possible to observe, with unprecedented precision, the frenzied dance of stars around Sgr A*.
Among them, S29 previously held the speed record, reaching nearly 8,740 km/s – 3 per cent of the speed of light – as it passed closest to the black hole. By way of comparison, the Earth orbits the Sun at just 29.8 km/s.
As observational techniques advance, astronomers are able to detect stars that are ever fainter and closer to the black hole. GRAVITY is thus evolving into GRAVITY+, a significantly improved version to which LIRA is actively contributing. New adaptive optics systems, laser guide stars and new methods of data analysis are pushing the instrument’s limits even further.
It was thus in the spring of 2023 that a previously invisible star appeared: S301.
By tracking its orbit, scientists discovered that it smashes the previous record held by S29, reaching 25,000 km/s – nearly three times as fast. This discovery directly illustrates how continuous improvements in observational techniques are revealing celestial bodies that were previously beyond our reach.
And S301 is breaking records left, right and centre. It completes one revolution around the black hole in just 8.7 years, on a highly elliptical orbit that brings it, at its closest point, to within just 12 astronomical units of Sgr A* — roughly the distance between the Sun and Saturn. It thus becomes the star with the shortest known orbital period and the tightest orbit around the galactic centre, passing ten times closer to the black hole than the closest-known star prior to this discovery.
This feat is all the more remarkable given that S301 shines two billion times fainter than Betelgeuse, in the midst of a region where light sources are almost indistinguishable from one another. Picking out its signal is like hearing the buzz of a fly amidst a symphony orchestra! Yet it was possible to distinguish it from its neighbours by just a few milliseconds of arc: a level of precision that would, from Earth, allow us to spot an object on the Moon the size of the Apollo lunar module.
“The discovery of S301 is no coincidence. It is the result of a long-term endeavour,” says Thibaut Paumard, co-investigator of the GRAVITY+ project and deputy director of the LIRA at the Paris Observatory - PSL, “with the development of several adaptive optics systems from the 1990s onwards, the GRAVITY instrument, which began operations in 2017 after more than ten years of development, followed by its upgrade, GRAVITY+, in 2024, and soon MICADO, which has also been under development for several years. At the same time, we are also developing the digital tools that enable us to compare observational data with theoretical predictions, such as the Gyoto software for calculating trajectories and gravitational lensing effects.”
S301 paves the way for the first measurement of a black hole’s rotation
To determine the orbit of S301, scientists analysed several years’ worth of observations in order to track its position precisely. Its proximity to the black hole significantly changes the picture: “Without this star, we would have to measure the orbits of the other stars for several more decades in the hope of detecting an effect caused by the rotation of Sagittarius A*”, explains Juan Osorno, a postdoctoral researcher at LIRA at the Paris Observatory – PSL and one of the co-authors of the study published in the journal Nature.
This is because, around a black hole, Newton’s laws no longer suffice. The orbit of S301 gradually deviates from the ellipse predicted by those laws, revealing the effects of general relativity.
- The first, Schwarzschild precession, is caused by the curvature of space-time due to the black hole’s mass: with each orbit, the point at which the star passes closest to Sgr A* shifts slightly, gradually tracing a sort of rosette pattern. This effect has already been observed with the star S2.
- But as it ventures much closer to the black hole, S301 becomes susceptible to a second, far more subtle phenomenon: Lense–Thirring precession, caused this time by the black hole’s rotation — or spin.
◽Read the rest of this article on the LIRA website
Scientific reference
‘Discovery of a star sensitive to the spin of Sagittarius A*’, by K. Abd El Dayem et al., Nature, 19 August 2026.