An Etymological Dictionary of Astronomy and Astrophysics

The solid, centrally located part of a → comet. The nucleus is a mass of dust and frozen gases. When heated by the → Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the → coma, which is later swept into an elongated tail.

Reliable measurements of cometary nuclei indicate sizes from a few km to 10 or 20 km. The nucleus of → Comet Hale-Bopp is one of the largest (perhaps 40 km).

The composition of the nucleus is determined by measuring the composition of the coma (except for 67P/Churyumov-Gerasimenko).
The dominant → volatile is → water, followed by → carbon dioxide (CO₂), → carbon monoxide (CO), → formaldehyde (H₂CO), → methanol (CH₃OH), → methane (CH₄) at a few percent level (with respect to water) and many other molecules at a lower level.

See also: → comet; → nucleus.