Astronomy & Celestial Mechanics
the sky, computed
Concept

Nuclear star cluster — where it appears

A dense cluster of millions of stars within a few parsecs of a galaxy's centre, often surrounding a massive black hole. Its density governs how often stars are scattered onto orbits that the black hole tears apart.

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

Ejected before touching, or touching before ejected. The separation at which a hardening pair of 0.7-solar-mass main-sequence stars is typically ejected — where one encounter's recoil exceeds an escape speed 4 times the dispersion, at binding energy about 120 mσ² — against the cluster's velocity dispersion, both axes logarithmic, beside the separation at which the two stars fill their Roche lobes and begin to merge, 0.0082 au (1.8 solar radii). The ejection separation falls as the inverse square of the dispersion, because a hotter cluster needs a harder binary to throw it out. The lines cross at a dispersion of about 18 km/s. In clusters slower than that — open clusters and nearly all globulars — a binary is ejected while its stars are still apart. In faster ones — the most massive globulars and the nuclear star clusters at galaxies' centres — it is driven into contact first, and its two stars become one.

Ejected from a globular, merged in a nucleus

A hard binary in a star cluster hardens until something ends it: an encounter that throws it out, or its own stars touching. Which comes first is decided by one comparison — the separation at which the cluster's escape speed is exceeded against the separation at which the stars meet — and the first shrinks as the square of the cluster's velocity dispersion. In a globular cluster binaries leave while their stars are still apart; in the nucleus of a galaxy they merge first, and the heating that holds the cluster up becomes a factory of stars more massive than they should be.

gravitation · Binary heating An excess the hole cannot afford for long. The mass a 10⁶-solar-mass hole gains from disrupted stars, in units of its own mass, against how long an excess in the disruption rate lasts, for excesses of 30, 100, 300 over the relaxed nucleus's 7.4·10⁻⁵ a year, with half of each Sun-like star accreted. An excess of 100 doubles the hole in 2.7·10⁸ years, an excess of 300 in 9·10⁷. The post-starburst phase that selects the hosts lasts about a billion years, so an excess of a hundred sustained through it would have grown every such hole several times over, and the galaxies that were post-starbursts a few billion years ago would carry holes several times heavier than their stars' motions predict. A binary's spike of 0.1 a year for 3·10⁵ years feeds it only 1.5%.

A flare rate a hole cannot afford for long

Tidal disruption flares are found in galaxies that stopped forming stars within the last billion years far more often than those galaxies' numbers allow — by factors estimated between thirty and two hundred. Each proposed cause can be priced. A dense cluster from the burst would need ten times the hole's mass of new stars inside a parsec; a triaxial nucleus would give twenty times too much; and any excess of a hundred, kept for the length of the phase, would feed a million-solar-mass hole several times its own mass.

galaxies · Tidal disruption

Named alongside it

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

Binary heatingBlack hole growthBlue stragglerEscape velocityGlobular clusterHard binaryHeggies lawLoss conePost-starburstRoche lobeStellar collisionSupermassive black hole

All concepts