DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Geroch group
(about 25 K/km near the surface) is usually taken
to be positive.
Geroch group In general relativity, an
infinite-parameter symmetry group of stationary
axisymmetric vacuum space-times. It is
generated by two noncommuting 3-parameter
symmetry groups; one is the group of constant
linear combinations of the space-like and timelike
Killing vectors. The other generating group
(called the Ehlers group) acts on the gravitational
potentials and is isomorphic to O(2,1).
Geroch–Hansen moments (1974) Gravitational
multipole moments of stationary and axisymmetric
isolated sources in the theory of general
relativity. They form a complex infinite
series. The real parts are the mass (or gravielectric)
moments and the imaginary parts are
the current or gravimagnetic moments. The
Schwarzschild spacetime has a mass monopole
moment equal to the mass m, and all other moments
vanish. The nth moment of the Kerr
spacetime with rotation parameter a is m(ia) n .
See Kerr black hole, Schwarzschild black hole.
Gershun’s law The conservation of energy
equation obtained by integrating the monochromatic
radiative transfer equation (for a medium
with no inelastic scattering or other sources)
over all directions; it states that the depth derivative
of the net plane irradiance equals the negative
of the absorption coefficient multiplied by
the scalar irradiance.
Gershun tube A tube used to limit the field
of view of a radiometer to a small solid angle;
used in measuring radiances.
GeV Giga electronvolt; a unit of energy equal
to a billion (10 9 ) electronvolts.
giant branch The collection of stars in which
a contracting helium core heats an overlying
hydrogen shell, accelerating hydrogen burning.
Stars develop a large hydrogen envelope and occupy
a specific region in the HR diagram, with
K to M spectral type (B−V= 1.0−−1.8, absolute
magnitude 2 to -2). The Red Giant phase
ends when the temperature of the core reaches
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10 8 K, and helium ignites explosively in its core
(the “helium flash”).
giant cells The largest of the discrete scales
governing convective motions on the sun; the
other three being granulation, mesogranulation,
and supergranulation. The existence of giant
cells is less conclusive than the other scales of
convection. However, they are believed to have
a dimension comparable with that of the depth
of the convection zone (∼ 300,000 km), surface
velocities of as little as 0.03 to 0.1 kms −1 and
lifetimes of ∼ 14 months.
giant planet One of Jupiter, Saturn, Uranus,
and Neptune in the solar system, or a similar extrasolar
planet. Typically much more massive
than terrestrial planets, giant planets are composed
of volatiles (hydrogen, helium, methane,
ammonia) with small solid cores, if any.
Gibbs free energy (Gibbs Potential) An extensive
thermodynamic potential H given by
H=U−ST −PV,
whereU is the internal energy,S is the entropy,
T is the temperature, P is the pressure, and V
is the volume of the system. For a reversible
process at constant T and P , work stored as
Gibbs Potential can be recovered completely.
gilvin See colored dissolved organic matter.
Ginzburg temperature In quantum (or classical)
systems with multiple minima in some potential,
separated by a barrier, the minimum temperature
needed to induce thermal jumps from
one minimum of the potential to another. Applications
include magnetism, superconductivity,
and the mechanism of formation of topological
defects in the early universe. See correlation
length, cosmic topological defect, Kibble mechanism.
glacial Referring to processes or features produced
by ice.
glaciation A geologic epoch when 30% of
the Earth’s land surface is covered with moving
ice. The most recent glaciation began in the