An Etymological Dictionary of Astronomy and Astrophysics

A nuclear reaction that takes place at high densities and relatively low temperatures. Pycnonuclear reactions are almost temperature independent and occur even at zero temperature. These reactions are extremely slow at densities typical for normal stars but intensify with increasing density. For example, carbon burns into heavier elements at densities over 10¹⁰ g cm^-3.

Etymology (EN): Pycnonuclear, from pycno- a combining form meaning “dense, thick,” from Gk. pyknos “dense, solid” + → nuclear;
→ reaction.

Etymology (PE): Vâžireš, → reaction; cagâl-hasteyi, from cagâl, → dense, + hasteyi, → nuclear.

General term for the class of → actinometers that measure the intensity of solar radiation received on the surface of the Earth. It functions by converting the heat of the sunlight into a voltage using a device called a thermopile, and a recording voltmeter.

See also: From pyr-, from → pyro- + → heliometer.

A yellow → mineral which is an iron sulfide (FeS₂). It is a source of → sulfur and is used in the manufacture of sulfuric acid.

See also: M.E., from O.Fr. pirite, from L. pyrites, from Gk. pyr “fire,” → pyro-, + → -ite.

A prefix meaning “fire, heat, high temperature,” used in the formation of compound words. → pyrheliometer; → pyrometer.

Etymology (EN): From Gk. pyro-, combining form of pyr “fire,” cognate with O.E. fyr, O.N. fürr, M.Du. vuur, Ger. Feuer), from PIE *paewr-;
cf. Mod.Pers. Lori porpor “blazing charcoal,” Gilaki bur, biur “smokeless red fire” (Lori perisk, periska “spark,” Kurd. biriske “spark,” Lârestâni pelita “spark”); Gk. pyr “fire;” Hitt. pahhur “fire;” Skt. pū- “to cleanse.”

Etymology (PE): Âzar, variants âtaš, taš “fire,” from Mid.Pers. âtaxš, âtur “fire;” Av. ātar-, āθr- “fire,” singular nominative ātarš-; O.Pers. ātar- “fire;” Av. āθaurvan- “fire priest;” Skt. átharvan- “fire priest;” cf. L. ater “black” (“blackened by fire”); Arm. airem “burns;” Serb. vatra “fire;” PIE base *āter- “fire.”

A device used for measuring high temperatures. By comparing a source whose temperature is to be measured to a standardized source of illumination, it determines the temperature of the former source.

See also: → pyro- + → -meter.

One of the major groups of silicate minerals found in many igneous and metamorphic rocks. Pyroxene minerals are also common in meteorites. There are many different types of pyroxene. All of the types contain Si₂O₆ but some have sodium (Na) while others have iron (Fe), magnesium (Mg), or a combination of these three elements.

See also: From → pyro- “fire,” + xeno- a combining form meaning “alien, strange, guest,” from Gk. xenos “stranger, guest.” Pyroxene was originally supposed to be a foreign substance when found in igneous rocks.

The proposition that the → square of the → hypotenuse of a
→ right triangle is equal to the → sum of the squares of the other two sides: a² + b² = c².

See also: After Pythagoras (c570 BC-c495BC), Greek philosopher and mathematician; → theorem.

Any group of three → integers that satisfy the relations specified by the → Pythagorean theorem. Some examples: 3, 4, and 5; 5, 12, and 13; 8, 15, 17.

See also: → Pythagorean theorem; → triple; → triplet.

The Compass Box. A faint constellation in the southern hemisphere, at 9h right ascension, 30° south declination, representing a mariner’s compass. Its brightest star, Alpha Pyxidis, is magnitude 3.7. Abbreviation: Pyx; genitive: Pyxidis.

Etymology (EN): L. pyxis, from Gk. pyxis “box.”

Etymology (PE): Qotbnamâ literally “pole indicator,” from qotb, → pole,

• namâ “displayer, indicator,” from nemudan “to show” (Mid.Pers. nimūdan, nimây- “to show,” from O.Pers./Av. ni- “down; into” (Skt. ni “down,” nitaram “downward,” Gk. neiothen “from below,” cf. E. nether, O.E. niþera, neoþera “down, downward, below, beneath,” from P.Gmc. *nitheraz,
  Du. neder, Ger. nieder; PIE *ni- “down, below”) + māy- “to measure;” cf. Skt. mati “measures,” matra- “measure;”
  Gk. metron “measure;” L. metrum; PIE base *me- “to measure”).