Fastest Star Ever Recorded Discovered Orbiting Milky Way's Central Black Hole
The discovery of S301 provides a rare opportunity to measure the spin of Sagittarius A* and test general relativity.
Astronomers have discovered a star, designated S301, orbiting the supermassive black hole Sagittarius A* at the center of the Milky Way. The find provides an extreme laboratory for testing the laws of physics in the strongest gravitational fields known to science.
Led by Stefan Gillessen, the research team identified S301 using the Very Large Telescope (VLT/VLTI) in Chile. First spotted in the spring of 2023, the star has since been monitored as it hurtles through the galactic center. S301 is now the closest known star to the black hole, orbiting approximately 10 times closer than the previous record-holder, S2. At its peak, the star reaches speeds of approximately 8% to 8.5% of the speed of light—roughly 25,000 kilometers per second—making it the fastest star ever spotted in this region. Its closest approach to the black hole is a distance similar to that between Saturn and the Sun.
Probing the Galactic Center
The center of the Milky Way is dominated by Sagittarius A*, a gravitational behemoth long studied via the "S-stars"—a group of stars whose orbits proved the black hole's existence and helped calculate its mass. However, S301's unprecedented proximity allows scientists to observe phenomena that were previously unreachable. Specifically, the star's orbit enables the study of "frame-dragging," also known as the Lense-Thirring effect. This occurs when a rotating massive object twists the fabric of space-time around it, dragging nearby matter and light along with the rotation.
Why the Discovery Matters
Measuring the spin of Sagittarius A* is one of the final major unknowns regarding the black hole. In physics, a black hole is defined by only three measurable properties: mass, spin, and charge. By tracking S301, researchers can finally determine the spin of the Milky Way's central engine, which helps scientists understand how black holes influence the evolution of their host galaxies.
Stefan Gillessen described the star as a tool for measurement, noting that a star is the "perfect leaf to drop" to see the movement of space-time, comparing the process to dropping a leaf in the wind to see how the air is moving. He added that for an observer on a planet orbiting S301, the black hole would appear as large as the full moon does from Earth at its closest approach.
The Path Forward
As astronomers continue to monitor S301, the data will be used to further refine the measurements of general relativity in extreme environments. While the star's orbit is now confirmed, the precise measurement of the black hole's spin remains the primary objective for the team. Future observations will focus on whether S301's trajectory reveals further anomalies that could challenge current gravitational models.