| parabola |
A conic section of eccentricity 1.0; the curve of the
intersection between a circular cone and a plane parallel to a straight
line in the surface of the cone. |
| parallax |
An apparent displacement of a nearby star that results
from the motion of the Earth around the Sun; numerically, the angle subtended
by 1 AU at the distance of a particular star. |
| parsec |
A unit of distance in astronomy, equal to 3.26 light
years. At a distance of 1 parsec, a star has a parallax of one arcsecond. |
| Pauli
exclusion principle |
Quantum mechanical principle by which no two particles
of the same kind can have the same position and momentum. |
| peculiar velocity |
The velocity of a star with respect to the local standard
of rest; that is, its space motion, corrected for the motion of the Sun
with respect to our neighboring stars. |
| penumbra |
The outer, not completely dark part of a shadow; the
region from which the source of light is not completely hidden. |
| perfect radiator or blackbody |
A body that absorbs and subsequently re-emits all radiation
incident upon it. |
| periastron |
The place in the orbit of a star in a binary-star sys-tem
where it is closest to its companion star. |
| perigee |
The place in the orbit of an Earth satellite where it
is closest to the center of the Earth. |
| perihelion |
The place in the orbit of an object revolving about
the Sun where it is closest to the Sun's center. |
| period-luminosity relation |
An empirical relation between the periods and luminosities
of certain variable stars. |
| perturbation |
The disturbing effect, when small, on the motion of
a body produced by a third body or other external agent. |
| photochemistry |
Chemical changes caused by electromagnetic radiation. |
| photometry |
The measurement of light intensities. |
| photon |
A discrete unit of electromagnetic energy. |
| photosphere |
The region of the solar (or a stellar) atmosphere from
which continuous radiation escapes into space. |
| pixel |
An individual picture element in a detector; for example,
a particular silicon diode in a CCD. |
| plage |
A bright region of the solar surface observed in the
monochromatic light of some spectral line. |
| Planck's constant |
The constant of proportionality relating the energy
of a photon to its frequency. |
| planet |
Any of the nine largest bodies revolving about the Sun,
or any similar bodies that may orbit other stars. Unlike stars, planets
do not (for the most part) give off their own light, but only reflect the
light of their parent star. |
| planetarium |
An optical device for projecting on a screen or domed
ceiling the stars and planets and their apparent motions in the sky. |
| planetary nebula |
A shell of gas ejected from, and enlarging about, a
certain kind of extremely hot star that is nearing the end of its life. |
| planetesimals |
The hypothetical objects, from tens to hundreds of kilometers
in diameter, that formed in the solar nebula as an intermediate step between
tiny grains and the larger planetary objects we see today. The comets and
some asteroids may be leftover planetesimals. |
| plasma |
A hot ionized gas. |
| polar axis |
The axis of rotation of the Earth; also, an axis in
the mounting of a telescope that is parallel to the Earth's axis. |
| Population I and II |
Two classes of stars (and systems of stars), classified
according to their spectral characteristics, chemical compositions, radial
velocities, ages, and locations in the Galaxy. |
| positron |
An electron with a positive rather than negative charge;
an antielectron. |
| potential energy |
Stored energy that can be converted into other forms;
especially gravitational energy. |
| precession (of Earth) |
A slow, conical motion of the Earth's axis of rotation,
caused principally by the gravitational pull of the Moon and Sun on the
Earth's equatorial bulge. |
| precession of the equinoxes |
Slow westward motion of the equinoxes along the ecliptic
that results from precession. |
| prime focus |
The point in a telescope where the objective focuses
the light. |
| prime meridian |
The terrestrial meridian passing through the site of
the old Royal Greenwich Observatory; longitude O°. |
| primitive |
In planetary science and meteoritics, an object or rock
that is little changed, chemically, since its formation, and hence representative
of the conditions in the solar nebula at the time of formation of the solar
system. Also used to refer to the chemical composition of an atmosphere
that has not undergone extensive chemical evolution. |
| primitive meteorite |
A meteorite that has not been greatly altered chemically
since its condensation from the solar nebula; called in meteoritics a chondrite
(either ordinary chondrite or carbonaceous chondrite). |
| primitive rock |
Any rock that has not experienced great heat or pressure
and therefore remains representative of the original condensates from the
solar nebula - never found on any object large enough to have undergone
melting and differentiation. |
| principle of equivalence |
Principle that a gravitational force and a suitable
acceleration are indistinguishable within a sufficiently local environment. |
| prism |
A wedge-shaped piece of glass that is used to disperse
white light into a spectrum. |
| prominence |
A phenomenon in the solar corona that commonly appears
like a flame above the limb of the Sun. |
| proper motion |
The angular change per year in the direction of a star
as seen from the Sun. |
| proton |
A heavy subatomic particle that carries a positive charge;
one of the two principal constituents of the atomic nucleus. |
| proton-proton cycle |
A series of thermonuclear reactions by which nuclei
of hydrogen are built up into nuclei of helium. |
| protoplanet or -star or -galaxy |
The original material from which a planet (or a star
or galaxy) condensed. |
| pulsar |
A variable radio source of small angular size that emits
very rapid radio pulses in very regular periods that range from fractions
of a second to several seconds. |
| pulsating variable |
A variable star that pulsates in size and luminosity. |
