“Decades 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,” says Nobel Prize winner Reinhard Genzel, Director at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, and founding member of the collaboration that made the new observations.
“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,” says Felix Mang, PhD student at MPE and author of the study published today in Nature.
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% 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. [1] 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.
S301 orbits Sagittarius A* every 8.7 years. At its closest approach — similar to the distance between the Sun and Saturn — the star moves at more than 8% the speed of light. The size of Neptune’s orbit is shown as a reference.
S301 passes so close to Sagittarius A* that its orbit could be affected by the rotation of the black hole, which twists space-time around it. The effect is small, but could be potentially measured in the next few years with the VLTI and ESO’s Extremely Large Telescope (ELT), currently under construction.
The orbit of S301 is displayed here with an elliptical curve. In each frame, the solid curve shows the path that the star has already completed up to that point, whereas the dashed one marks the path it will follow afterwards. Credit:ESO/GRAVITY collaboration
“With this star we hope to measure, within the next 10 years, the spin of the black hole,» says Mang. MPE researcher 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 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 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 Betelgeuse (the orange star in the constellation 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 instrument, now known as GRAVITY+ following an infrastructure upgrade. [2] 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 observatory in the world has four 8-metre telescopes that can act together as an interferometer,” says co-author Frank Eisenhauer, GRAVITY+ Principal Investigator and Director at MPE.
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*. In the process, S301 became trapped by the black hole’s gravity while its companion star was kicked out with high velocity, most likely enough to leave the galaxy altogether.
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. “That would be a dream come true,” says Mang.
More information: ESO
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