naked singularity: A singularity that is not surrounded by an event horizon and therefore kept from our view.


neap tides: The tides when the gravitational pulls of the Sun and Moon are perpendicular, making them relatively low.


Near-Earth object: A comet or asteroid whose orbit is sufficiently close to Earth’s that a collision is possible or even likely in the long term.  See Amor asteroids, Apollo Asteroids.


nebula: (plural: nebulae) Interstellar regions of dust or gas.


nebula hypothesis: The particular nebular theory for the formation of the Solar System advanced by Laplace.


nebular theories: The theories that the Sun and the planets formed out of a cloud of gas and dust.


neutrino: A spinning, neutral elementary particle with little rest mass, formed in certain radioactive decays.


neutron: A massive, neutral elementary particle, one of the fundamental constituents of an atom.


neutron star: A star that has collapsed to the point where it is supported against gravity by neutron degeneracy.


New General Catalogue: A New General Catalogue of Nebulae and Clusters of Stars by J. L. E. Dreyer, 1888


new moon: The phase when the side of the Moon facing the Earth is the side that is not illuminated by sunlight.


Newtonian: (telescope) A reflecting telescope where the beam from the primary mirror is reflected by a flat secondary mirror to the side.


NGC: New General Catalogue.


non-thermal radiation: Radiation that cannot be characterized by a single number (the temperature).  Normally, we derive this number from Planck’s law, so that radiation that does not follow Planck’s law is called nonthermal.


nova (plural: novae): A star that suddenly increases in brightness; an event in a binary system when matter from the giant component falls on the white dwarf component.


nuclear bulge: The central region of spiral galaxies.


nuclear burning: Nuclear fusion.


nuclear force: The strong force, one of the fundamental forces.


nuclear fusion: The amalgamation of lighter nuclei into heavier ones.


nucleosynthesis: The formation of the elements.


nucleus: (Plural: nuclei) (a) Of an atom, the core, 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, no more than a few km across, at the center of the head; (c) of a galaxy, the innermost region.



O and B association: A group of O and B stars close together.


objective: The principal lens or mirror of an optical system.


oblate: With equatorial greater than polar diameter.


occultation: The hiding of one astronomical body by another.


Occam’s razor: The principle of simplicity, from the medieval philosopher William of Occam (approximately 1285-1349): the simplest explanation for all the facts will be accepted.


Olbers’s paradox: The observation that the sky is dark at night contrasted to a simple argument that shows that the sky should be uniformly bright.


one atmosphere: The air pressure at the Earth’s surface.


one year: The length of time the Earth takes to orbit the Sun.


Oort comet cloud: The trillions of incipient comets surrounding the Solar System in a 50,000 A.U. sphere.


open cluster: A galactic cluster, a type of star cluster.


open universe: A big-bang cosmology in which the Universe has infinite volume and will expand forever.


opposition: An object’s having a celestial longitude 180˚ from that of the Sun.


optical: In the visible part of the spectrum, 4000-7000 A, or having to do with reflecting or refracting that radiation.


optical double: A pair of stars that appear extremely close together in the sky even though they are at different distances from us and are not physically linked.


organic: Containing carbon in its molecular structure.


Orion Molecular Cloud: The giant molecular cloud in Orion behind the Orion Nebula, containing many young objects.


oscillating universe: The version of a closed universe in which our cycle is but one of many.


Ozma: One or two projects that searched nearby stars for radio signals from extraterrestrial civilizations.


ozone layer: A region in the Earth’s upper stratosphere and lower mesosphere where O3 absorbs solar ultraviolet radiation.