DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

granulation
have been broken at extremely high energies.
Thus, many gauge particles would be very massive,
explaining why we have not yet observed
them. See spontaneous symmetry breaking.
granulation Cellular structure of the photosphere
visible at high spatial resolution. The
characteristic scale of the granulation is ∼
1000 km, ranging widely from∼ 300 km to over
2000 km. Typical velocities present in the granules
are horizontal outflows ∼ 1.4 km s −1 and
upflows in the center of the granules ∼ 1.8 km
s −1 . Identifiable granules live for∼ 5to10min
and at any one time ∼ 3 to 4× 10 6 granules
cover the surface of the sun.
grating spectrograph An instrument for the
analysis of radiation at different wavelengths, in
which light is dispersed by a diffraction grating.
In a typical grating spectrograph design, light
focused on the focal plane of the telescope is
collimated (i.e., the rays of the beam are made
parallel) on a blazed diffraction grating. Light
of different wavelengths is thus diffracted along
different directions, and it is then re-focused on
a detector, for example a photographic plate or
a CCD, by a lens or a mirror. Several designs
exist, based on different choices of the focusing
and collimating elements, or on the use of a
reflection or a transmission grating. The spectral
resolving power, i.e., the ability to separate
two close spectral lines spaced by λ at wavelengthλ
is usuallyλ/λ < ∼ 10 4 . See diffraction
grating.
gravitation The interaction between objects
that in Newtonian theory depend only on their
distances and masses:
F=Gm1m2ˆrr −2 ,
where F is the attractive force,m1 andm2 are the
masses, r is their separation, ˆr is a unit vector
between the masses, and G is Newton’s gravitational
constant.
In the more accurate description in general
relativity, the gravitational field is a symmetric
rank two tensor in four dimensions, which obeys
Einstein’s equations, which have as a source
the stress-energy tensor of the matter. General
relativity reduces to the Newtonian description
in weak-field, quasi stationary situations. See
© 2001 by CRC Press LLC
208
Newtonian gravity, general relativity, Newtonian
gravitational constant.
gravitational collapse One of several possible
final episodes of stellar evolution. Stars
are supported against collapse under their own
weight by gas pressure resulting from high temperature.
The equilibrium of cold matter can
be supported by the pressure of degenerate electron
gas. The equilibrium of white dwarf stars is
thoughttobeduetothis. Chandrasekharshowed
in 1939 that the maximal mass of a white dwarf
is 1.4 solar mass and that stars more massive
than this will collapse (to a neutron star or to a
black hole).
gravitational constant G In Newtonian
physics, the acceleration of a particle towards
the Earth isGm/r 2 wherem andr are the mass
and radius of the Earth, respectively and
G= 6.670× 10 −8 cm 3 /g· sec 2
is the gravitational constant.
gravitational deflection of light The presence
of a central mass (e.g., the sun) causes local
curvature of spacetime, and trajectories of photons
(the quanta of light) are deflected (attracted
to the mass). In the case of a spherical central
body, for small angle deflection, the angle deflected
is
θ= 4GM/dc 2 ,
whereM is the central mass,G is Newton’s constant,c
is the speed of light, andd is the impact
parameter of the light past the mass. If the deflection
were as if the photon were a particle
in Newtonian gravity traveling at the speed of
light, the deflection would be half the relativistic
result. Deflection of light has been verified
to parts in 1000 by observations of the direction
to extragalactic radio sources, as the sun passes
near their position in the sky. In the case of distant
sources, deflection by intervening galaxies
or clusters of galaxies causes lensing, leading to
the appearance of multiple images, and of rings
or arcs of distorted images. See gravitational
lens, light deflection.
gravitational equations The field equations
of the gravitational interaction. The basic tenet