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 R_{M}
(ρ_{M}/ρ_{m})^{1/3},
where R_{M} is the radius of the → primary body,
ρ_{M} is the → density of the
primary, and ρ_{m} is the density of
the secondary body. This formula can also be
expressed as:
d = 1.26 R_{m}
(M_{M}/M_{m})^{1/3}, where
R_{m} is the radius of the secondary. As an example,
for the Earth-Moon system, where R_{M} = 6,378 km,
ρ_{M} = 5.5 g cm^{-3}, and ρ_{m} = 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

حد ِ ادینگتون ِ چرخشی

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).

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

حد ِ هورپهی ِ خورشید

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.

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.

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.