Stars at the Milky Way's core lost their companions to tidal shear from Sagittarius A*
Why the stars on the innermost orbits around the Milky Way's central black hole travel alone has been an open question. Kinematic surveys and simulations now point to the Hills mechanism: a double star that strays within milliparsec distances of Sagittarius A* comes apart, one member flung outward while the other is trapped on a path stretched far from a circle. The survivors are the fast-moving S-cluster stars mapped around the galactic centre.
Science··Evening
Companionless stars on the innermost orbits
At the centre of the Milky Way sits Sagittarius A*, a supermassive black hole circled by a tight swarm of fast-moving stars known as the S cluster. A census of those orbits finds something the wider galaxy does not show: the stars there carry no stellar companions. Astrophysical simulations, together with kinematic surveys of the same region, attribute that absence to the black hole's tidal shear, the difference in pull across an object close to a very large mass. It acts on any pair straying within milliparsec distances, thousandths of a parsec, of the centre.[1]
A black hole's tide breaks the pair apart
The process carries a name, the Hills mechanism, and it describes what follows when a bound pair of stars drifts too close to a massive black hole. The tide pulls harder on the near star than on the far one, and the two separate. One member is thrown outward at high speed; the survivor falls inward onto a narrow path that is highly eccentric, meaning stretched far from a circle. Around Sagittarius A* this happens systematically to arriving pairs rather than once in a while.[1]
Stretched orbits mark the captured survivors
The evidence comes from tracking orbital eccentricity, a measure of how far a path departs from a circle, among the high-velocity stars of the S cluster. A pair that arrives intact and is torn apart leaves a signature on whichever star stays: an orbit both strongly elongated and very close in, with no partner beside it. Those are the orbits the surveys and the simulations reproduce, and the solitary population of stars observed near Sagittarius A* is made of exactly such survivors.[1]
