| 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. |