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virial mass jerm-e viriyâl Fr.: masse du viriel The mass of a cluster of stars or galaxies in statistical equilibrium derived by using the → virial theorem. |
virial parameter pârâmun-e viriyâl Fr.: paramètre du viriel A dimensionless parameter that measures the ratio of thermal plus kinetic energies to gravitational energy of a physical system, such as a molecular cloud. The virial parameter is expressed as: α_{vir} = 5σ^{2}R / GM, where R and M are the radius and mass of the cloud respectively, σ is the one-dimensional → velocity dispersion inside the cloud, and G the → gravitational constant. It indicates whether a cloud could be bound or not. For molecular clouds that are confined by their surface pressure and for which self-gravity is unimportant, α_{vir} is much larger than unity, whereas α_{vir} is ~ 1 when the gravitational energy of a clump becomes comparable to its kinetic energy. See, e.g., Bertoldi & McKee, 1992 (ApJ 395, 140). See also → virial theorem. |
virial radius šo'â'-e viriyâl Fr.: rayon du viriel The radius centered on a galaxy containing matter at 200 times the → critical density of the Universe. |
virial temperature damâ-ye viriyâl Fr.: température du viriel The mean temperature at which a gravitationally → bound system would satisfy the → virial theorem. For a system of mass M and radius R with constant density, the gravitational energy per unit mass is W = GM/R. The kinetic energy per unit mass is E = (3/2)kT_{vir}/μ, where k is → Boltzmann's constant and μ the mean molecular weight. According to the virial theorem, E = W/2, which leads to the virial temperature T_{vir} = (1/3)(GM/kR). → virial; → temperature. |
virial theorem farbin-e viriyâl Fr.: théorème du viriel A general equation applicable to a gravitationally → bound system of equal mass objects (stars, galaxies, etc.), which is stable against → dynamical disruption. It states that in such a system the average → gravitational potential energy (W_{vir}) is twice the average → kinetic energy (K_{vir}) of the system: W_{vir} = -2K_{vir}. This general proposition, first derived by Rudolf Clausius (1822-1888), has important applications in a variety of fields ranging from statistical mechanics to astrophysics. See also → virialization, → virial equilibrium, → virialized. |
virialization viriayleš Fr.: virialisation The process whereby a system of gravitationally interacting particles attains stability. The comparable mass components interact with each other, but the whole system does not expand or collapse. Virialization occurs when the → potential energy is twice the negative → kinetic energy: - W_{vir} = 2 K_{vir} (→ virial theorem). In the case of a → galaxy cluster, when the cluster is virialized the merging process and the collapse of matter have finished and the formation process of the galaxy cluster is considered to be done. A cluster has formed by → hierarchical clustering. Virialized clusters, in other words finished clusters, can be found by looking at their radius and density. A cluster is virialized when it satisfies the condition: R_{vir} ~ R_{max}/2, where R_{vir} is the radius when the cluster is virialized and R_{max} is the radius when the collapse starts. From this condition it follows that the object is 8 times denser at virialization than when the collapse started. Verbal noun of → virialize. |
virialize viriyâlidan Fr.: virialiser To undergo → virialization. |
virialized viriyâlidé Fr.: virialisé That has undergone → virialization. Past participle of → virialize. |
virtual virâgin Fr.: virtuel 1) General: Having the efficacy without the material part; unreal but capable
of being considered as real for some purpose. M.E., from M.L. virtualis, from L. virtus "manliness, excellence, potency, efficacy," from vir "man, human, husband, soldier," cf. Mid.Pers. vīr, wīr "man, hero;" Av. vīra- "man, human;" Skt. vīrá- "man, hero;" Lith. vyras "man, husband;" O.Ir. fer "man;" Goth. wair "man;" O.E. wer "man." In Roman philosophy, virtue became associated with virility and strength of character. Virâgin from vir "intellect, mind, memory," variants bar, bir (Mid.Pers. vir, varm, vârom "mental faculty, memory, mind;" Av. vārəma, vārəm "according to one's wishes," from var- "to choose") + -âgin a suffix denoting "consisting of, similarity, possession." |
