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pair production Process in which a very high-energy photon interacts with another photon or a strong magnetic field to give rise to a particle-anti-particle pair.

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 due to the Earth's orbit around the Sun is exactly 1 arc second, equal to 3.3 light years.

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

period The time needed for a complete cycle of repetitive motion; for example the time for an orbiting body to complete one revolution about another body, or the time needed for a pendulum to make a complete swing.

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.

photoelectric effect A basic physical phenomenon that demonstrates radiation to be composed of particles.

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.

planet One of eight 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, with a hot central star that is in the process of becoming a white dwarf star. [More Info: Field Guide]

plasma A state of matter wherein all atoms are ionized; a mixture of free electrons and free atomic nuclei.

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. This positron is the antiparticle of the electron. Positrons and electrons annihilate each other when they meet, producing pure energy in the form of gamma rays.

potential energy Stored energy that can be converted into other forms; especially gravitational energy.

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

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

proper motion The angular movement of a star across the sky, as seen from the 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 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 cloud of gas is sufficiently hot and dense that it becomes opaque to its own radiation, but not hot enough for the onset of nuclear reactions. [More Info: Field Guide]

pulsar Object that emits radiation in the form of rapid pulses with a characteristic pulse period and duration. Generally used to describe the pulsed radiation from a rotating neutron star. [More Info: Field Guide]

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

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