Full Breakdown
Fastest Known Star S301 Circles Milky Way’s Central Black Hole
8/19/2026, 8:40:25 PM
Core Discovery
On August 19, astronomers announced the detection of S301, the fastest-moving star ever recorded in the Milky Way. Orbiting the supermassive black hole Sagittarius A* (Sgr A*), the star reaches speeds of roughly 25,000 km s?¹—about 8 % of the speed of light. Its highly elliptical orbit takes 8.7 years, bringing it within ? 11.5 AU of Sgr A* (comparable to Saturn’s orbit) before swinging out to ? 1,360 AU at apastron.
Background & Context
Sagittarius A* lies at the Galactic Center, about 26,000 light-years from Earth, and contains ~4 million solar masses. S301’s proximity makes it uniquely sensitive to the black hole’s gravitational field and spin, a long-sought parameter for testing Einstein’s general relativity.
Data & Statistics
| Property | Value |
|---|---|
| Designation | S301 |
| Mass | ? 1.5 × Solar mass |
| Luminosity | ? 5 × Solar luminosity |
| Peak speed | ? 25,000 km s?¹ |
| Orbital period | 8.7 years |
| Closest approach | ? 11.5 AU |
| Farthest distance | ? 1,360 AU |
| Instrumentation | ESO’s VLTI with the GRAVITY+ instrument |
Official Statements & Responses
Stefan Gillessen, senior scientist at the Max Planck Institute for Extraterrestrial Physics (MPE), noted that S301’s orbit will not decay soon, allowing prolonged observation of the black hole’s spin effects. He added that the star’s motion directly feels the spin of Sgr A*, offering a rare chance to infer the rotation from stellar dynamics. Felix Mang, a doctoral student at MPE and co-author, said the tight orbit opens a new window onto the black hole’s fundamental properties and likely resulted from a binary disruption by a three-body interaction with Sgr A*.
Verbatim Quotes
- “One star shoots out from the center of the Milky Way traveling at high speed and even leaving the galaxy. The other star is stuck forever on an orbit close to the black hole. That is S301,” — Stefan Gillessen
- “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,” — Stefan Gillessen
Why It Matters
Because S301 passes so close to Sgr A*, its trajectory should be subtly altered by the black hole’s spin. Detecting these minute orbital shifts over the next decade could provide the first direct measurement of a supermassive black hole’s rotation, a critical test of relativistic predictions and a boost to models of black-hole-driven galaxy evolution.
What’s Next
The team will continue monitoring S301 through upcoming close passages, aiming to capture cumulative orbital changes that betray the black hole’s spin. Determining Sgr A*’s rotation would mark a milestone in astrophysics and refine estimates of how black holes grow and interact with surrounding matter.
