S301: The Fastest Known Star at the Galactic Center May Let Us Measure Sagittarius A*'s Spin

On August 19, 2026, *Nature* published (DOI 10.1038/s41586-026-10894-w) a paper by the ESO GRAVITY collaboration — led by the Max Planck Institute for Extraterrestrial Physics and Paris Observatory, among others — announcing the discovery of a new star, S301, orbiting Sagittarius A* (Sgr A*), the 4.3-million-solar-mass black hole at the center of the Milky Way. One full orbit takes only 8.7 years, the pericenter lies just ~136–142 Schwarzschild radii from the black hole (~12 AU), and the pericenter velocity reaches 25,000 km/s, more than 8% of the speed of light.

S2 Was the 30-Year "Beacon"; S301 Is the "Accelerator"

S2 (also known as S0-2) has dominated Galactic-center stellar dynamics for the past three decades:

• Tracked since 1992, completing more than two full orbits;
• 16-year orbital period;
• Its observations confirmed two hallmark effects of general relativity: gravitational redshift of starlight climbing out of the black hole's gravity well, and the slow rotation of the orbital ellipse — Schwarzschild precession.

But S2's orbit is not close enough to the black hole to measure the black hole's spin.

Black hole spin is an independent physical quantity:

• A non-rotating Schwarzschild black hole is described by a single parameter, M;
• A rotating Kerr black hole has two parameters, M and J; rotation drags spacetime around with it — the "frame-dragging" effect;
• The strength of this drag falls off with the cube of the distance — the closer you get, the stronger the effect.

To detect frame dragging at the Galactic center, a star more than 10 times closer than S2 is needed. Enter S301.

This Star Wasn't "Seen Optically" — It Was Reconstructed from Large Data Sets

S301 first appeared in spring 2023 as a very faint northwestern spot in new images from the GRAVITY interferometer on ESO's VLT in Chile. The team then dug into the archives:

• First estimated a preliminary 2023 orbit;
• Back-searched older images of the same field;
• Re-identified S301 in 2017 and 2021 archival data;
• Combined 19 positional measurements spanning 8 years to trace a complete, closed ellipse.

It is not a new star that suddenly appeared near the Galactic center — it has always been there, only hidden by brighter stars such as S2 and S0-102 over the past 30 years. In other words, a hidden dynamical beacon uncovered by the combination of improved instrument sensitivity and upgraded data-inversion algorithms.

Key Parameters at a Glance

| Parameter | S301 | Notes | |---|---|---| | Orbital period | 8.7 years | Previously the shortest was 12 years | | Pericenter distance | 12 AU | ≈ 136–142 Schwarzschild radii | | Pericenter velocity | 25,000 km/s | ≈ 8% of light speed | | Eccentricity | Extremely elongated ellipse | Far more extreme than S2 | | Schwarzschild precession | ≈ 2 degrees per orbit | Too small to resolve for S2, clear for S301 | | Stellar type | 1.1–1.5 solar-mass main-sequence star | About 2 billion times fainter than S2 | | Origin | Hills mechanism | Originally a binary, torn apart, half retained | | First detection | Spring 2023, GRAVITY | Published 2026-08-19 in Nature |

The Hills mechanism explains its highly elongated orbit: it was once part of a binary torn apart by the black hole's gravity; the companion was flung out of the Galaxy as a hypervelocity star, while S301 was left behind on an extreme orbit.

Unlocking the "Can't Directly Measure Black Hole Spin" Problem

Why has Sgr A*'s spin been unmeasurable so far?

• S2 is too far away; frame-dragging effects are too weak;
• Prograde and retrograde orbits respond differently to frame dragging, but S2's orbit is still not close enough;
• In-plane precession alone cannot distinguish general relativity from alternative gravity theories.

S301 unlocks these locks:

• Close enough (12 AU ≈ 136–142 Rs) — frame-dragging effects ~1000 times stronger than for S2;
• Highly eccentric orbit — ≈ 2 degrees of Schwarzschild precession per orbit already observed in 8 years of data;
• GRAVITY+ upgrade — 10–100 times the sensitivity of the previous generation, enabling retrieval of a star 2 billion times fainter than Betelgeuse;
• S301 is the first star predicted to be able to distinguish general relativity from fifth-force candidate theories.

Can We Get "Sgr A*'s Spin Number" Within 10 Years?

The joint team predicts that over the next 10 years, S301 observations can pin down Sgr A*'s spin — enough to distinguish between "rapidly spinning" and "nearly non-spinning" black hole models.

What is still needed:

• Measuring S301's line-of-sight (radial) motion — the missing half of the current data;
• Combined observations using GRAVITY (transverse) plus upcoming spectrographs (radial);
• Continued data accumulation so spin-induced orbital precession emerges from the noise.

If the spin is measured, it would mean:

• Sgr A* is a Kerr black hole — consistent with all known stellar-mass and supermassive black holes that rotate;
• The orbital dynamics of millions of stars near the Galactic center gain a second parameter;
• The "fifth-force" candidates of alternative gravity theories face tighter constraints — the Einstein camp gains another decade.

Implications for Nobel-Level Questions

• 2002–2020 Nobel recognition (Penrose, Genzel, Ghez) for black hole evidence rested heavily on S2 and the other S-stars; S301 further strengthens that chain of evidence.

S301 means:

• The experimental entry point for the next Nobel-level question is on the table: "the spin of the Milky Way's central black hole" and "empirical tests of alternative gravity theories";
• A quantitative answer is expected within 10 years — unprecedented precision for a 4.3-million-solar-mass black hole;
• "Invisible does not mean unmeasurable" has been proven again — a star 2 billion times fainter than Betelgeuse now has its full orbit mapped.

What This Actually Changes

This is not just another bright star at the Galactic center. The experimental window for "measuring the Galactic-center black hole" has shifted from "measuring mass" to "measuring spin."

S2's 16-year orbit answered whether a black hole exists.

S301's 8.7-year orbit will answer how it spins.

And "how it spins" directly bears on:

• The growth history of the entire supermassive black hole family,
• The long-term dynamics of stellar populations in galactic-center gravitational potential wells,
• Whether alternative gravity theories can stand in the most extreme observable environment.