The solid, icy core of a comet, typically a few kilometers across, composed of frozen gases, rock, and dust. When heated by the Sun, the nucleus sublimates and creates the comet's visible coma and tail.
From Latin 'cometa' meaning 'long-haired star' and 'nucleus' meaning 'kernel' or 'core'. The term evolved as astronomers realized comets had solid centers rather than being purely gaseous phenomena, with nucleus adopted in the 19th century to describe the central body.
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