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. |
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. |
solar radius šo'â'-e xoršid Fr.: rayon solaire A unit of length, representing the radius of the → Sun, used to express the size of stars in astrophysics. It is equivalent to: 695,700 km, 0.00465047 → astronomical units, 7.35355 × 10^{-8} → light-years, and 2.32061 → light-seconds. |