virtual displacement jâbejâyi-ye virâgin Fr.: déplacement virtuel In → analytical mechanics, any infinitesimal change in the configuration of a material system, consistent with any constraints acting on the system at a given instant. If the constraints are stationary (→ scleronomous), then the actual displacement of the system, in an infinitesimal length of time dt, coincides with one of its virtual displacements. In the case of time-dependent (→ rheonomous) constraints, the actual displacement of the system does not coincide with any of the virtual ones, since the conditions imposed by the constraints vary during the time dt. → virtual; → displacement. |
virtual image tasvir-e virâgin Fr.: image virtuelle Optics: An image formed inside an instrument at the point where diverging rays would cross if they were extended backward into the instrument. Such an image cannot be obtained on a screen placed at its apparent position, since the rays do not pass through that point. → real image. |
virtual observatory nepâhešgâh-e virâgin Fr.: observatoire virtuel An international initiative by the astronomical community to allow global electronic access to the available astronomical data archives of space and ground-based observatories. It also aims to enable data analysis techniques through a coordinating entity that provides common standards, wide-network bandwidth, and state-of-the-art analysis tools. The Virtual Observatory is also intended for re-using data for scientific objectives different from the original ones, in order to optimize the science return of astronomical observations. The Virtual Observatory's capabilities are enabled through the use of standard protocols for registering the existence and location of data and for requesting data that satisfies the user's interests. These standards are developed on an international basis through the → IVOA. The cornerstone of the Virtual Observatory is → interoperability. → virtual; → observatory. |
virtual particle zarr-ye virâgin Fr.: particule virtuelle A subatomic particle that, according to the uncertainty principle, comes into being out of energy fluctuations of the "vacuum" and lasts for extremely short periods of time. An electron-positron pair can exist only about 4 x 10^{-21} seconds. The lifetime increases as the mass and energy involved decreases. Virtual particles are real and have measurable effects, but cannot be directly observed, according to the uncertainty principle. → vacuum polarization. |
virtual work kâr-e virâgin Fr.: travail virtuel In → analytical mechanics, an element of work performed in a → virtual displacement by the → forces acting on all n particles of a → holonomic system with s degrees of freedom (→ degree of freedom). |
virtual work principle parvaz-e kâr-e virâgin Fr.: principe du travail virtuel In → analytical mechanics, a principle whereby it is necessary and sufficient for the equilibrium of any material system with ideal constraints that the sum of the elements of work, performed by the applied forces acting on the system in any virtual displacement, be equal to zero (if all constraints are bilateral) or less than zero (if some of the constraints are unilateral). |
viscometer vošksanj Fr.: viscosimètre An instrument used to measure the → viscosity of a liquid. Same as viscosimeter. |
viscosimeter vošksanj Fr.: viscosimètre Same as → viscometer. |
viscosity vošksâni (#) Fr.: viscosité The property of a → fluid that resists the force tending to cause the fluid to flow. Viscosity may be thought of as the internal → friction of two fluid layers which flow parallel to each other at different speeds. The cause of viscosity is the transport of → momentum by the molecules from one layer to the other. Viscosity is given by η = φ.u.λ.ρ, where φ is a coefficient which depends on the nature of the interaction between the molecules, u is the average velocity of thermal motion of the molecules, λ is the → mean free path, and ρ the → density of the fluid. Also called → dynamic viscosity or → absolute viscosity. See also → kinematic viscosity. |
viscous vošksân (#) Fr.: visqueux Having the property of → viscosity. See also: → viscous dissipation, → viscous fluid, → viscous force, → nonviscous. M.E., from M.Fr. viscous, from L. viscosus "sticky," from viscum "anything sticky; mistletoe." From vošk "a kind of sticky gum" + -sân suffix of similarity, from sân "way, manner." |
viscous dissipation eftâl-e vošksân Fr.: dissipation visqueuse A degradation of → mechanical energy that is irreversibly converted to → thermal energy due to → viscous forces in the → fluid. Viscous dissipation occurs in → turbulent flows. → viscous; → dissipation. |
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