An Etymological Dictionary of Astronomy and Astrophysics

1. The time in which a → white dwarf cools to half its temperature. It depends on the composition, the mass, and the actual luminosity at some point in time. Cooling time is given by the relation:

t = 8.8 × 10⁶ (12/A) (M)^5/7 (μ/2)^-2/7 (L)^-5/7 in years, where M and L are mass and luminosity in solar units, A the mean → atomic mass, and μ the → mean molecular weight (Iben & Tutukov, 1984, ApJ 282, 615). See also → Mestel theory; → white dwarf crystallization.

2. The time needed by a → plasma to radiate its thermal energy. The cooling time is directly proportional to the square root of the temperature and inversely proportional to the density. It turns out that for the → intercluster medium in a → cluster of galaxies this time is longer than the → age of the Universe. At the centers of some clusters, however, the cooling time is smaller than the age of the Universe due to the presence of a → cooling flow.

See also: → cooling; → time.