nucleus - (n.)

(a.) Of an atom, the core of an atom, which has a positive charge, contains most of the mass, and takes up only a small part of the volume; (b) of a comet, the chunks of matter, taking up a volume no more than a few kilometers across, at the center of the head of a comet; (c) of a galaxy, the innermost regions of a spiral galaxy; it does not show spiral structure and is visible from the sky as a bulge in the otherwise flat disk of the galaxy.