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parabola A conic section of eccentricity 1.0; the curve of the intersection between a circular cone and a plane parallel to a straight line in the surface of the cone.
parallax An apparent displacement of a nearby star that results from the motion of the Earth around the Sun; numerically, the angle subtended by 1 AU at the distance of a particular star.
parsec A unit of distance in astronomy, equal to 3.26 light years. At a distance of 1 parsec, a star has a parallax of one arcsecond.
Pauli exclusion principle Quantum mechanical principle by which no two particles of the same kind can have the same position and momentum.
peculiar velocity The velocity of a star with respect to the local standard of rest; that is, its space motion, corrected for the motion of the Sun with respect to our neighboring stars.
penumbra The outer, not completely dark part of a shadow; the region from which the source of light is not completely hidden.
perfect radiator or blackbody A body that absorbs and subsequently re-emits all radiation incident upon it.
periastron The place in the orbit of a star in a binary-star sys-tem where it is closest to its companion star.
perigee  The place in the orbit of an Earth satellite where it is closest to the center of the Earth.
perihelion The place in the orbit of an object revolving about the Sun where it is closest to the Sun's center.
period-luminosity relation An empirical relation between the periods and luminosities of certain variable stars. 
perturbation The disturbing effect, when small, on the motion of a body  produced by a third body or other external agent.
photochemistry Chemical changes caused by electromagnetic radiation.
photometry The measurement of light intensities.
photon A discrete unit of electromagnetic energy. 
photosphere The region of the solar (or a stellar) atmosphere from which continuous radiation escapes into space.
pixel An individual picture element in a detector; for example, a particular silicon diode in a CCD.
plage A bright region of the solar surface observed in the monochromatic light of some spectral line.
Planck's constant The constant of proportionality relating the energy of a photon to its frequency.
planet Any of the nine largest bodies revolving about the Sun, or any similar bodies that may orbit other stars. Unlike stars, planets do not (for the most part) give off their own light, but only reflect the light of their parent star.
planetarium An optical device for projecting on a screen or domed ceiling the stars and planets and their apparent motions in the sky.
planetary nebula A shell of gas ejected from, and enlarging about, a certain kind of extremely hot star that is nearing the end of its life.
planetesimals  The hypothetical objects, from tens to hundreds of kilometers in diameter, that formed in the solar nebula as an intermediate step between tiny grains and the larger planetary objects we see today. The comets and some asteroids may be leftover planetesimals.
plasma A hot ionized gas.
polar axis The axis of rotation of the Earth; also, an axis in the mounting of a telescope that is parallel to the Earth's axis.
Population I and II Two classes of stars (and systems of stars), classified according to their spectral characteristics, chemical compositions, radial velocities, ages, and locations in the Galaxy.
positron An electron with a positive rather than negative charge; an antielectron.
potential energy Stored energy that can be converted into other forms; especially gravitational energy.
precession (of Earth) A slow, conical motion of the Earth's axis of rotation, caused principally by the gravitational pull of the Moon and Sun on the Earth's equatorial bulge.
precession of the equinoxes Slow westward motion of the equinoxes along the ecliptic that results from precession.
prime focus The point in a telescope where the objective focuses the light.
prime meridian The terrestrial meridian passing through the site of the old Royal Greenwich Observatory; longitude O.
primitive In planetary science and meteoritics, an object or rock that is little changed, chemically, since its formation, and hence representative of the conditions in the solar nebula at the time of formation of the solar system. Also used to refer to the chemical composition of an atmosphere that has not undergone extensive chemical evolution.
primitive meteorite A meteorite that has not been greatly altered chemically since its condensation from the solar nebula; called in meteoritics a chondrite (either ordinary chondrite or carbonaceous chondrite).
primitive rock Any rock that has not experienced great heat or pressure and therefore remains representative of the original condensates from the solar nebula - never found on any object large enough to have undergone melting and differentiation.
principle of equivalence Principle that a gravitational force and a suitable acceleration are indistinguishable within a sufficiently local environment.
prism A wedge-shaped piece of glass that is used to disperse white light into a spectrum.
prominence A phenomenon in the solar corona that commonly appears like a flame above the limb of the Sun.
proper motion The angular change per year in the direction of a star as seen from the Sun.
proton A heavy subatomic particle that carries a positive charge; one of the two principal constituents of the atomic nucleus.
proton-proton cycle A series of thermonuclear reactions by which nuclei of hydrogen are built up into nuclei of helium.
protoplanet or -star or -galaxy The original material from which a planet (or a star or galaxy) condensed.
pulsar A variable radio source of small angular size that emits very rapid radio pulses in very regular periods that range from fractions of a second to several seconds.
pulsating variable A variable star that pulsates in size and luminosity.
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