paraboloid: A 3-dimensional surface formed by revolving a parabola around its axis.
parallax: (a) Trigonometric parallax, half the angle through which a star appears to be displaced when the Earth moves from one side of the Sun to the other (2 A.U.); it is inversely proportional to the distance. (b) Other ways of measuring distance, as in spectroscopic parallax.
parallel light: Light that is neither converting nor diverging.
parent molecules: Molecules in a comet that break up into daughter molecules.
parsec: The distance from which 1 A.U. subtends one second of arc. (approximately 3.26 ly.).
particle physics: The study of elementary nuclear particles.
peculiar motion: The motion of a galaxy with respect to the Hubble flow.
penumbra: (a) For an eclipse, the part of the shadow from which the Sun is only partially occulted; (b) of a sunspot, the outer region, not as dark, as the umbra.
perfect cosmological principle: The assumption that on a large scale the Universe is homogenous and isotropic in space and unchanging in time.
perihelion: The near point to the Sun of the orbit of a body orbiting the Sun.
period: The interval over which something repeats.
phase: (a) of a planet, the varying shape of the lighted part of a planet or moon as soon from some vantage point; (b) the relation of the variations of a set of waves.
phase transitions: Changes from one state of matter to another, as from solid to liquid or liquid to gas; phase transitions in the early Universe marked the separation of the fundamental forces.
photometry: The electronic measurement of the amount of light.
photomultiplier: An electronic device that through a series of internal stages multiplies a small current that is given off when light is incident on it; a large current results.
photon: A packet of energy that can be thought of as a particle traveling at the speed of light.
photon sphere: The sphere around a black hole, 3/2 the size of the vent horizon, within which exit cones open and in which light can orbit.
photosphere: The region of a star from which most of its light is radiated.
plage: The part of a solar active region that appears bright when viewed in Hα.
Planck’s constant: The constant of a proportionality, h, between the frequency of an electromagnetic wave and the energy of an equivalent photon. E-hv – hc/λ.
Planck’s law: The formula that predicts, for an opaque object at a certain temperature, how much radiation there is at every wavelength.
Planck time: The time very close to the big bang, 10-43 seconds, before which a quantum theory of gravity would be necessary to explain the Universe and which therefore thus currently inaccessible to our computations.
planet: A celestial body of substantial size (more than about 1000 km across), basically non-radiating and of insufficient mass for nuclear reactions ever to begin, ordinarily in orbit around a star.
planetary nebulae: Shells of matter ejected by low-mass stars after their main-sequence lifetime, ionized by ultraviolet radiation form the star’s remaining core.
planetesimal: One of the small bodies into which the primeval solar nebula condensed and from which the planets formed.
plasma: An electrically neutral gas composed of exact equal numbers of ions and electrons.
plates: Large flat structures making up a planet’s crust.
plate tectonics: The theory of the Earth’s crust, explaining it as plates moving because of processes beneath.
plumes: Thin structures in the solar corona near the poles.
point objects: Objects in which no size is distinguishable.
polar axis: The axis of an equatorial telescope mounting that is parallel to the Earth’s axis of rotation.
pole star: A star approximately at a celestial pole, Polaris is now the pole star; there is no south pole star.
poor cluster: A cluster of stars or galaxies with few members.
positive ion: An atom that has lost one or more electrons.
positron: An electron’s antiparticle (charge of +e), where e is the unit of electric charge.
precession: The very slowly charging position of stars in the sky resulting from variations in the orientation of the Earth’s axis; a complete cycle takes 26,000 years.
precession of the equinoxes: The slow variation of the position of the equinoxes (intersections of the ecliptic and celestial equator) resulting from variations in the orientation of the Earth’s axis.
pre-main-sequence star: A ball of gas in the process of slowly contracting to become a star as it heats up before beginning nuclear fusion.
pressure: Force per unit area.
primary cosmic rays: The cosmic rays arriving at the top of the Earth’s atmosphere.
primary distance indicators: Ways of measuring distance directly, as in trigonometric parallax.
prime focus: The location at which the main lens or mirror of the telescope focuses an image without being reflected or refocused by another mirror or other optical element..
primeval solar nebula: The early stage of the Solar system in nebular theories.
primordial background radiation: Isotropic millimeter and sub-millimeter radiation following a black-body curve for about 3 K; interpreted as a remnant of the big bang.
primordial nucleosynthesis: The formation of the nuclei of isotopes of hydrogen (such as deuterium), helium, and lithium in the first 10 minutes of the Universe.
primum mobile: In Ptolemaic and Aristotelian theory, the outermost sphere around the Earth, which give its natural motion to inner spheres.
principal quantum number: The integer n that determines the main energy level in an atom.
prograde motion: The apparent motion of the planets when they appear to move forward (from west to east) with respect to the stars; see also retrograde motion.
prolate: Having the diameter along the axis of rotation longer than the equatorial diameter.
prominence: Solar gas protruding over the limb, visible to the naked eye only at eclipses but also observed outside the eclipses by its emission-line spectrum.
proper motion: Angular motion across the sky with respect to a framework of galaxies or fixed stars.
proteins: Long chains of amino acids; fundamental components of all cells of life as we know it.
proton: Elementary particle with positive charge +e, where e is the unit of electric charge, one of the fundamental constituents of an atom.
proton-proton chain: A set of nuclear reactions by which four hydrogen nuclei combine one after the other to form one helium nucleus, with a resulting release of energy.
protoplanets: The loose collections of particles from which the planets formed.
protosun: The Sun in formation.
pulsar: A celestial object that gives of pulses of radio waves; thought to be a rotating neutron star with a very strong magnetic field.