nebula General term used for any "fuzzy" patch on the sky, either light or dark; a cloud of interstellar gas and dust.
neutrino An electrically neutral elementary particle that is one of the products of nuclear fusion reactions. Neutrinos have little or no mass, move at close to the speed of light, and interact very weakly with matter.
neutrino oscillations Possible solution to the solar neutrino problem, in which the neutrino has a very tiny mass. In this case, the correct number of neutrinos can be produced in the solar core, but on their way to Earth, some can "oscillate," or become transformed into other particles, and thus go undetected.
neutron An elementary particle with roughly the same mass as a proton, but which is electrically neutral. Along with protons, neutrons form the nuclei of atoms.
neutron capture The primary mechanism by which very massive nuclei are formed in the violent aftermath of a supernova. Instead of fusion of like nuclei, heavy elements are created by the addition of more and more neutrons to existing nuclei.
neutron star A dense stellar remnant produced by the collapse of the core of a massive star as part of a supernova that destroys the rest of the star. Typically neutron stars are composed almost entirely of neutrons, are about 20km in diameter, are about 1.5 times as massive as the Sun. [More Info: Field Guide]
Newtonian mechanics The basic laws of motion, postulated by Newton, which are sufficient to explain and quantify virtually all of the complex dynamical system behavior found on Earth and elsewhere in the universe.
non-thermal radiation Radiation released by virtue of a fast-moving charged particle (such as an electron) interacting with a magnetic force field or other particles; this process has nothing to do with heat.
north celestial pole Point on the celestial sphere directly above the Earth's north pole.
nova A star that suddenly increases in brightness, often by a factor of as much as 10,000, then slowly fades back to its original luminosity. A nova is the result of an explosion on the surface of a white dwarf star, caused by matter falling onto its surface from the atmosphere of a binary companion. [More Info: Field Guide]
nuclear force The force that binds protons and neutrons within atomic nuclei, and which is effective only at distances less than about 10-13 centimeter.
nuclear fusion A nuclear process that releases energy when lightweight nuclei combine to form heavy-weight nuclei.
nucleosynthesis The building up of heavy elements from lighter ones by nuclear fusion.
nucleus Dense, central region of an atom, containing both protons and neutrons, and orbited by one or more electrons.