An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

Hydrodynamics → conservation laws (which can be extended to → magnetohydrodynamics, MHD) which describe the physical conditions of material across a → shock front. A fluid is completely described by its velocity, density, pressure, specific heat ratio, and magnetic field (in the MHD case). Mass, momentum, and energy fluxes are conserved in the shock, leading to the Rankine-Hugoniot relations. Also called Rankine-Hugoniot jump conditions. See also → jump condition.

Named after William John Rankine, → Rankine scale, and Pierre Henri Hugoniot, → Hugoniot curve; → condition.

The → vapor pressure of an ideal → solution is dependent on the vapor pressure of each chemical component and the → mole fraction of the component present in the solution. This means that the addition of → solute to a liquid lessens the tendency for the liquid to become a → solid or a → gas. For example, the addition of → salt to water causes the water to freeze below its normal → freezing point (0°C) and to boil above its normal → boiling point (100°C).

After François-Marie Raoult (1830-1901), the French chemist who studied the physical properties of chemical solutions; → law.

Occurring within a short time; happening speedily; moving or acting with great speed; swift (Dictionary.com).

From L. rapidus "tearing away, seizing, swift," from rapere "to hurry away, seize, plunder;"

Tond "swift, rapid, brisk; fierce, severe" (Mid.Pers. tund "sharp, violent;" Sogdian tund "violent;" cf. Skt. tod- "to thrust, give a push," tudáti "he thrusts;" L. tundere "to thrust, to hit" (Fr. percer, E. pierce, ultimately from L. pertusus, from p.p. of pertundere "to thrust or bore through;" PIE base *(s)teud- "to thrust, to beat").

An object with technical designation MXB 17302335 which is characterized by erratic and extremely intense → X-ray emissions. The Rapid Burster is a → binary system comprising a → low-mass star as its → primary and a → secondary  → neutron star. The → gravitational attraction of the neutron star strips its → companion of some of its gas, which then forms an → accretion disk and spirals toward the neutron star. The Rapid Burster is located at a distance of 110 kpc in the highly reddened → globular cluster Liller 1. It is a → low-mass X-ray binary (LMXB) and a recurrent → X-ray transient. So far less than 200 LMXBs have been detected in the → Galaxy and the → Magellanic Clouds. All produce a persistent flux of X-rays, the result of a release of → gravitational potential energy. Approximately 40 of them also exhibit → Type I bursts which are due to → thermonuclear flashes on the surface of a neutron star. The Rapid Burster is unique among the LMXBs in that it produces X-ray bursts in quick succession. These are called → Type II bursts, and they result from a spasmodic release of gravitational potential energy, which is due to some unknown → accretion disk instability (Lewin et al., 1996, ApJ 462, L39).

→ rapid; → burster.

A chemically peculiar star characterized by the presence of high-frequency non-radial oscillations, with periods that range between about 4 and 16 min. These variations have periods from about 5 to 20 minutes and low amplitudes (B < 10 mmag). They are consistent with acoustic (→ p mode) pulsations of low degree and high radial overtone.

→ rapid; → -ly; → oscillate; → -ing; → Ap star.

A part of a river where the current runs very swiftly. See also → torrent.

→ rapid.

Tondâb, from tond, → rapid, + âb, → water.

Not occurring very often; not found in large numbers.

M.E., from O.Fr. rere "sparse" from L. rarus "thinly sown; not thick; having intervals between."

Perz, in several Iranian languages and dialects "little, few, small, minute part of any thing" (Khonsâri: perz, Qatrâni: perzema, Birjandi: porzu, Farâmarzâni: pesk, Kurd petik, Tabari: perik "minute quantity, particle"); maybe ultimately from Proto-Ir. *paraka- "small part," → part.
Kamyâb "rare, difficult to find," from kam "little, few; deficient, wanting; scarce" (Mid.Pers. kam "little, small, few," O.Pers./Av. kamna- "small, few"); + yâb present stem of yâftan, yâbidan "to find, discover; to obtain, acquire" (Mid.Pers. ayâftan, ayâpênitan "to reach, attain;" Manichean Mid.Pers. 'y'b "to attain;" Parthian, Sogdian (+ *pati-) pty'b "to reach, obtain;" Av. ap- "to reach, overtake," apayeiti "achieved, reached;" Skt. âp- "to reach, gain," âpnoti "reaches, gains;" Gk. hapto, haptomai "to touch, cling to, adhere to;" L. apiscor "touch, reach;" PIE base *ap- "to take, reach").

Any of the group of metallic → chemical elements with → atomic numbers between 57 and 71 inclusive. The name is an inappropriate terminology, since they are neither rare nor earth; preferred name → lanthanide.

→ rare; → earth; → element.

Another name for → inert gas.

M.E., from O.Fr. rere "sparse," from L. rarus "loose, wide apart, thin, infrequent;" &rar; gas.

Kamyâb "rare, difficult to find," from kam "little, few; deficient, wanting; scarce" (Mid.Pers. kam "little, small, few," O.Pers./Av. kamna- "small, few"); + yâb present stem of yâftanyâftan, yâbidan "to find, discover; to obtain, acquire" (Mid.Pers. ayâftan, ayâpênitan "to reach, attain;" Manichean Mid.Pers. 'y'b "to attain;" Parthian, Sogdian (+ *pati-) pty'b "to reach, obtain;" Av. ap- "to reach, overtake," apayeiti "achieved, reached;" Skt. âp- "to reach, gain," âpnoti "reaches, gains;" Gk. hapto, haptomai "to touch, cling to, adhere to;" L. apiscor "touch, reach;" PIE base *ap- "to take, reach").

