S301 Is the Fastest Star in the Milky Way, and It Can Feel Sagittarius A* Spin

Twenty-five thousand kilometres per second. That is how fast S301 is moving, which works out at roughly 8 percent of the speed of light, and it makes S301 the fastest known star in the Milky Way.

Astronomers using the Very Large Telescope Interferometer at the European Southern Observatory found it orbiting Sagittarius A*, the four-million-solar-mass black hole at the centre of our galaxy. One lap takes 8.7 years.

What makes S301 the fastest star in the Milky Way

Speed at the galactic centre is a function of how close you get. At its nearest point, S301 passes about as far from Sagittarius A* as Saturn sits from the Sun. In a solar system that would be a leisurely outer orbit. Around four million solar masses, it is a slingshot.

Resolving a star that close required interferometry rather than a single dish, which is why this took the VLTI rather than a conventional telescope run. The team has been tracking stars in that inner region for years, and S301 is the current record holder rather than a one-off detection.

Why the orbit matters more than the record

We already know the mass of our black hole. We have seen its shadow, and instruments like Webb keep filling in the environment around it. Spin is the last major property still unmeasured.

S301 is close enough to feel frame dragging, the effect where a rotating black hole drags spacetime around with it. A star inside that region does not trace a clean, repeating ellipse. The orbit precesses, and the precise way it wobbles encodes how fast the black hole is turning.

Track two complete orbits and the trajectory is pinned down tightly enough to extract the spin. At 8.7 years a lap, that is roughly a decade of patient observation. The team expects to get there.

What the answer would tell us

Spin is a growth record. A fast-spinning black hole implies steady accretion from a disc, material arriving from broadly the same direction over a long stretch of time, each parcel adding angular momentum in the same sense. A slow spin implies chaotic feeding, matter falling in from all angles and cancelling itself out.

That distinction reaches well past our own galaxy. Models of how supermassive black holes assembled, including the dramatic stellar disruptions we now catch in real time, depend on assumptions about spin that nobody has been able to check locally. Measuring it once, properly, on the one black hole we can watch star by star, calibrates everything else.

For a broader sense of how much of the galaxy’s mass budget is still guesswork, our piece on dark matter possibly coming in two particle types covers the other end of the same problem.

Seeing the galactic centre yourself

To be clear about what is possible from a back garden: S301 is not observable with amateur equipment, and neither is Sagittarius A*. The galactic centre itself is another matter. On a dark summer night the core of the Milky Way through Sagittarius is one of the better things a pair of wide-field binoculars will show you, and a set like the Celestron Cometron 7x50 is available on Amazon for the price of a takeaway or two. You will be looking in the right direction, at least.

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