A commonly accepted set of scientific ideas and assumptions.
parallax
The apparent change in position of an object due to the change in the position of the observer. Astronomical parallax is measured in seconds of arc (arcseconds).
parsec
The distance to a hypothetical star whose parallax is 1 second of arc. 1 pc = 206,265 AU = 3.26 ly.
perihelion
The orbital point of closest approach to the Sun.
photon
A quantum of electromagnetic energy. Carries an amount of energy that depends inversely on its wavelength.
planetesimal
One of the small bodies that formed from the solar nebula and eventually grew into proto-planets.
population I
Stars rich in atoms heavier than helium. Nearly always relatively young stars found in the disk of the galaxy.
population II
Stars poor in atoms heavier than helium. Nearly always relatively old stars found in the halo, globular clusters, or the nuclear bulge.
precession
The slow change in the direction of Earth's axis of rotation. One cycle takes nearly 26,000 years.
proper motion
The rate at which a star moves across the sky. Measured in seconds of arc per year.
protein
Complex molecule composed of amino acid units.
proton
A positively charged atomic particle contained in the nucleus of atoms. The nucleus of a hydrogen atom.
protoplanet
Massive object resulting from the coalescence of planetesimals in the solar nebula and destined to become a planet.
protostar
A collapsing cloud of gas and dust destined to become a star.