S301: the fastest known star

A star that races round our galaxy's black hole every 8.7 years, at over 8% of the speed of light at its closest

Step by step

The heart of the Milky Way

Around it swarm the S-stars, a few dozen stars on tilted, stretched orbits. S2, the brightest, takes 16 years to go round. S301, found in 2023, goes round in under 9 years and comes closer and faster than any other known star. What is measured and what is drawn: the orbits are the measured ones; the lines, the labels and the stars of the sky are drawn.

How S301 was found

At the galaxy's centre a thousandth of an arcsecond is about 8 AU. A single big telescope blurs everything within about 60 thousandths of an arcsecond into one smudge. GRAVITY combines the light of the four telescopes of ESO's Very Large Telescope Interferometer in Chile, and since 2017 the team has watched the hole for about 100 hours a year.

Why it orbits

Newton's picture: throw a ball sideways hard enough and it falls round the Earth instead of into it. Gravity bends the star's path towards the hole all the time. Far out it moves slowly and falls back in; close in it moves fast enough to swing round and climb out again. Nothing takes away its energy, so it goes round the same way again and again.

Feeling the hole spin

Einstein's theory says a spinning mass drags space and time round with it: the Lense–Thirring effect. Round Earth it is tiny; the satellite Gravity Probe B needed years to measure it. Near a black hole it grows quickly as you come closer, and S301 is the first star known to come close enough for its orbit to show it.

How much the orbit turns tells how fast the hole spins. Measuring two full orbits, about ten years of watching, should be enough. The answer bears on how black holes grow: by steady feeding from one side, which spins them up, or by many small meals from all directions, which leave them spinning slowly.

Why it isn't torn apart

So S301 is safe: at its closest the tide is only about a thousandth of its own pull, and it heats the star far too little to matter. A red giant, hundreds of times bigger, would have its outer layers torn away on such an orbit, and a star swung inside its tidal radius would be shredded, as seen in tidal disruption flares in other galaxies.