A newly identified star, S301, is the fastest known object in the Milky Way and the closest yet observed to the Galaxy’s central black hole, Sagittarius A*. Astronomers report the star reaches velocities of about 25,000 km/s (roughly 15,500 miles per second) as it sweeps through a highly eccentric orbit that completes in 8.7 years. At closest approach it comes within a distance comparable to about 12 times that between Earth and the Sun, making it an exceptional probe of the black hole’s immediate environment.
Why S301 matters
Objects in very tight orbits around massive black holes are valuable because they sample the strongest gravitational fields accessible to direct observation. The combination of S301’s speed, short orbital period and close pericentre means astronomers can observe relativistic effects — including the twisting of spacetime expected around a spinning black hole — on timescales far shorter than previously possible.
“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,”
The comment comes from Felix Mang, a PhD student at the Max Planck Institute for Extraterrestrial Physics (MPE) and a co-author of the study, which is published in the journal Nature. Stefan Gillessen, a senior scientist at MPE and co-author, noted that the discovery could allow, for the first time, a direct measurement of the black hole’s spin — a key prediction of Einstein’s general theory of relativity. Previously, determining that spin would likely have required tracking multiple stars for many decades.
Observations and provenance
The orbit of S301 has been traced back to observations from 2017, using data including high-resolution imaging from the European Southern Observatory’s Very Large Telescope Interferometer (VLTI). The VLTI sequence shows several stars following paths close to Sagittarius A*, with S301 standing out for both its speed and its exceptionally tight trajectory.
| Parameter | Value (reported) |
|---|---|
| Peak velocity | ~25,000 km/s (~15,500 miles/s) |
| Orbital period | 8.7 years |
| Closest approach | ≈ 12× Earth–Sun distance |
The rapid orbit has practical advantages: because S301 completes its path in under a decade, repeated pericentre passages — where relativistic effects are greatest — will be available on human timescales, enabling refined tests of gravity and black-hole properties in the near future.
What astronomers hope to learn
- Measure the spin of Sagittarius A* directly by tracking how the black hole’s rotation drags spacetime and alters S301’s orbit.
- Study the stellar dynamics and extreme tidal environment close to a massive black hole.
- Improve constraints on general relativity in the strong-field regime, by comparing observed orbital precession and other relativistic signatures with theoretical predictions.
Because the orbit is both highly eccentric and compact, effects such as Lense–Thirring precession — the nodal and periapsis shifts caused by a spinning mass — become observable with fewer years of data than would otherwise be required. The discovery therefore compresses decades of potential observation into a much shorter interval.
Published in Nature, the study was conducted by a team including researchers at the Max Planck Institute for Extraterrestrial Physics. The ability to map S301’s orbit back to 2017 underscores the value of long-term monitoring programmes of the Galactic Centre and the high angular resolution provided by instruments such as the VLTI.
In practical terms, S301 provides astronomers with a new, fast-moving test particle in the deepest part of the Milky Way’s gravitational well. Continued observation of its motion promises to sharpen our empirical picture of black-hole physics and may offer one of the cleanest direct measurements yet of a massive black hole’s spin.
