- The central, positively-charged, part of an atom, containing almost all of its mass. The nucleus’s positive charge arises because it consists of positive-charged protons and neutral neutrons, and is balanced by the negative charge of orbiting electrons.
- The small solid body at the centre of the head of a comet. Comet nuclei in the inner solar system are hidden from view by glowing gas and dust. The first detailed view of one was obtained by sending the Giotto spacecraft into Halley’s Comet on 13 March 1986.
- The positively-charged dense central core of an atom. All atomic nuclei, except protium, which is hydrogen-1, consist of protons and neutrons. The nucleus of hydrogen-1 consists of a single proton. In an uncharged atom, the number of electrons is the same as the number of protons. When two or more atoms have the same atomic number, but different atomic masses, they are isotopes of the same element. A nucleus is held together by the nuclear force, and comprises almost all of the mass of an atom. Also called atomic nucleus.
- The central or key part of something.
- The part of a control program, such as an operating system, which is memory resident.
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