ASTR 103 - Astronomy

Glossary - P

P (seismic) waves

Longitudinal waves of an earthquake that cause Earth's inner material to expand and contract alternately. Also called primary, or pressure, or compression waves.

Pangaea

Supercontinent into which all the present continents were merged over 200 million years ago.

panspermia

Theory that microorganisms floating in space or attached to interstellar dust particles can germinate and start the evolutionary chain of life when they encounter a hospitable sterile planet.

parabola

Conic section formed by a plane passing parallel to one side of a cone. The eccentricity of a parabola equals 1.

parallax

Apparent shift in the position of an object when observed from two different placed.

parsec

Unit of distance corresponding to the distance of a body whose parallax equals one arc second.

partial eclipse

 

Pauli exclusion principle

 

peculiar galaxy

abnormally shaped galaxy that emits nonthermal radiation.

penumbra

portion of the shadow from which part of the light source is excluded when a body passes over the light source.

perfect cosmological principle

proposition in the steady state theory that the Universe looks the same everywhere at all times.

perfect gas

ideal gas whose pressure increases directly with the temperature and the density.

perigee

 

perihelion

point in the path of a body orbiting the Sun where it is closest to the Sun.

period

 

period table

 

period-luminosity relation

 

perturbation

disturbance in the normal movement of an orbiting body arising from an external force, usually gravitational.

phase

the repeating portions of a cyclic phenomena, such as the varying shape of the sunlit portion of the Moon during its monthly orbit or the relationship between the crests and trough of a wave.

photometry

 

photometer

instrument used to measure the intensity of a light source.

photometric system

 

photon

unit carrier of electromagnetic radiation.

photosphere

light-emitting, visible surface of the Sun.

photosynthesis

buildup of organic compounds within plants by absorption of water, carbon dioxide, and solar energy.

pion

subatomic particle with a mass equal to 270 electron masses. It may be neutral or charged positively or negatively. Also known as pi-meson.

plage

bright, disturbed area of the solar surface.

Planck's constant (h)

universal constant that connects the energy of the photon (E) to its frequency (f) through the equation E=hf.

Planck's law

formula giving the relation between the temperature and the energy emitted by a blackbody at any wavelength.

planet

one of the principal nonluminous bodies in orbit around the Sun or another star. There are nine in the Solar System.

planetary nebula

slowly expanding envelope of gas surrounding a small, hot central star.

planetesimals

small solid bodies believed to have formed during the condensing stage of the solar nebula.

plasma

hot ionized gas that is electrically conductive.

plate tectonics

the motions of large segments of the Earth's surface, having the form of plates, over the mantle.

polar axis

 

polarized radiation

electromagnetic radiation whose transverse vibration is confined to a fixed place (plane-polarized light) or to one that rotates (circularly polarized light).

Population I

younger stars found in greatest numbers in the outer portions of the Galactic disk.

Population II

older stars inhabiting mainly the central and halo portions of the Galactic system.

population types

classification of the stars into two main groups, Population I and Population II, and intermediate types based on differences in age, chemical composition, spectral properties, velocities, and location in a galaxy.

positive space curvature

space continuum whose curvature is spherical or ellipsoidal, resulting in a closed universe.

positron

positively charged electron or antielectron.

potential energy

energy acquired as a result of position in a gravitational field.

Precambrian

geologic era between the time when Earth's crust formed about 4.5 billion years ago to about 1 billion years ago.

precession of equinoxes

conical movement of Earth's axis of rotation about the vertical to the plane of the ecliptic in a period of 25,800 years. The phenomenon causes the equinoxes to slide westward along the ecliptic about 50 seconds of arc per year.

pressure

the force per unit area exerted by a mechanic force or a field of force.

prime focus

 

primeval fireball

high-powered explosion from a superhot, superdense state of condensed matter (the big band) that supposedly initiated the expansion of the Universe.

principle of equivalence

Einstein's declaration that a gravitational force cannot be distinguished from an inertial force; hence a gravitational field can be replaced by an accelerated system.

prism

 

prominence

 

proper motion

angular change of a star's direction from the Sun in one year.

protein

large molecule composed of hundreds to thousands of amino acids joined together by peptide links making up the DNA molecule.

proton

positively charged particle that is part of the nucleus of every atom. It is 1836 times heavier than the electron.

proton-proton (p-p) chain

sequence of thermonuclear reactions that builds up helium from hydrogen with the release of energy inside the cores of main sequence stars. Also known as the p-p chain.

pulsar

very small, highly condensed, rapidly spinning star emitting a narrow beam of electromagnetic radiation, observed as a fast pulse from Earth. Believed to be the neutron core remnant of a supernova explosion.

pulsating variable