Astronomers have discovered the fastest known star in our galaxy, the Milky Way, orbiting the black hole at its centre. The star, named S301, was detected with the European Southern Observatory’s Very Large Telescope Interferometer (ESO’s VLTI) and reaches speeds of 25,000 kilometres per second as it travels around the four-million-solar-mass black hole. It comes closer to it than any other observed before — so close that it feels the effects of the black hole’s rotation.
Reinhard Genzel, Nobel Prize winner, Director at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, and founding member of the collaboration that made the new observations, stated:
“Decades of carefully tracking stars orbiting our galaxy’s central black hole, Sagittarius A, have led to this breakthrough discovery of a very promising star. Because it orbits so close to Sagittarius A, S301 opens a new window to the fundamental properties of spacetime in this extreme black-hole environment.”
Felix Mang, PhD student at MPE and author of the study published in Nature, added:
“What is special about this star is that it’s orbiting Sagittarius A* on a very tight orbit, taking just 8.7 years to complete it, and is approaching the black hole at a mere 12 times the distance of Earth to the Sun. That is unprecedented.”
During its closest passage to the black hole, the star travels at around 25,000 kilometres per second — 100,000 times faster than a commercial plane, or over 8 percent of the speed of light — making it the record holder for the fastest star in the Milky Way. S301 also comes closer to Sagittarius A* than any other star observed so far, approaching the black hole at around the distance of Saturn to the Sun. Because S301 comes so close to Sagittarius A*, it is the first star known that could be used to directly measure the rotation of a black hole.
Like most things in our Universe, astronomers predict that Sagittarius A* spins. According to Einstein’s general theory of relativity, a spinning black hole drags spacetime along with it and twists it, which impacts the orbits of surrounding stars. The effect is felt more strongly for objects orbiting fast-rotating black holes at close range.
Felix Mang stated:
“With this star, we hope to measure, within the next 10 years, the spin of the black hole.”
Stefan Gillessen, MPE researcher who also had a leading role in the new study, added:
“For the first time, we would actually be able to measure very directly 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, commented:
“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 Betelgeuse in the sky, was no easy feat. The team used the VLTI, a facility at ESO’s Paranal Observatory in Chile, and its GRAVITY instrument, now known as GRAVITY+ following 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.
Frank Eisenhauer, co-author, GRAVITY+ Principal Investigator and Director at MPE, stated:
“Worldwide, Paranal is the only place where you can do this type of observation because no other observatory in the world has four 8-metre telescopes that can act together as an interferometer.”
With GRAVITY, and later with GRAVITY+, the team managed to catch a first glimpse of the new star in spring 2023 and have followed it since to constrain its orbit. They could also trace S301’s orbital history back to 2017, finding that it last made its closest approach to the central black hole in early 2023. S301’s orbital properties, and the fact that stars cannot form so close to a massive black hole, indicate that the star was likely part of a binary pair that was torn apart by the tidal forces of Sagittarius A*.
Follow-up observations with GRAVITY+, and with the MICADO instrument on ESO’s upcoming Extremely Large Telescope (ELT), will be crucial for tracing S301’s path over the next decade, as it makes its next closest passage in 2031. Observing at least two complete orbits of S301 allows its trajectory to be constrained with high enough precision to enable the team to directly determine the spin of Sagittarius A* for the first time.
Felix Mang concluded:
“That would be a dream come true.”
