Alfvén radius šo'â'-e Alfvén Fr.: rayon d'Alfvén 1) In theories of magnetized → accretion disks,
the distance from a non-rotating star where the → free fall
of a spherical accretion flow is stopped, which occurs where the
→ ram pressure of the infalling matter equals the
→ magnetic pressure of the star. → Alfvén wave; → radius. |
Alfven radius šo'â'-e Alfvén Fr.: rayon d'Alfvén 1) In theories of magnetized → accretion disks,
the distance from a non-rotating star where the → free fall
of a spherical accretion flow is stopped, which occurs where the
→ ram pressure of the infalling matter equals the
→ magnetic pressure of the star. → Alfvén wave; → radius. |
Bohr radius šo'â'-e Bohr Fr.: rayon de Bohr The radius of the orbit of the hydrogen electron in its ground state (0.529 Å). |
Bondi-Hoyle accretion radius šo'â'-e farbâl-e Bondi-Hoyle Fr.: rayon de l'accrétion de Bondi-Hoyle In the → Bondi-Hoyle accretion process, the radius where the gravitational energy owing to star is larger than the kinetic energy and, therefore, at which material is bound to star. The Bondi-Hoyle accretion radius is given by R_{BH} = 2 GM / (v^{2} + c_{s}^{2}) where G is the gravitational constant, M is the stellar mass, v the gas/star relative velocity, and c_{s} is the sound speed. → Bondi-Hoyle accretion; → radius. |
corotation radius šoâ'-e ham-carxeš Fr.: rayon de corotation 1) In the → X-wind model of → accretion,
the distance from the star where the → centrifugal force
on a particle corotating with the star balances the
→ gravitational attraction; in other words, where the
→ accretion disk rotates at the same
→ angular velocity as the star. → corotation; → radius. |
cyclotron radius šo'â'-e siklotron Fr.: rayon de cyclotron Same as → Larmor radius. |
de Vaucouleurs radius šo'â'-e de Vaucouleurs Fr.: rayon de Vaucouleurs An → isophotal radius of a galaxy, where the → surface brightness in the B band falls to 25 mag arcsec^{-2}. After the French-born American astronomer Gérard de Vaucouleurs (1918-1995); → radius. |
Earth radius šo'â'-e zamin (#) Fr.: rayon terrestre The distance from the Earth's center to its surface, about 6,371 km. |
effective radius šo'â'-e oskarmand Fr.: rayon effectif Of a galaxy, the distance from its center within which half of the total luminosity is included. |
Einstein radius šo'â'-e Einstein Fr.: rayon d'Einstein In gravitational lens phenomenon, the critical distance from the → lensing object for which the light ray from the source is deflected to the observer, provided that the source, the lens, and the observer are exactly aligned. Consider a massive object (the lens) situated exactly on the line of sight from Earth to a background source. The light rays from the source passing the lens at different distances are bent toward the lens. Since the bending angle for a light ray increases with decreasing distance from the lens, there is a critical distance such that the ray will be deflected just enough to hit the Earth. This distance is called the Einstein radius. By rotational symmetry about the Earth-source axis, an observer on Earth with perfect resolution would see the source lensed into an annulus, called Einstein ring, centered on its position. The size of an Einstein ring is given by the Einstein radius: θ_{E} = (4GM/c^{2})^{0.5} (d_{LS}/(d_{L}.d_{S})^{0.5}, where G is the → gravitational constant, M is the mass of the lens, c is the → speed of light, d_{L} is the angular diameter distance to the lens, d_{S} is the angular diameter distance to the source, and d_{LS} is the angular diameter distance between the lens and the source. The equation can be simplified to: θ_{E} = (0''.9) (M/10^{11}Msun)^{0.5} (D/Gpc)^{-0.5}. Hence, for a dense cluster with mass M ~ 10 × 10^{15} Msun at a distance of 1 Gigaparsec (1 Gpc) this radius is about 100 arcsec. For a gravitational → microlensing event (with masses of order 1 Msun) at galactic distances (say D ~ 3 kpc), the typical Einstein radius would be of order milli-arcseconds. |
electron radius šo'â'-e elektron Fr.: rayon de l'électron The classical size of the electron given by r_{e} = e^{2}/m_{e}c^{2} = 2.81794 × 10^{-13} cm, where e and m_{e} are the → electron charge and → electron mass, respectively, and c is the → speed of light. |
equatorial radius šo'â'-e hamugâri Fr.: rayon équatorial Of a planet, the distance from the center to the equator. For Earth it is 6,378.1370 km. Jupiter has an equatorial radius 11.2 times Earth's value. → equatorial; → radius. |
gyroradius leršo'â' Fr.: gyrorayon Same as → Larmor radius. |
Hubble radius šo'â'-e Hubble (#) Fr.: rayon de Hubble The size of the observable Universe as derived from the ratio c/H_{0}, where H_{0} is the → Hubble constant and c the → speed of light. Same as → Hubble distance, → Hubble length, and → cosmic horizon. |
isophotal radius šo'â'-e izošidi Fr.: rayon isophotal The size attributed to a galaxy corresponding to a particular level of → surface brightness. The reason is that galaxies do not have sharp edges. |
Larmor radius šoâ'-e Larmor (#) Fr.: rayon de Larmor The radius of the circular motion of a → charged particle moving in a → uniform magnetic field. Same as → gyroradius, → radius of gyration, → cyclotron radius. The Larmor radius (r_{L}) is obtained by equating the → Lorentz force with the → centripetal force: qvB = mv^{2}/r_{L}, which leads to r_{L} = p/(ZeB), where p is → momentum, Z is → atomic number, e is the → electron charge, and B is → magnetic induction. The frequency of this circular motion is known as the → gyrofrequency. → Larmor frequency; → radius. |
radius šo'â' (#) Fr.: rayon Of a circle, any straight line segment extending from the center to a point on the
circumference. From L. radius "staff, spoke of a wheel, beam of light," of unknown origin. Šo'â', loan from Ar. |
radius of gyration šo'â'-e lereš Fr.: gyrorayon Same as → Larmor radius. |
radius vector bordâr-e šo'â'i (#) Fr.: rayon vecteur Math.: In a system of polar or spherical coordinates, a line joining a point
to the origin. |
Schwarzschild radius šo'â'-e Schwarzschild Fr.: rayon de Schwarzschild The critical radius at which a massive body becomes a → black hole, i.e., at which light is unable to escape to infinity: R_{s} = 2GM / c^{2}, where G is the → gravitational constant, M is the mass, and c the → speed of light. The fomula can be approximated to R_{s}≅ 3 x (M/Msun), in km. Therefore, the Schwarzschild radius for Sun is about 3 km and for Earth about 1 cm. |