Astronomers have found a star that passes closer to the black hole at the center of our galaxy than any star known before. The discovery could finally reveal how fast that black hole spins.
The star, called S301, was tracked with the GRAVITY instrument on the Very Large Telescope Interferometer in Chile. A sequence of images shows several stars orbiting Sagittarius A*, the supermassive black hole at the heart of the Milky Way.
A star on an extreme path
S301 does not just orbit the black hole. It skims it. The star's closest approach is tighter than that of any other known star, bringing it deep into the region where gravity bends light and warps time.
That extreme orbit makes S301 a natural probe. As the star swings past the black hole, its motion carries information about the black hole's rotation. The effect is small, but GRAVITY can measure it.
Why spin matters
A black hole's spin is one of its most basic properties. It records how the black hole grew, whether it swallowed stars whole, and how it merged with other black holes over billions of years.
But spin has stayed hidden. Most black holes are too far away and too small on the sky to measure. Sagittarius A* is the best target we have, and S301 finally gives astronomers the tool they needed.
The spin of Sagittarius A* could also tell us about the history of the Milky Way itself. A fast spin suggests the galaxy merged with another galaxy long ago. A slow spin points to a quieter past.
What comes next
The team plans to track S301 through more of its orbit in the coming years. Each pass will add data and tighten the measurement.
Other instruments are joining the hunt. The upcoming Extremely Large Telescope, also in Chile, will give astronomers an even sharper view of the galactic center.
For now, S301 is a reminder that the biggest discoveries often start with a single star doing something no star has done before.