Free Astronomy Magazine November-December 2026

“With this star we hope to measure, within the next 10 years, the spin of the black hole,” says Mang. MPE re- searcher Stefan Gillessen, who also had a leading role in the new study, adds: “For the first time, we would actually be able to measure very di- 44 NOVEMBER-DECEMBER 2026 ASTRO PUBLISHING T his image shows the path of the S301 star around Sagittarius A*, the super- massive black hole at the centre of our galaxy. The diamonds show measure- ments performed with the GRAVITY instrument. The size of Neptune’s orbit is shown as a reference. S301 orbits Sagittarius A* every 8 years. As it does so, its orbit rotates forming a rosette. This effect, known as Schwarzschild precession, had already been measured around Sagittarius A*, and it happens regardless of whether the black hole rotates or not. However, if the black hole rotates there’s a different effect at play. As the black hole spins it drags and twists space-time around it, a phenomenon known as Lense-Thirring effect. The S301 star ventures so close to the black hole that it could be affected by this phenomenon. The twisted space-time around the black hole would slightly change the orbit of S301 at each close passage. After several years, the difference between the pre- dicted orbits for a non-rotating black hole and a rotating one would be small, about the size of Earth’s orbit around the Sun, but large enough that it could be measured with the VLTI and ESO’s Extremely Large Telescope (ELT), currently under construction. [ESO/GRAVITY collaboration/L. Calçada] rectly the spin of a massive black hole, which would be a key test of Einstein’s theory.” Juan Osorno, an astronomer at LIRA Observatoire de Paris–PSL, France, who also had a key role in the study adds: “Without this star, we would need to measure the motion of other stars for several more decades to get anywhere close to measuring the spin of the black hole.” Finding S301, which appears two billion times fainter than Betel- geuse (the orange star in the con- stellation Orion) in the sky, was no easy feat. The team used the VLTI, a facility at ESO’s Paranal Observatory in Chile and its GRAVITY instru- ment, now known as GRAVITY+ fol- lowing an infrastructure upgrade. The VLTI’s superpower lies in its ability to combine the light from four 8-metre telescopes to create a ‘virtual’ telescope with 15 times the spatial resolution of a single 8- metre telescope. “Worldwide, Paranal is the only place where you can do this type of observations because no other ob- servatory in the world has four 8-