Fr.: densité optique
The transmittance of a point on a photographic negative equal to the log to the base 10 of the reciprocal of the transmittance through the negative at that point.
Fr.: vitesse orbitale
The velocity of an object in a given orbit around a gravitating mass. For a perfect circular orbit, the velocity is described by the formula V =√[G(M + m)/r], where G is the gravitational constant, M the mass of the primary gravitating body, m the mass of the orbiting object, and r the radius of the orbit.
1) The property of → orthogonal functions.
Fr.: instabilité de paire
An instability arising from the → pair production inside a → massive star leading to energetic → supernova explosions. The pair instability occurs when, late in the star's life, the core reaches a sufficiently high temperature after → carbon burning, a condition in which the pair production can take place. The pairs of electron and positron annihilate to form a neutrino and an anti-neutrino. Consequently, the pressure drops and the outer layers fall in onto the core. The temperature and pressure increase rapidly and eventually titanic nuclear burning causes an extraordinary explosion with energies higher than 1051 erg. See also → pair-instability supernova and → pulsational pair-instability supernova.
Fr.: instabilité de paire
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.
Fr.: vitesse parabolique
The speed necessary to form a parabolic orbit around a gravitational center. It is also the minimum speed necessary to escape from the gravitational pull of a body.
Fr.: inégalité parallactique
An irregularity in the Moon's motion caused by the Sun's gravitational attraction, which sets the Moon ahead or behind its normal orbital position. The Moon is about 2 arcminutes ahead of its expected position at first quarter, and a similar amount behind at last quarter.
1) General: Equality, as in amount, status, or character;
equivalence; correspondence; similarity; analogy. Opposite of disparity.
From M.Fr. parité, from L.L. paritas "equality," from L. adj. par "equal."
Hamâli, quality noun of hamâl, → pair (equivalent 2).
Fr.: conservation de parité
In quantum mechanics, the condition of parity in strong and electrodynamic interactions, where it remains constant and does not change with time. In other words, parity conservation implies that Nature is symmetrical and makes no distinction between right- and left-handed rotations or between opposite sides of a subatomic particle. Thus, for example, two similar radioactive particles spinning in opposite directions about a vertical axis should emit their decay products with the same intensity upward and downward. Same as → parity symmetry.
Fr.: symétrie de parité
The invariance of physical laws under a transformation that changes the sign of the space coordinates. Parity symmetry is sometimes called mirror symmetry. It is known that the parity symmetry is violated in some weak interactions, while it is well preserved in all other three interactions (gravitational, electromagnetic, strong). Same as → P-symmetry and → parity conservation.
Fr.: violation de la parité
In quantum mechanics, the condition of → parity in the → weak interaction. For example, the emitted → beta particles in → radioactive decay of → cobalt-60 nuclei are not equally distributed between the two poles of cobalt-60. More beta particles emerge from one pole than the other, and it would be possible to distinguish the mirror image nuclei from their counterparts.
Fr.: instabilitÃ© 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.
path of totality
Fr.: bande de totalité
The path (up to 320 km wide) that the Moon's shadow traces on the Earth during a total solar eclipse.
Fr.: luminosité du pic
The → bolometric luminosity of a → supernova corresponding to the highest brightness in its → light curve. The peak luminosity occurs after the → supernova explosion; it is directly linked to the amount of radioactive 56Ni produced in the explosion and can be used to test various explosion models. Following → Arnett's rule, one can derive the 56Ni mass from the peak luminosity of a → Type Ia supernova.
Fr.: vitesse particulière
1) Velocity with respect to the Local Standard of Rest.
The quality or condition of being peculiar.
Noun form of → peculiar.
Fr.: relation période-luminosité
A → correlation between the periods and luminosities of → Cepheid variable stars: Cepheids with longer periods are intrinsically more luminous than those with shorter periods. The relation was discovered by Henrietta Leavitt in 1912 when studying Cepheids in the → Small Magellanic Cloud. Once the period of a Cepheid variable is determined from observations, the period-luminosity relation can be used to derive its luminosity. Since luminosity is a function of → distance, the distance can then be calculated with the luminosity. The period-luminosity relation is an invaluable tool for the measurements of distances out to the nearest galaxies and thus for studying the structure of our own Galaxy and of the Universe.
period-mean density relation
bâzâneš-e dowré-cagâli-ye miyângin
Fr.: relation période-densité moyenne
A relation that gives a rough estimate of the oscillation period of a → pulsating star as a function of its mean density. This relation is obtained by considering how long it would take a sound wave to travel across the diameter of a model star: Π ≅ (3π/2γGρ)1/2, where ρ is the mean density, γ the ratio of → specific heats (Cp/Cv), and G the → gravitational constant. This relation shows that the pulsation period of a star is inversely proportional to the square root of its mean density. And this is the reason why the pulsation periods decrease along the → instability strip from the luminous, very tenuous → supergiants to the faint, very dense → white dwarfs.
A state or condition characterized by regular repetition in time or space.