New star orbiting close to Milky Way’s central black hole could let astronomers probe its spin

Astronomers say they have found a star circling close enough to the Milky Way’s central black hole that its path should be directly affected by the black hole’s spin, opening a possible new way to test one of the more subtle predictions of Einstein’s theory of gravity.

In a paper posted to arXiv on July 14 and described there as accepted for publication in Nature, researchers report the discovery of the star, called S301, near Sagittarius A, the roughly 4.3 million-solar-mass black hole at the center of the galaxy. In the paper, first-listed author K. Abd El Dayem and a large collaboration including members of the GRAVITY and Galactic Center teams write that “S301’s motion is directly sensitive to the spin of Sgr A.”

That is the scientific significance of the find. Stars orbiting Sagittarius A* have already been used to confirm lower-order relativistic effects, including gravitational redshift, the transverse Doppler effect and Schwarzschild precession. But spin-related effects from a rotating black hole — described by the Kerr metric and often discussed in terms of frame-dragging — are smaller and harder to detect. If S301 can be tracked precisely enough, its orbit could provide an independent dynamical probe of the black hole’s spin, distinct from horizon-scale imaging methods.

The paper does not report a spin measurement. Instead, it identifies a star whose orbit appears unusually well suited to making one possible.

According to the authors, S301 is a faint main-sequence star with a K-band magnitude of 19.3. They report that it completes an orbit in about 8.7 years and follows a highly elongated path with an eccentricity of about 0.98. At its closest approach, the star comes within roughly 140 to 149 Schwarzschild radii of Sagittarius A* and reaches about 25,000 kilometers per second, or around 8% of the speed of light.

Those are extreme conditions by the standards of known stars near the galactic center, and they are what make the orbit so valuable. The closer and faster the star gets, the more strongly relativity should shape its motion, including effects tied specifically to black-hole spin.

The team found S301 using near-infrared interferometry with VLTI/GRAVITY, an instrument that combines light from multiple telescopes to sharpen measurements of objects in the crowded region around Sagittarius A*. The paper says the star was first identified in spring 2023 in GRAVITY images, then followed with additional observations in 2024 and 2025. The researchers also report likely detections in archival data from 2017 and 2021, giving them about 19 astrometric measurements for their orbital fit.

The author list includes several leading figures in galactic center astronomy, among them R. Abuter, S. Gillessen, R. Genzel and F. Eisenhauer. Their paper says S301’s motion is within reach of current near-infrared interferometry and future spectroscopy on an extremely large telescope, a combination that could constrain the spin of Sagittarius A* through stellar dynamics.

The researchers also suggest the star’s extreme orbit may be consistent with the Hills mechanism, in which a binary star wanders too close to a black hole, is torn apart, and leaves one star tightly bound while the other is flung away.

For now, the result is a new target rather than a final measurement. But if follow-up observations deliver the needed precision, S301 could turn the nearest massive black hole into an even sharper test of how rotating black holes behave.