Astronomy & Celestial Mechanics
the sky, computed
Concept

Runaway star — where it appears

A star moving through space much faster than its neighbours, typically at tens to hundreds of kilometres a second. Runaways are thrown out either by close encounters in dense young clusters or by the supernova of a binary companion.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

Two bodies at a mass ratio of 2.6 to 1. Both bodies orbit their common centre of mass, on similar ellipses whose sizes are in inverse proportion to the masses — here 2.6 to 1, so the heavier body's path is 2.6 times smaller.

The companion survives, and it is moving

When one star of a close pair explodes, the other is left holding an orbital velocity and no orbit. Whether the pair stays bound turns on a single question — did more than half the total mass leave? — and the stars that answer no are running across the Galaxy at a hundred kilometres a second with nothing visible behind them.

stars · Supernovae Which of three bodies is thrown out. Of 60 integrations of three bodies of mass 1, 2, 3 released from rest in random triangles, 57 broke up within the time integrated, each leaving one body unbound and a binary behind. The escaper was the body of mass 1 in 46 runs (81 per cent), 2 in 6 runs (11 per cent), 3 in 5 runs (9 per cent). The bars beside them are the shares the statistical theory of break-up predicts to leading order, in proportion to the escaper's mass to the power −3: 86, 11, 3 per cent. Every run is deterministic and none of them could be predicted without integrating it, but the ensemble is: the lightest body leaves most often when the masses differ, because the energy of the binary left behind is shared between the escaper and the binary's recoil in inverse proportion to their masses, and the lightest body gets the most speed. Energy is conserved in every run to better than 3·10⁻⁴.

The lightest body leaves

Three stars close together, with no hierarchy between them, cannot stay together. One of them is thrown out, which one is unpredictable in any single case, and across many cases it is almost always the lightest — a statistical law emerging from a deterministic chaos that cannot be integrated far enough to forecast.

gravitation · The three-body problem

Named alongside it

The objects these essays reach for when they reach for this one.

Binary starBinary starsCentre of massChaosDemocratic tripleErgodic hypothesisEscapeEscape velocityHalf-lifeMass lossNatal kickNeutron star

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