DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

HI region
sure and temperature characteristic of the uppermost
part of the lower mantle (up to 30 GPa and
2,300 K), and diamond-anvil apparatus which
produces pressure up to 300 GPa and temperature
over several thousand K. A quench method
has been used to identify the phases for phase
equilibrium experiments, and in situ observation
under high-pressure and high-temperature state
is now being done employing X-rays.
H I region A part of the interstellar medium
where hydrogen atoms and most other atoms
remain neutral. Most HI regions have important
concentrations of molecules and dust. The main
chemical constituents are hydrogen atoms and
molecules. The temperature can vary from 10 2
to 10 3 K while the density is of the order of 100
or less particles cm −3 . See interstellar medium.
H II region A part of the interstellar medium
where all hydrogen is ionized by ultraviolet light
from one or many main sequence stars of effective
temperature between 3 × 10 4 to 5 × 10 4 K.
Mostotherelementsarealsosinglyionized. HII
regions can have varying amounts of mass up
to several thousand solar masses and temperatures
between 6 × 10 3 to 1.5 × 10 4 K. The
density varies from 10 to 10 4 particles cm 3 , and
up to 10 6 particles cm −3 in ultracompact HII
regions. Their shapes are often irregular, and
determined by the surrounding medium, and
are generally associated with molecular clouds.
The mass typically varies from 0.1 to 10 4 solar
masses with sizes from 0.01 to 10 parsecs. They
are located mostly in the spiral arms of galaxies.
Many of them, associated with particular
phases of stellar evolution, are also called supernova
remnants, planetary nebulae, and nova
remnants. See interstellar medium. Compare
with planetary nebula.
Himalia Moon of Jupiter, also designated
JVI. Discovered by C. Perrine in 1904, its orbit
has an eccentricity of 0.158, an inclination of
27.63 ◦ , and a semimajor axis of 1.148 ×10 7 km.
Its radius is 93 km, its mass 9.56 × 10 18 kg,
and its density 2.83 g cm −3 . It has a geometric
albedo of 0.03, and orbits Jupiter once every
250.6 Earth days.
© 2001 by CRC Press LLC
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hindcasting (wave) A procedure of determining
wave conditions which existed at a prior
time, using observations of atmospheric pressure
or wind measurements.
historical climate The accumulation of
weather records analyzed for long-term trends
over periods of years. Climate records kept
beginning about 2000 years ago, until very recently,
were over land only.
hoarfrost Frost that crystallizes onto vegetation
and surfaces by direct deposition from saturated
air during a hard freeze. See hard freeze.
Holmberg radius The length of the semimajor
axis of a galaxy, either expressed in angular
or linear units, measured from the center
to a minimum surface brightness of 26.5 photographic
magnitudes per square second of arc
(approximately 1.5% the surface brightness of
the night sky).
homogeneity A system is homogeneous if all
points in it are equivalent with regard to relevant
properties.
In cosmology, independence of position in
space. The Euclidean space, used as a background
for Newtonian physics, is homogeneous
even though its contents are not. The spacetime
describing a realistic cosmology may be
spatially homogeneous, i.e., its geometry may
be independent of position in each space of constant
cosmological time, but in a correct (general
relativistic) description, this requires the matter
stress tensor to be homogeneous under the same
symmetry that describes the homogeneity of the
space. It cannot be fully homogeneous as a
space-time because homogeneity in four dimensions
would imply independence of time, while
there is observational evidence for the expansion
of the universe. Whether our actual spacetime
is even approximately spatially homogeneous is
a question that can, in principle, be answered
by observations. However, the observations are
difficult and, thus far, inconclusive (see cosmological
principle). At small scales (of clusters
of galaxies at least) the universe is obviously inhomogeneous.
If our spacetime is not spatially
homogeneous, even on long scale average, then
it must be modeled by inhomogeneous models.