Astronomy & Celestial Mechanics
the sky, computed
Concept

Wide binary — where it appears

A pair of stars bound to each other at separations of thousands of astronomical units or more, orbiting in hundreds of thousands to millions of years. Their weak binding makes them sensitive probes of passing masses and of the Galaxy's tide.

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

How much a dying star stirs its companion's orbit, against their separation. The rms change in orbital eccentricity of a 0.8 M☉ companion when a 2 M☉ star becomes a 0.6 M☉ white dwarf, against the binary's separation before the loss, both logarithmic. Blue: the mass loss alone — twelve thermal-pulse episodes 500 years wide and a superwind 10,000 years wide, each adding a kick of random sign whose size falls off exponentially once it lasts longer than about a sixth of an orbit. Below 100 au the orbit is slow to every episode and the eccentricity is untouched. The pulses switch on near 229 au, and by 3,000 au the change is 0.26 and the superwind too has become sudden. Dashed: the change a velocity kick to the white dwarf of 0.05 or 0.2 km/s would make on its own, which grows as the square root of the separation. A 0.05 km/s kick outweighs the pulses out to 251 au; a 0.2 km/s kick outweighs the pulses at every separation drawn.

A kick that writes over the record of the pulses

A star that becomes a white dwarf loses half its mass in pulses, and a companion far enough away feels each pulse as a sudden jolt to its orbit's shape. The wide binaries of white dwarfs should therefore carry the pulses' record in their eccentricities. They would, except that the white dwarf itself is born moving at a few tenths of a kilometre a second, and a kick that small writes more eccentricity than all the pulses together.

orbits · Effective potential How long a wide pair survives a halo of heavy objects. The time for a binary of one solar mass to be dissolved by passing compact objects, against its separation, both logarithmic, if the whole local dark-matter halo — 0.01 M☉ per cubic parsec, moving past at 300 km/s — is made of objects of 10, 100, 1000 M☉. A pair 0.1 pc apart (about 20,000 AU) lasts 477, 48 and 4.8 billion years respectively; at 1 pc, 47.7, 4.8 and 0.48. The heavier the objects at fixed total density, the fewer and harder the kicks, and the sooner the widest pairs go. The dashed line is 10 billion years, the age of the halo's oldest binaries.

Pairs of stars too loosely bound to survive heavy dark matter

Two stars a tenth of a parsec apart are held together by a force so weak that a passing mass of a few hundred suns, at a distance of a parsec, can loosen them. If the dark matter round the Milky Way were made of heavy compact objects, such pairs would have been unbound long ago. Some have survived ten billion years in the halo. The widest of them sets a ceiling on how heavy the dark matter's pieces can be — and the ceiling has moved twice, both times because of one pair.

gravitation · Impulse approximation

Named alongside it

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

Adiabatic invariantAsymptotic giant branchBinding energyDark matterDiffusionEccentricityImpulse approximationImpulsive approximationMachoMass lossNatal kickPrimordial black hole

All concepts