Fr.: rayon de Roche
The smallest distance at which a → satellite under the influence of its own → gravitation and that of a central mass about which it is describing a → Keplerian orbit can be in equilibrium. This does not, however, apply to a body held together by the stronger forces between atoms and molecules. At a lesser distance the → tidal forces of the → primary body would break up the → secondary body. The Roche limit is given by the formula d = 1.26 RM (ρM/ρm)1/3, where RM is the radius of the → primary body, ρM is the → density of the primary, and ρm is the density of the secondary body. This formula can also be expressed as: d = 1.26 Rm (MM/Mm)1/3, where Rm is the radius of the secondary. As an example, for the Earth-Moon system, where RM = 6,378 km, ρM = 5.5 g cm-3, and ρm = 2.5 g cm-3 is 1.68 Earth radii.
Named after Edouard Albert Roche (1820-1883), the French astronomer who first calculated this theoretical limit in 1848; → limit.
rotational Eddington limit
hadd-e Eddington-e carxeši
Fr.: limite d'Eddington avec rotation
The → Eddington limit of luminosity for a → rotating star in which both the effects of → radiative acceleration and rotation are important. Such objects mainly include → OB stars, → LBV, → supergiants, and → Wolf-Rayet stars. It turns out that the maximum permitted luminosity of a star is reduced by rotation, with respect to the usual Eddington limit (Maeder & Meynet, 2000, A&A, 361, 159).
Fr.: limite de Schönberg-Chandrasekhar
During the → main sequence stage, a star burns the hydrogen in its core and transforms it into helium. When the helium mass amounts to about 10% of the initial stellar mass, the star can no longer maintain the → hydrostatic equilibrium in its core; the star increases its volume and leaves the main sequence in order to become a → red giant.
Named after the Brazilian astrophysicist Mario Schönberg (1914-1990) and Subramahmanyan Chandrasekhar, → Chandrasekhar limit, who were the first to point out this limit and derive it (1942, ApJ 96, 161).
solar ecliptic limit
hadd-e hurpehi-ye xoršid
Fr.: limite écliptique du Soleil
The greatest angular distance from a → lunar orbit node within which a → solar eclipse may occur when the Sun and Moon are in conjunction there. The solar ecliptic limit extends about 17° on each side of the node.
Fr.: limite stationnaire
Same as → stationary limit.
stationary limit surface
ruye-ye hadd-e istvar
Fr.: surface limite stationnaire
A property of → space-time outside a → rotating black hole, which consists of a surface which geometrically bounds the → ergosphere outward. At the stationary limit a particle would have to move with the local light velocity in order to appear stationary to a distant observer. This is because the space here is being dragged at exactly the speed of light relative to the rest of space. Outside this limit space is still dragged, but at a rate less than the speed of light. Also known as → static limit.
Fr.: limite sous-stellaire
The mass limit below which → hydrogen fusion cannot take place, and the cloud collapse cannot lead to the formation of a star. The limit is 0.075 → solar masses, corresponding to about 80 Jupiter masses.
hadd-e bâlâ (#), ~ zabarin (#)
Fr.: limite supérieure
Of an integral operator, the point at which the integration ends.
upper mass limit
hadd-e bâlâyi-ye jerm, ~ zabarin-e ~
Fr.: limite supérieure de masses
The highest mass range admitted in a star formation model. The high mass end of the → initial mass function. The upper mass limit is a critical parameter in understanding → stellar populations, → star formation, and → massive star feedback in galaxies.
bardid bâ gonj-e hyaddmand
Fr.: relevé limité en volume
A survey in which the observed objects are contained in a given volume of space.