S301 orbits Sagittarius A* at 8% light speed and can finally measure its spin

Something is orbiting the Milky Way’s central black hole so fast and so close that the physics of spinning objects — not just massive ones — finally becomes testable. S301, identified by a team at the Max Planck Institute for Extraterrestrial Physics using the European Southern Observatory’s VLTI, passes within 1.78 billion kilometres of Sagittarius A* — a distance comparable to the gap between the Sun and Saturn. It does so at roughly 25,000 km/s, or 8% of the speed of light. No other star tracked in this region of the galaxy has moved this fast or come this close.

What separates S301 from the other fast-moving stars near the galactic centre is not the speed alone — it is the consequence. At that proximity, Einstein’s general relativity predicts that the spinning black hole drags the surrounding spacetime with it, an effect called frame-dragging or the Lense-Thirring effect. Because S301 passes so deep into this region, its orbit should carry measurable imprints of how fast Sagittarius A* is rotating. That has never been achievable with any known star.

“With this star we hope to measure, within the next 10 years, the spin of the black hole,” said Felix Mang, lead author of the study and researcher at the Max Planck Institute for Extraterrestrial Physics. No direct measurement of Sagittarius A*’s rotation currently exists. S301 could produce one.

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A different class of S star

Sagittarius A* is surrounded by a cluster of fast-moving stars called S stars, catalogued over three decades and used to map the black hole’s gravitational influence. The best-known, S2, provided the first direct evidence that Sagittarius A* has a mass of four million solar masses — work that earned Reinhard Genzel and Andrea Ghez the 2020 Nobel Prize in Physics. S2 reaches about 7,650 km/s at closest approach, roughly 2.5% of the speed of light, on a 16-year orbit.

S301 completes a full orbit in 8.7 years — less than half the time — and at closest approach moves three times faster than S2 at its own pericenter. More to the point, it passes approximately 12 astronomical units from Sagittarius A*, around eight times closer than any previously known S star. At that distance, general relativistic effects do not merely exist; they accumulate in the orbit at a rate that the instruments at Paranal can track year by year.

The discovery was made using GRAVITY+, a next-generation upgrade to the original GRAVITY interferometer at ESO’s Paranal Observatory in Chile. GRAVITY+ combines light from four 8.2-metre telescopes to reach the angular resolution needed to resolve individual stars within the dense, dust-obscured stellar cluster around the galactic centre. By tracking S301’s position over multiple observation epochs, the team reconstructed its full orbit with enough precision to establish the pericenter distance and peak velocity.

Why the spin matters

The mass of Sagittarius A* has been known for years. But mass is only one of the two fundamental properties a black hole can have, alongside electric charge — which astrophysical black holes are expected to shed quickly in real environments. The remaining property is spin. According to the Kerr solution of Einstein’s field equations, a rotating black hole carries angular momentum, and that rotation shapes the geometry of spacetime around it in ways that go beyond what the mass alone produces.

How fast Sagittarius A* rotates, and in which direction, governs the structure of its event horizon and the orbits of anything nearby. Current estimates from indirect methods — modelling of the accretion disc emission observed in the 2022 Event Horizon Telescope image — suggest it probably spins at a moderate rate, but the uncertainties remain large enough to cover much of the allowed range. A measurement from S301’s orbital precession would be the first direct determination.

The Lense-Thirring effect causes the plane of a star’s orbit around a spinning body to precess — to slowly rotate — at a rate that scales with the black hole’s spin parameter. For S301, the team calculates that this precession will accumulate to a detectable level over the coming years of observation with GRAVITY+.

What the data does not yet settle

Several important limits apply. S301’s orbit — its speed, period, and pericenter distance — is now well-established. The frame-dragging precession has not yet been detected; it is a future measurement that requires watching the star over several more passes around Sagittarius A*.

The galactic centre is also environmentally noisy by astrophysical standards. The distribution of stars, gas, and dark matter in the region generates Newtonian gravitational perturbations that can shift an orbit in ways that mimic relativistic precession. Extracting the Lense-Thirring signal requires modelling and subtracting these contributions, which adds complexity and uncertainty to the eventual measurement.

Finally, the discovery of S301 is itself a prompt to search more carefully. The inner stellar cluster around Sagittarius A* is heavily obscured by dust, and GRAVITY+ can only probe so far. The possibility remains that additional stars orbit even closer, with orbital signatures that contain stronger or different relativistic imprints.

Common questions about S301 and Sagittarius A*

How did scientists find a star this close to the galactic centre?

The Very Large Telescope Interferometer’s GRAVITY+ instrument combines light from four 8-metre telescopes to achieve the resolution needed to resolve individual stars in the dense cluster around Sagittarius A*. By tracking tiny positional shifts over multiple years, the team reconstructed S301’s full orbit.

What is frame-dragging, and why does it matter?

A rotating mass drags the fabric of spacetime around it — the faster the spin, the stronger the drag. Close to a spinning black hole, this alters the orbital paths of nearby objects in a way that depends on both the magnitude and direction of the rotation. Measuring it from a stellar orbit gives a direct readout of the black hole’s angular momentum — something no other current method has achieved for Sagittarius A*.

How does S301 compare to other S stars near Sgr A*?

S2, the most studied S star, reaches a closest approach of roughly 100 astronomical units on a 16-year cycle. S301 approaches eight times closer, in 8.7 years, at three times the pericenter speed. That combination of proximity and short period makes it uniquely sensitive to frame-dragging.

Will this change what we know about the Milky Way’s black hole?

Sagittarius A*’s spin is currently unconstrained by direct measurement. Determining it would complete the basic physical description of our galaxy’s central black hole and open tests of general relativity in a strong-field, rotating environment that stellar orbits have not previously reached.

Observations with GRAVITY+ will continue across multiple S301 orbital periods. The team expects the frame-dragging precession to reach detectable levels within roughly a decade — a timeline short enough that the same instruments and team that found S301 are likely to make the spin measurement themselves.

Reference: Mang et al., “S301: a new S star orbiting close to Sagittarius A*,” Nature, 2026. DOI: 10.1038/s41586-026-10894-w

Tags: astronomy, black holes, Sagittarius A*, ESO, frame-dragging, GRAVITY+

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