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near-Earth object (NEO)  A comet or asteroid whose path intersects the orbit of the Earth. 
nebula  Cloud of interstellar gas or dust. 
neutrino  A fundamental particle that has little or no rest mass and no charge but that does have spin and energy. Neutrinos rarely interact with ordinary matter. 
neutron A subatomic particle with no charge and with mass approximately equal to that of the proton. 
neutron star A star of extremely high density composed almost entirely of neutrons.
nova A star that experiences a sudden outburst of radiant energy, temporarily increasing its luminosity by hundreds to thousands of times.
nuclear Referring to the nucleus of the atom. 
nuclear bulge Central part of our Galaxy.
nuclear transformation The change of one atomic nucleus into another, as in nuclear fusion.
nucleosynthesis The building up of heavy elements from lighter ones by nuclear fusion.
nucleus (of atom) The heavy part of an atom, composed mostly of protons and neutrons, and about which the electrons revolve.
nucleus (of comet) The solid chunk of ice and dust in the head of a comet.
nucleus (of galaxy) Central concentration of matter at the center of a galaxy.
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