ASTR 103 - Astronomy

Glossary - N

nadir

The point on the celestial sphere opposite the zenith or directly below the observer.

natural selection

Chance adaptations that involve a continual adjustment of organisms to their environment, permitting them to survive.

nebula

Bright or dark cloud of gas and dust, which may contain stars.

nebular hypothesis

Theory that supposes the Solar System originated from a condensed cloud of gas and dust; first put into mathematical form by Laplace (1796) and modified by a number of modern physicist and astronomers.

negative space curvature

Infinite space continuum whose curvature is hyperbolic and open-ended. In the three-dimensional version, the surface is saddle-shaped.

neutrino

Elementary particle without mass or charge emitted during a nuclear reaction.

neutron

Neutral elementary particle of about the same mass as the proton. Protons and neutrons are the basic constituents of atomic nuclei.

neutron star

Gravitationally collapsed star of very small dimensions and enormously high density, composed mainly of neutrons that may be the core remnant of a supernova.

newton (N)

 

Newtonian focus

 

Newton's laws (of motion)

Three laws formulated by Isaac Newton: (1) a body remains at rest or moves at constant speed along a straight line so long as no external force acts upon it; (2) the acceleration of a body is proportional to the force acting on it and inversely proportional to its mass; (3) to every force there is an equal and opposite counterforce.

node

One of the two points of intersection, 180 o apart, between the orbit of a celestial body and a plane of reference such as the ecliptic.

nonstatic cosmological models

 

nonthermal radiation

Radiation produced by a body that is not related to its thermal energy.

nova

Star that suddenly erupts into an object of great brilliance, surpassing the Sun's luminosity by a factor of hundreds of thousands to millions of times and then fading more slowly.

nuclear bulge

 

nuclear force

That force exerted by subatomic particles that is responsible for form, shape, and motion in the subatomic world of the nucleus.

nucleic acid

Substance of the DNA molecule.

nucleon

Either the proton or neutron inside the atomic nucleus.

nucleotide

Repeating section in the double-twisted DNA chain, consisting of three units: a base, a phosphate, and a sugar, constituting half of the cross-rung of the DNA helix.

nucleus (of an atom)

Central portion of an atom, a comet, a galaxy, or a cell.

nucleus (of a comet)

 

nucleus (of a galaxy)

 

nutation

Small 18.6-year period of oscillation superimposed on Earth's 25,800-year period of precession.