An Etymological Dictionary of Astronomy and Astrophysics
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The most important phenomenon occurring during → white dwarf evolution, which results from its cooling. Crystallization is a → phase transition whereby → latent heat is released. At the cooler end of a white dwarf's life (→ cooling time), the → thermal energy of nuclei, which are positively charged ions, becomes small and the effects of electrostatic interaction on the motion of ions become important. The ions repel each other and their distribution will be such that the → Coulomb energy per ion is a minimum. This will cause the ions to form crystal-like lattice structures. As the star cystallizes it releases latent heat, providing an additional energy source that slows the cooling process compared to the → Mestel theory. Once the bulk of the white dwarf is crystalline, heat can travel through the star more easily and the white dwarf cools faster.

→ white; → dwarf; → crystallization.

A type of ultra-cool → brown dwarf with an → effective temperature lower than 500 K. → Near infrared spectra of these objects show deep absorption bands of H₂O and CH₄. So far only seven brown dwarf candidates belonging to this class have been found, all Y0 subtypes. These objects are very dim, with H magnitudes 19-23. See Cushing et al. 2011 (arXiv:1108.4678). The precise definition of the Y class requires new findings in the future about these objects.

For the choice of the letter Y, see Kirkpatrick et al. 1993, ApJ 406, 701; → dwarf.