A Star Will Test the Spin of Our Black Hole
Astronomers have found a star racing close enough to the Milky Way's central black hole to feel it twist spacetime. The next close pass could turn that distortion into a measurement.
The Discovery
Astronomers have discovered a faint star on an extreme 8.7-year orbit around Sagittarius A*, the supermassive black hole at the centre of the Milky Way. The star, named S301, is predicted to whip through its closest approach at roughly 25,000 kilometres per second.
The open-access Nature study reports that the GRAVITY instrument at the Very Large Telescope first picked S301 out in 2023. Researchers then traced it through observations from 2017 to 2025, building an orbit from 19 measured positions.
What Makes It Strange
S301 dives much deeper into the black hole's gravitational field than S2, the famous star already used to test Einstein's predictions. That matters because a rotating mass should drag nearby spacetime around with it, producing an effect known as Lense-Thirring precession.
The distortion is tiny at a distance. S301's sharply elongated orbit carries it close enough that current infrared interferometry, paired with future Extremely Large Telescope spectroscopy, should be able to detect the black hole's spin directly through the star's changing path.
The star is drawing the shape of the abyss around it.
The Implications
Sagittarius A* has a mass about 4.3 million times the Sun's, but its spin remains difficult to pin down. A dynamical measurement would test the Kerr description of a rotating black hole using a star as a natural probe rather than inferring spin from glowing gas.
The paper does not claim the measurement has already been made. It identifies S301 as the first known Galactic-centre star whose orbit is sensitive enough for the test with available and near-future instruments.
What Happens Next
S301 is expected to make its next closest passage around April 2027. Intensive observations before, during and after that sweep should refine its velocity and reveal whether its orbital plane shifts as rotating spacetime predicts.
Its extreme eccentricity also hints at a violent origin. The researchers suggest S301 may be the survivor of a binary system ripped apart by Sagittarius A*, with its former companion thrown away while S301 was captured into this near-plunge orbit.