Dancing on the edge of a black hole: Meet S301

GRAVITY+ at ESO’s Very Large Telescope Interferometer has discovered S301, a star so faint it is about two billion times dimmer than Betelgeuse, yet on an extreme orbit around Sagittarius A*, the supermassive black hole at the centre of the Milky Way. At closest approach it comes within roughly the Sun–Saturn distance of the black hole, completing an orbit in about 8.7 years at nearly 8% of the speed of light.

That path takes S301 into a region where the black hole’s rotation should drag spacetime itself — the Lense–Thirring or frame-dragging effect. Combined with the larger Schwarzschild precession already seen in the star S2, further tracking of S301 may yield the first direct measurement of how fast Sgr A* is spinning. I am a co-author of the Nature paper and a member of the GRAVITY+ consortium.

This is an incredible measurement. We’ve tracked a star orbiting the Milky Way’s central black hole at nearly 8% of the speed of light. Using Einstein’s theory of general relativity, we’ve measured its motion with unprecedented precision, showing that further observations may provide the first direct measurement of how fast the black hole at the center of the galaxy is spinning.

— Denis Defrère, KU Leuven co-author

Read the KU Leuven Institute of Astronomy press release, the Nature paper, and the companion MPE release.