An Etymological Dictionary of Astronomy and Astrophysics

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory



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Number of Results: 30 Search : stability
pair-instability supernova
  اَبَر-نووای ِ ناپایداری ِ جفت، اَبَر-نو‌اختر ~ ~   
abar-novâ-ye nâpâydâri-ye joft, abar-now-axtar-e ~ ~

Fr.: supernova à instabilité de paires   

A special type of → supernova that would result from the → pair instability in → supermassive stars with a mass range between 140 and 260 Msun in a low → metallicity environment. Such objects descended from the → Population III stars in the early history of the Universe. Such supernovae are the most powerful thermonuclear explosions in the Universe. Pair-instability supernovae may have played an important role in the synthesis of → heavy elements. Moreover, the energetic feedback of the processed elements to their surroundings could have affected the structure and evolution of the early Universe (See, e.g., Fryer et al. 2001, ApJ 550, 372; Heger & Woosley 2002, ApJ 567, 532). See also → pulsational pair-instability supernova.

pair; → instability; → supernova.

Parker instability
  ناپایداری ِ پارکر   
nâpâydâri-ye Parker

Fr.: instabilité de Parker   

A type of instability found in some astrophysical phenomena involving → magnetic fields; it arises if a gas layer is supported by the horizontal magnetic fields against → gravity. Also called → magnetic buoyancy. Briefly, this instability works as follows. Consider a uniform disk of gas which is coupled to a magnetici field that is parallel to the disk. Suppose that the disk is gravitationally stratified in the vertical direction, and is in dynamical equilibrium under the balance of gravity and pressure (thermal and magnetic). Now consider a small perturbation which causes the field lines to rise in certain parts of the disk and sink in others. Because of gravity, the gas loaded onto the field lines tends to slide off the peaks and and sink into the valleys. The increase of mass loads in the valleys makes them sink further, while the magnetic pressure causes the peaks to rise as their mass load decreases. Consequently, the initial perturbation is amplified, causing the production of density fluctuations in an initially uniform disk. The characteristic scale for the Parker instability is ~4πH, where H is the scale height of the diffuse component of the disk. For the Milky Way, where H ~ 150 pc, this scale is about 1-2 kpc. Numerical simulations show that the density contrast generated by the Parker instability is generally of order unity before the instability saturates. This implies that the Parker instability on its own may not be able to drive collapse on large scales. Nevertheless, it may trigger gravitational instability in a marginally unstable disk and/or induce strong motions in the medium, thereby acting as a source of turbulence on large scales (see, e.g., Houjun Mo, Frank van den Bosch, Simon White, 2010, Galaxy Formation and Evolution, The University Press, Cambridge, UK).

First studied by E. N. Parker, 1966, ApJ 145, 811; → instability.

pulsational instability
  ناپایداری ِ تپشی   
nâpâydâri-ye tapeši

Fr.: instabilité pulsationnelle   

A term used to describe irregularly spaced, fine-scale structure in optically thick rings. The process relies on a combination of viscosity and self-gravity of ring material to produce this fine structure. Also known as overstability (Ellis et al., 2007, Planetary Ring Systems, Springer).

pulsational; → instability.

pulsational pair-instability supernova
  ابر-نووای ِ ناپایداری ِ تپشی ِ جفت، اَبَر-نو‌اختر ~ ~ ~   
abar-novâ-ye nâpâydâri-ye tapeši-ye joft, abar-now-axtar-e ~ ~ ~

Fr.: supernova à instabilité pulsationnelle de paires   

A → supernova resulting from the → pair instability that generates several successive explosions. According to models, a first pulse ejects many solar masses of hydrogen layers as a shell. After the first explosion, the remaining core contracts and searches for a stable burning state. When the next explosion occurs a few years later, several solar masses of material are again ejected, which collide with the earlier ejecta. This collision can radiate 1050 erg of light, about a factor of ten more than an ordinary → core-collapse supernova. After each pulse, the remaining core contracts, radiates neutrinos and light, and searches again for a stable burning state. Later ejections have lower mass, but have higher energy. They quickly catch up with the first shell, where the collision dissipates most of their kinetic energy as radiation. The first SNe from → Population III stars are likely due to pulsational pair instability (Woosley et al. 2007, Nature 450, 390). See also → pair-instability supernova.

pulsational; → pair; → instability.

Rayleigh-Taylor instability
  ناپایداری ِ ریلی-تیلر   
nâpâydâri-ye Rayleigh-Taylor

Fr.: instabilité Rayleigh-Taylor   

A type of hydrodynamical instability between two fluids of different densities, which occurs when the heavy fluid lies above the lighter fluid in a gravitational field. More generally a material interface is said to be Rayleigh-Taylor unstable whenever the fluid acceleration has an opposite direction to the density gradient.

rayleigh; → Taylor number; → instability.

secular instability
  ناپایداری ِ دیریاز   
nâpâydâri-ye diryâz

Fr.: instabilité séculaire   

Instability caused by a slow dissipation of energy.

secular; → instability.

secular stability
  پایداری ِ دیریاز   
pâydâri-ye diryâz

Fr.: stabilité séculaire   

1) The condition in which the equilibrium configuration of a system is stable over long periods of time.
2) The condition of a star when it is stable against arbitrary adiabatic perturbations.

secular; → stability.

pâydâri (#)

Fr.: stabilité   

A condition in which a dynamical system slightly displaced from its equilibrium configuration always tends to return to this configuration. → instability, → instability strip.

Noun from adj. → stable.

Taylor-Couette instability
  ناپایداری ِ تیلر-کویءت   
nâpâydâri-ye Taylor-Couette

Fr.: instabilité de Taylor-Couette   

A hydrodynamic instability in the → Taylor-Couette flow that arises when the rotation velocity of the fluid exceeds a critical value. The instability arises for → Taylor numbers larger than about 1700. At the beginning the fluid elements will move in simple rolls, but turbulence in the form of complex spirals will appear with increasing rotation velocity.

Taylor-Couette flow; → instability.

âzmunpaziri (#)

Fr.: testabilité   

The state or fact of being → testable. See also → falsifiability.

testable; → -ity.

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