An Etymological Dictionary of Astronomy and Astrophysics
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Same as → PG 1159 star.

→ post-; → degenerate; → star.

A technique in spectroscopy which uses a combination of several dispersive elements (prisms in series or a grism) before focusing the light on the primary disperser, usually a grating, in order to achieve high spectral resolutions.

→ pre-; → dispersion.

A → binary system whose components are → pre-main sequence stars.

→ pre-; → main sequence; → binary.

A → Herbig AeBe star displaying → forbidden emission lines in its spectrum.

→ pre-; → main; → sequence; → B[e] star.

A star that evolves in the → Hayashi phase and has not yet reached the → zero-age main sequence.

→ pre-; → main sequence, → star.

The stage of a star before its eruption to become a nova.

→ pre-; → nova; → stage.

An adjective relating to a stage before the formation of a → protostar. → pre-stellar core.

→ pre-; → stellar.

A small, gravitationally unstable molecular → clump of typical size of less than 0.1 pc resulting from → gravitational collapse and → fragmentation of a larger → molecular cloud. It is a centrally concentrated structure which evolves into a → class 0 object, where eventually a single star or a stellar system is formed. Core masses range between 0.5 and 5 solar masses, with a mean number density of at least 10⁴-10⁵ cm^-3, and a temperature as low as about 10 K. A pre-stellar core evolves into a → Class 0 object. Also called dense core.

→ pre-stellar; → core.

A → post-planetary nebula star that is approaching the top of the → white dwarf sequence. These stars have exhausted the capacity of → nuclear burning in their cores.

→ pre-; → white; → dwarf.

Existing before or making possible the appearance of living organisms. Prebiotic molecules are biologically essential components such as amino acids, sugars, and nucleic-acid bases which are precursors of the origin of life.

→ pre- + → bio- + -tic equivalent of → -ic.

1) To correct celestial coordinates for → precession.
2) To undergo → precession.

Back formation from → precession.

Back formation from pišâyân, → precession.

The apparent position of a celestial object corrected for the epoch → precession.

Precessed, p.p. of → precess; → coordinates.

The periodic motion of the → rotation axis of a body such as a → spinning top in which the axis of rotation gradually sweeps out a conical shape. In the case of the spinning Earth, it is due to the combined → gravitational attractions of the → Sun, the → Moon, and → planets on Earth's → equatorial bulge. Since the Earth's axis is tilted to its → orbital plane or → ecliptic, the gravitational force of the Sun and the Moon on the Earth's equatorial bulge tend to pull it back toward the plane of ecliptic. As a result, the axis → precesses. Earth's axis of rotation precesses with a period of about 25,770 years, describing one complete circle on the → celestial sphere (→ precession constant). This circle has a radius of approximately 23°.5, equal to the → inclination of the Earth's orbit. Since the → vernal equinox is the reference direction for the → equatorial coordinate system, the coordinates of "fixed" objects change with time and must therefore be referred to an → epoch at which they are correct. → sign of zodiac.
See also → general precession; → precession of the ecliptic; → precession of the equator; → precession of the equinoxes; → precessional circle; → precession time; → geodetic precession; → general precession in longitude; → general precession in right ascension; → orbital precession; → perihelion precession; → planetary precession; → precession period.

L.L. præcissionem "a coming before," from L. præcessus, p.p. of præcedere "to happen before," from the fact that the equinoxes occur earlier each year with respect to the preceding year, from præ- "before," → pre-, + cedere "to walk, to go, to happen."

Pišâyân, literally "coming before," from piš- "before" → pre- + ây- (present stem of âmadan "to come, arrive, become"), from Av. ay- "to go, to come," aēiti "goes;" O.Pers. aitiy "goes;" Skt. e- "to come near," eti "arrival;" L. ire "to go;" Goth. iddja "went," Lith. eiti "to go;" Rus. idti "to go;" + -ân suffix of space and time.

The amount by which the equinoctial points drift westward annually due to precession. Its value for epoch J2000.0 is 50''.26, resulting from the westward → precession of the equator (50".38), and the eastward → precession of the ecliptic (0".12).

→ precession; → constant.

The component of general precession caused by the gravitational attraction of the planets on the Earth's center of mass. It causes the equinox to move eastward by about 0''.12 per year in the opposite direction to the → precession of the equator. This terminology replaces → planetary precession, according to an IAU resolution adopted in August 2006.

→ precession; → ecliptic.

That component of general precession caused by the combined effect of the Moon, the Sun and the planets on the equatorial protuberance of the Earth, producing a westward motion of the equinoxes along the ecliptic about 50'' per year. According to an IAU resolution adopted in August 2006, the present terminology replaces lunisolar precession.

→ precession; → equator.

The slow motion of the equinoxes along the ecliptic, resulting from the combined motion of the equator (→ precession of the equator) and the ecliptic (→ precession of the ecliptic), or in other words the precession of the Earth's axis of rotation. Also know as → general precession. The First Point of Aries moves westward along the ecliptic at 50.38 arcseconds per year (1 degree every 71.6 years), causing the equinoxes to occur about twenty minutes earlier each sidereal year. See also → nutation.

→ precession; → equinox.

The gradual change in he orbital planes of a binary system.

→ precession; → node.

The interval with which a rotating body precesses. The precession period of the Earth is 25,770 years. For a → spinning top it is given by: T_p = (4π²I)/(mgrT_s), where I is the → moment of inertia, m the mass of the top, g gravity, r the distance between the center of mass and the contact point, and T_s is the spinning period of the top.

→ precession; → period.

A time interval over which an orbit precesses by 2π radians in its plane.

→ precession; → time.