The state of being rarefied, less dense.
An instantaneous reduction in density of a gas resulting from passage a sound wave; opposite of → compression.

M.E. rarefien, from M.Fr. rarefier, from L. rarefacere "make rare," from rarus "loose, wide apart, thin, infrequent."

Verbal noun from âlar present stem of âlaridan→ rarefy + -š, a suffix.

A pressure wave in a fluid generated by rarefaction. It travels in the opposite direction to that of a shock wave in the medium.

→ rarefaction; → wave.

A gas whose pressure is much less than a reference pressure.

Past participle of → rarefy; → gas.

1) To make less dense.
2) To become less dense; become thinned.
Related concepts: → attenuate, → dilute, → decompress.

M.E. rarefien, from M.Fr. rarefier, from rare, combining form of rarus "loose, wide apart, thin, infrequent."

Alaridan, infinitive of âlar, from intensive/nuance prefix â- + lar "thin, meagre" (Dehxodâ), Lori, Laki larr, larrek "thin cow or sheep," Kurd. lerr "thin, lean," variants of laqar (Torbat-Heydariyé), lâqar "lean, meagre, slender; weak."

The brightest star in the constellation → Hercules. It is a → red supergiant of type M5 (surface temperature about 3300 K) lying at a distance of 380 light-years. Rasalgethi is a variable star with a mean magnitude of V = 3.48. It has a fifth magnitude companion 5'' away. This secondary is itself a double that consists of a 4 solar-mass class G5 giant star with a temperature about that of the Sun and a 2.5 solar-mass F2 dwarf star (somewhat hotter than the Sun) in orbit around each other separated by 0.4 AU with a 52 day period.

Rasalgethi, from Ar. Ra's al-Jathi (رأس‌الجاثی) "the Kneeler's Head," in reference to an early name for the constellation, the figure of the man seen upside down, his head toward the south, from ra's "head" + jathi "kneeling."

The brightest star (V = 2.08) in the constellation → Ophiuchus. Rasalhague is a → giant star of type A5 lying 47 light-years from Earth. It has a faint, very close companion only 0''.5 away, 7 A.U., that orbits with a period of 8.7 years.

Rasalhague, from Ar. Ra's al-Hayyah (رأس‌الحیه), literally the "head of the snake," from ra's "head" + Hayyah "snake," referring to the creature in Gk. mythology.

Any of various long-tailed rodents resembling mice but larger, especially one of the genus Rattus (TheFreeDictionary.com).

From M.E. rat, rotte, from O.E. ræt, of unknown origin.

Gerz, from (Lori, Laki) gerza "rat, big mouse," Gilaki (Lâhijân, Gâleš, Bandar-Anzali, etc.) gerze "rat; mouse," variant (Mâzandarân) gal "mouse," of unknown origin.

The amount of change of some quantity during a time interval divided by the length of the time interval.

M.E. rate "monetary value," M.Fr. rate "price, value," from M.L. rata (pars) "fixed (amount)," from L. rata "fixed, settled," p.p. of reri "to reckon, think," → reason.

Nerx "rate, price, tariff."

Math.: The quotient of two quantities arrived at by dividing one by the other.

From L. ratio "reckoning, calculation," also "reason," from rat-, p.p. stem of reri "to reckon, calculate," also "think," → reason.
Fr. rapport, back-formation from rapporter "bring back," from → re- "back, again" + apporter "to bring," from L. apportare "to bring," from → ad- "to" + portare "to carry."

Vâbar, on the model of Fr. rapport "bringing back," as above, from vâ- prefix meaning "back to the original place, again," → re-, + bar present stem of bordan "to carry, lead" (Mid.Pers. burdan, O.Pers./Av. bar- "to bear, carry," barəθre "to bear (infinitive)," Skt. bharati "he carries," Gk. pherein, L. fero "to carry;" PIE base *bher- "to carry").

1) Having or exercising the ability to → reason.
2) Consistent with or based on reason; logical; in accordance with sound reasoning.
3) Math.: Capable of being expressed as a quotient of integers.

M.E. racional, from O.Fr. racionel, from L. rationalis "of or belonging to reason, reasonable," from ratio (genitive rationis) "reckoning, calculation, reason," from rat-, pp. stem of reri "to reckon, calculate; consider, think."

1) Râyani, of or pertaining to râyan, → reason.
2) Xeradâné, xeradmandâné, of or pertaining to xerad "understanding, judgement, intellect, wisdom;" Mid.Pers. xrad "reason, intellect, intelligence, wisdom, understanding;" O.Pers. xraθu- "wisdom;" Av. xratu- "intelligence, undestanding, wisdom; will, purpose, council;" cf. Skt. krátu- "power, will-power;" Gk. kratos "power, strength."
3) vâbari, of or pertaining to vâbar, → ratio.

Any number that can be expressed as a ratio of two integers, providing the second number is not zero.

→ rational; → number.