Milky Way's fastest star orbits our supermassive black hole so closely it feels its spin

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 and. The institutional report frames the development in practical terms and ties it to the broader mission or observing effort.

It is relevant because astrophysics becomes persuasive only when an observed signal can be tied to a physically defensible explanation. Compact objects such as neutron stars and black holes are natural laboratories for extreme physics, but the distance and complexity of these systems make interpretation difficult without multi-wavelength coverage and careful modeling. A detection without a mechanism is only half a result. the other half comes from showing that the signal fits quantitatively inside a coherent physical picture rather than merely being consistent with a broad family of models. 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 km/s as it travels around. 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.

That is unprecedented,” says Felix Mang, PhD student at MPE and author of the study published today in Nature. 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. 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.

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. 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.

The broader interest lies in turning an observational clue into something that can be weighed against competing models of the underlying physics. Astrophysics does not have the luxury of controlled experiments; everything is inferred from radiation that traveled across cosmic distances under conditions that cannot be reproduced in a terrestrial laboratory. This makes the interpretation chain longer and more uncertain than in bench science, but it also means that a well-constrained measurement of an extreme object carries theoretical information that no earthbound experiment can provide.

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. At its closest approach, the star passes just 1.78 billion km from the black hole, around 12 times the Sun-Earth distance or just 20% larger than the Sun-Saturn distance.

Because the account originates with ESO Press Releases, it functions best as a primary institutional report that is close to the data and operations, not as independent scientific validation. Institutional communications are produced by organizations with legitimate interests in presenting their work in a favorable light, which does not make them unreliable but does make them partial. Details that complicate the narrative, including instrument limitations, unexpected failures and results below projections, tend to be minimized relative to progress messages. Technical documentation and peer-reviewed publications, where they exist, provide the complementary layer that institutional releases cannot substitute.

The next step is to see whether independent datasets and physical modeling converge on the same interpretation. Multi-wavelength follow-up, combining X-ray, radio and optical data where possible, is typically what separates a compelling detection from a robust physical characterization. In high-energy astrophysics, results that initially looked definitive have been revised when data from a second messenger arrived; the current result should be read with that history in mind.

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