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Fr.: disque protoplanétaire
A → circumstellar disk of gas and dust surrounding a → pre-main sequence star from which planetary systems form. Protoplanetary disks are remnants of → accretion disks which bring forth stars. Typically, their sizes are ~100-500 AU, masses ~10-2 solar masses, lifetimes ~106-107 years, and accretion rates ~10-7-10-8 solar masses per year. According to the standard theory of planet formation, called core accretion, planets come into being by the growth of → dust grains which stick together and produce ever larger bodies, known as → planetesimals. The agglomeration of these planetesimals of 100 to 1000 km in size into rocky Earth-mass planets is the main outcome of this theory. Beyond the → snow line in the disk, if the masses of these cores of rock and ice grow higher than 10 times that of Earth in less than a few million years, gas can rapidly accrete and give rise to giant gaseous planets similar to → Jupiter. If core building goes on too slowly, the disk gas dissipates before the formation of → giant planets can start. Finally the left-over planetesimals that could not agglomerate into rocky planets or core of giant planets remain as a → debris disk around the central object that has become a → main sequence star. An alternative to core accretion theory is formation of planets in a massive protoplanetary disk by → gravitational instabilities. The validity of these two theories is presently debated. See also → protoplanet.
Fr.: pré-nebuleuse planétaire
The fluid substance within the living cell that consists of two major divisions, the cytoplasm and the nucleoplasm (cell nucleus). It is composed mainly of nucleic acids, proteins, lipids, carbohydrates, and inorganic salts.
Fr.: abondance protosolaire
A stage in the process of → star formation, after the → gravitational collapse of the dense → pre-stellar core and before the initiation of → nuclear fusion in the central object which will eventually become a star. Protostars are classified into four groups: → Class 0, → Class I, → Class II, and → Class III.
Of or pertaining to → protostars.
Fr.: effondrement protostellaire
Fr.: disque protostellaire
Fr.: jet protostellaire
A high-velocity and highly → collimated jet associated with the earliest phase of → star formation that propagating along the polar axis of the → protostar-→ accretion disk system. Protostellar jets are usually detected in the [S II], [O I], and Hα lines and are therefore referred to as optical jets. They may have more than a parsec in length. Their formation is related to the → magnetohydrodynamics (MHD) of accretion disks. These jets are detected in protostellar sources over a wide range of masses, from the very early stages of formation (sources associated with infalling envelopes whose mass exceeds that of the growing star) all the way to the → classical T Tauri stars, whose envelopes have already dispersed. This phenomenon is thought to play a key role in regulating the star formation process by removing the excess → angular momentum of disk material and enabling matter to flow toward the center. Protostellar winds also provide an important → feedback mechanism between the forming star and the surrounding medium, to which they return mass and energy. Protostellar jets are at the origin of → bipolar outflows. The et sweeps up ambient → molecular cloud material into two thin shells, which manifest themselves as the observed bipolar lobes of → carbon monoxide (CO) emission. Once the molecular cloud material has been swept away (on a timescale of 105 years), the bipolar outflow disappears, leaving the protostellar jet to erratically fire away for a further 106-107 years.
The Sun at its protostellar formation stage, before becoming a main sequence star, nearly 5 billion years ago. The protosun was more luminous than today and larger, with a radius comparable to that of the orbits of the inner planets
The original or model on which something is based or formed.
Capable of being demonstrated or proved.
To supply proof of, to establish or demonstrate the truth or validity of.
A short popular saying, usually of unknown and ancient origin, that expresses effectively some commonplace truth or useful thought (Dictionary.com).
Verbal noun of → provoke.
The forepart of a ship or boat; bow; opposite to stern or poop→ Puppis.
From M.Fr. proue, from Upper It. (Genoese) prua, from L. prora "prow," from Gk. proira, related to pro "before, forward," → pro-.
Farâl, from farâ "forward" (farâ raftan "to go forward, proceed," farâ rândan "to drive forward"), equivalent to → pro-, + relation suffix -âl, → -al. Compare farâl with prow "bow," Fr. la proue "prow, bow," from dialectal It. proa, prua, from L. prora "bow," from Gk. proira, related to pro "before, forward."
Fr.: Proxima b
An → extrasolar planet orbiting our nearest stellar neighbor → Proxima Centauri. The planet was detected through a long-term → radial velocity campaign and found to have an → orbital period of ~ 11.2 days, a → semi-major axis of ~ 0.05 → astronomical units (20 times closer to Proxima than the Earth is to the Sun), and a minimum mass 1.3 → Earth masses (M sin i = 1.3 M_Earth), i.e. ~ 30% larger than the Earth (Anglada-Escudé et al. 2016, Nature 536, 437). The planet's surface temperature should allow it to support liquid water, and its mass suggests that it might have a rocky surface. With a semi-major axis of ~ 0.05 AU, it lies in the center of the classical habitable zone for Proxima. However, Proxima Centauri is a → flare star and the → X-ray flux received by the planet is 400 times greater than the flux that Earth receives from the Sun. Energetic particles associated with the flares may erode the atmosphere or hinder the development of primitive forms of life. It is also not known whether the → exoplanet has a magnetic field, like Earth, which could shield it from the dangerous stellar radiation.
proksimâ Kentâwros, nazdiktarin ~
Fr.: Proxima du Centaure
The closest star to the Sun, lying 4.24 → light-years away. Other designations: α Centauri C, GL 551, HIP 70890, or simply Proxima. It is the faintest of the three stars that make up the → Alpha Centauri system. Proxima Centauri is a → red dwarf of → spectral type M6 Ve. It has a magnitude of +11.0, but undergoes sudden brightness increases of up to 1 mag lasting several minutes. Proxima is a late-type → flare star with a rotation period of ~ 84 days. Its mass is about 0.123 → solar masses or 129 → Jupiter masses. Proxima orbits the binary system AB at a distance of about 15,000 → astronomical unit (AU)s, with a period of approximately 550,000 years (Kervella et al., 2016, arXiv:1611.0349). In about 200,000 years it will be at the same distance as AB and in 240,000 years it will be farther to Sun than AB. It has an → effective temperature of only around 3,050 K, a luminosity of 0.15 per cent of that of the Sun, a measured radius of 14 per cent of the radius of the Sun and a mass of about 12 per cent of the mass of the Sun. An → exoplanet, named → Proxima b, has been discovered orbiting our nearest neighbor star. Proxima experiences a seven-year activity cycle, similar to the Sun's 11-year cycle (B. J. Wargelin, B. J. et al., 2016, arXiv:1610.03447). But unlike the Sun's relatively moderate flares, Proxima's outbursts of X-ray and ultraviolet radiation could prove deadly for any hypothetical life on its planet, Proxima b.
Proksimâ, from L., as above; Kentâwros, → Centaurus; nazdiktarin, superlative of nazdik "near," from Mid.Pers. nazdik "near," from nazd "close" (Mid.Pers. nazd, nazdik "near," nazdist "first;" O.Pers. ašna- "close;" Av. nazdišta- "nearest, next," nazdyo "nearer to," nas- "to come near, approach, reach;" cf. Skt. nédīyas- "closer, very close," nas- "to approach, to reach") + -ik, → -ic.