Glossary -- P

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paradigm

a general agreement of belief of how the world works; what could be called ``common sense''.

parallax

an apparent shifting of an object's position resulting from observing the object from two different vantage points. Stellar parallaxes are seen when we view nearby stars from opposite sides of the Earth's orbit.

parsec

(pc): distance at which an object would have a parallax of one arc second. Equals approximately 3.26 light years or about 206,265 astronomical units.

penumbra

region of partial shadow that is outside the umbra; the light source is partially blocked.

perfect cosmological principle

an assumption that the universe is everywhere uniform and looks the same in any direction in all space and time---it is the same everywhere and does not change throughout time.

perihelion

point in an object's orbit around the Sun that is closest to the Sun.

period-luminosity relation

how the average luminosity of Cepheid variable stars depends on their period of pulsation.

photon

a distinct ``chunk'' or particle of electromagnetic radiation.

photosphere

the thin layer of the Sun where the gas just becomes thin enough for the photons from the interior can escape to space. It is the ``surface'' of the Sun.

photosynthesis

a process used by plants to convert water, carbon dioxide, and sunlight into carbohydrates and oxygen. The oxygen in the Earth's atmosphere is produced by this process.

planetary nebula

final mass-loss stage for a dying low-mass star in which the outer layers are ejected during the core's collapse to form a white dwarf.

poor cluster

galaxy cluster with only a few tens of galaxies.

Population I

(stars): younger stars including the hot blue stars that have slightly elliptical orbits closely aligned with the disk plane of the Milky Way Galaxy. The youngest stars are found in the spiral arms of the galactic disk.

Population II

(stars): older, redder stars that have very elliptical orbits randomly oriented and are found in the stellar halo and bulge of the Milky Way Galaxy.

precession

slow wobble of an object's rotation axis or an object's orbit. The precession of the Earth's rotation axis is caused by the gravitational pulls of the Sun and the Moon on the Earth's equatorial bulge.

pressure

amount of force per unit area: pressure = force/area.

primitive

in studies of the solar system, an object or rock that has remained chemically unchanged since it formed (solidified) about 4.6 billion years ago. The object holds a record of the very early conditions from which the rest of the solar system (Sun, planets, moons) formed.

proper motion

angular distance an object moves across the sky (perpendicular to your line of sight) in a given amount of time.

proton

positively-charged subatomic particle that is found in the nucleus of an atom. It has about 1800 times more mass than its negatively-charged electron counterpart.

proton-proton chain

a nuclear fusion chain reaction used by most stars to generate energy. In a chain process involving three or more reactions, the net result is four hydrogen nuclei are fused together to form a helium nucleus plus energy.

protostar

collapsing clump of dust and gas that will later become a star. The protostar is warm enough to produce a lot of infrared and some microwave radiation. Microwave energy is produced by the surrounding cocoon cloud.

pulsar

young neutron star with a strong magnetic field and rapid rotation that produces beams of radiation out of its magnetic poles. If the beams cross our line of sight, we see the star ``pulsate'' (flash on and off).

Pythagorean paradigm

``common sense'' belief articulated by Pythagoras about the universe that says all objects move in perfectly circular orbits at perfectly uniform speeds and the Earth is at the center of the motions of celestial bodies.

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last update: 06 August 1999