Fastest Star Orbits Black Hole in Cosmic Discovery

Astronomers have identified a star, designated S301, orbiting the Milky Way's central black hole at a record-breaking distance and speed.

• 25,000 km per second: S301's orbital speed, or 8% of the speed of light.
• 8.7 years is S301's orbital period around Sagittarius A*.
• 12 times the Earth-Sun distance: S301's closest approach to the black hole.
• Scientists first spotted S301 in 2023 using the Very Large Telescope Interferometer.

Why it matters: S301 orbits deep enough in Sagittarius A*'s gravity well that the black hole's spin physically drags spacetime around it — a phenomenon called frame-dragging.

• Measuring that drag would allow the first direct calculation of a black hole's spin, a key test of Einstein's general theory of relativity.
• Spin is the one property of Sagittarius A* scientists currently cannot measure; mass and charge are already known.

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What’s next: S301's closest approach in 2031 is the primary opportunity to measure Sagittarius A*'s spin directly.

• The research team plans to continue tracking S301 and search for other stars with similar orbits.
• S301 alone cannot confirm the black hole's spin; whether additional stars or data can close that gap remains unknown.
• How S301 formed so close to Sagittarius A* — likely torn from a binary pair — has not been confirmed.
• Whether the spin measurement, once possible, will match general relativity's predictions is still an open question.