parallax The apparent motion of a relatively close object with respect to a more distant background as the location of the observer changes.

parsec The distance at which a star must lie in order that its measured parallax is exactly 1 arc second, equal to 206,000 A.U.

pair production The process in which two photons of electromagnetic radiation give rise to a particle—anti-particle pair.

partial eclipse Celestial event during which only a part of the occulted body is blocked from view.

penumbra Portion of the shadow cast by an eclipsing object in which the eclipse is seen as partial.

perihelion The closest approach to the Sun of any object in orbit about it.

period The time needed for an orbiting body to complete one revolution about another body.

period-luminosity relation A relation between the pulsation period of a Cepheid variable and its absolute brightness. Measurement of the pulsation period allows the distance of the star to be determined.

permafrost Layer of permanently frozen water ice believed to lie just under the surface of Mars.

photoelectric effect Experiment concerning the detection of electrons from a metal surface, whose speed off the surface was dependent on the frequency of light striking the surface. The theoretical explanation rests on viewing light as made up of photons, or individual bullets of energy.

photometer A device that measures the total amount of light received in all or part of the image.

photometry Branch of observational astronomy in which intensity measurements are made through each of a set of standard filters.

photon Individual packet of electromagnetic energy that makes up electromagnetic radiation.

photosphere The visible surface of the Sun, lying just above the uppermost layer of the Sun's interior, and just below the chromosphere.

pixel One of many tiny picture elements, organized into an array, making up a digital image.

Planck curve see blackbody curve

planet One of nine major bodies that orbit the Sun, visible to us by reflected sunlight.

planetary nebula The ejected envelope of a red giant star, spread over a volume roughly the size of our solar system.

planetary ring system Material organized into thin, flat rings encircling a giant planet, such as Saturn.

planetesimal Term given to objects in the early solar system that had reached the size of small moons, at which point their gravitational fields were strong enough to begin to influence their neighbors.

plate tectonics The motions of regions of Earth's crust, which drift with respect to one another. Also known as continental drift.

polarization The alignment of the electric fields of emitted photons, which are generally emitted with random orientations.

positron Atomic particle with properties identical to those of a negatively charged electron, except for its positive charge. The positron is the antiparticle of the electron. Positrons and electrons annihilate one another when they meet, producing pure energy in the form of gamma rays.

precession The slow change in the direction of the axis of a spinning object, caused by some external influence.

primary atmosphere The chemical components that would have surrounded Earth just after it formed.

prime focus The point in a reflecting telescope where the mirror focuses incoming light to a point.

primordial nucleosynthesis The production of elements heavier than hydrogen by nuclear fusion in the high temperatures and densities which existed in the early universe.

Principle of Cosmic Censorship A proposition to separate the unexplained physics near a singularity from the rest of the well-behaved universe. The principle states that nature always hides any singularity, such as a black hole, inside an event horizon, which insulates the rest of the universe from seeing it.

prominence Loop or sheet of glowing gas ejected from an active region on the solar surface, which then moves through the inner parts of the corona under the influence of the Sun's magnetic field.

proper motion The angular movement of a star across the sky, as seen from Earth, measured in seconds of arc per year. This movement is a result of the star's actual motion through space.

proton An elementary particle carrying a positive electric charge, a component of all atomic nuclei. The number of protons in the nucleus of an atom dictates what type of atom it is.

proton-proton chain The chain of fusion reactions, leading from hydrogen to helium, that powers most main-sequence stars.

protoplanet Clump of material, formed in the early stages of solar system formation, that was the forerunner of the planets we see today.

protostar Stage in star formation when the interior of a collapsing fragment of gas is sufficiently hot and dense that it becomes opaque to its own radiation. The protostar is the dense region at the center of the fragment.

protosun The central accumulation of material in the early stages of solar system formations, the forerunner of the present-day Sun.

Ptolemaic model Geocentric solar system model, developed by the second century astronomer Claudius Ptolemy. It predicted with great accuracy the positions of the then known planets.

pulsar Object that emits radiation in the form of rapid pulses with a characteristic pulse period and duration. Charged particles, accelerated by the magnetic field of a rapidly rotating neutron star, flow along the magnetic field lines, producing radiation that beams outward as the star spins on its axis.

pulsating variable star A star whose luminosity varies in a predictable, periodic way.