Astronomy & Celestial Mechanics
the sky, computed
Concept

White dwarf cooling — where it appears

The steady loss of stored heat by a white dwarf, which has no fuel and shines only by cooling. Its luminosity falls in a predictable way with age, so the faintest white dwarfs in a population date it.

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

A 1.1 solar-mass white dwarf held up for 2.8 extra billion years. The time a white dwarf takes to reach the crystallisation luminosity, and the two delays that follow, against mass. The lower band is bare Mestel cooling — thermal energy of the ions leaking out through an envelope whose opacity is Kramers'. On top of it sits the latent heat of crystallisation, 0.85 kT per ion released when the liquid interior freezes into a lattice; and on top of that the gravitational energy of ²²Ne settling through what is left, taken here as 0.6 kT per ion. Neither is fuel: both are energy the star already had, released late and radiated at the low luminosity it has by then, which is why so little of it buys so much time. The delay rises from 1.88 billion years at 0.5 solar masses to 2.81 at 1.1 — 58 per cent of the cooling already done. A white dwarf age computed from the bare law is too young, and it is too young by more the heavier the star is, which is exactly the direction that matters, because the massive white dwarfs are the ones used to date the oldest populations.

A clock that stops while its interior freezes

A white dwarf has nothing left to burn, so its brightness is a record of how long it has been cooling — until the interior crystallises. The latent heat of that phase change, and the settling of a heavy isotope through what is left, hold a massive white dwarf up for nearly three extra billion years.

stars · White dwarf cooling A period that lengthens by a few femtoseconds a second. How late the pulses of a 215-second mode arrive compared with a constant period, in seconds, against years of timing, for period derivatives of 2, 4, 6 × 10⁻¹⁵ seconds per second. The lateness grows as the square of the time: 0.9 s after ten years at 4 × 10⁻¹⁵, 19 s after forty-five. The dots are simulated annual measurements with one second of scatter, as good as a season's timing of a bright pulsator; the curvature emerges only after two decades.

A cooling rate read off a pulsation clock

A pulsating white dwarf rings at a period of a few minutes, steady enough to keep time like a good clock, and the period lengthens as the star cools. The lengthening is a few femtoseconds a second — a second in ten million years — and after forty years of timing it has been measured. It is the one rate of cooling anywhere in astronomy that is read off the star itself rather than inferred from a model, and anything that carried extra energy out of the star's core would have made the clock run fast.

stars · White dwarf cooling

Named alongside it

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

Cooling ageCrystallisationLuminosity functionAsteroseismologyCoulomb couplingDebye coolingLatent heatNeon-22 sedimentationNumber densityPhase transitionWhite dwarf

All concepts