DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Herbig–Haro object An object that is apparently
a forming star, surrounded by still accreting
matter. These objects have strong stellar
winds, and mass loss including jets. Herbig–
Haro objects are powerful emiters of infrared radiation;
thespectrumconsistsofstrongemission
lines presumably produced by internal shocks
within protostellar jets or winds.
herringbone burst See type II radio burst.
Hertz (Hz) Unit of frequency equal to s −1 .
Hertzsprung Gap The region on a
Hertzsprung–Russell Diagram between the
main sequence and red giant branches in which
relatively few stars are found. The reason is that
stars evolve across the gap (at roughly constant
luminosity) very quickly because the hydrogen
fuel in their cores has been exhausted and the
hydrogen around the core is not yet hot enough
to fuse. Thus, the only energy source is gravitational
potential energy from core contraction,
some of which is used up in simultaneously expanding
the outer layers of the star. The gap
is particularly conspicuous in the HR diagram
of a young open cluster of stars, where all the
stars are the same age. See Hertzsprung–Russell
diagram, main sequence star, open cluster, red
giant.
Hertzsprung–Russell diagram See HR diagram.
Hesperian Geophysical epoch on the planet
Mars, 1.8 to 3.5 Gy BP. Channels on Mars give
evidence of large volumes of water flow at the
end of the Hesperian and the beginning of the
Amazonian epoch.
H I 21-cm line Spectral line emitted in the
radio domain, at a wavelength of 21 cm (corresponding
to a frequency of 1420 GHz), due
to the hyperfine transition between two energy
states in the ground level of the hydrogen atom,
the lower state with electron spin and proton spin
anti-parallel, the higher state with the two spins
parallel. The HI 21-cm emission line was first
detected in 1951; since then it has been used to
map the distribution of neutral hydrogen within
the galaxy, and in external galaxies, with radio
© 2001 by CRC Press LLC
high-pressure and high-temperature experiment
telescopes and interferometers. See forbidden
lines.
Higgs mechanism Mechanism responsible
for the existence of massive particles in the standard
model of quantum field theory. The mechanism
arises in a spontaneous breakdown of symmetry.
For instance, far before a gauge U(1)
symmetry the symmetry breakdown (i.e., at high
temperatures), we have a complex scalar fieldφ
and a massless gauge field A µ (like the photon,
for example) with two degrees of freedom
(two polarizations). As the temperature lowers,
φ selects one possible phase in the range
[0 − 2π]. This is called spontaneous symmetry
breakdown. After the breakdown, φ represents
a massive scalar field andA µ becomes a massive
gauge field (three degrees of freedom), totaling
again exactly four degrees of freedom as before
the transition. Such events were presumably important
in the very early universe. See Abelian
Higgs model, cosmic topological defect, Goldstone
model, spontaneous symmetry breaking.
higher derivative theories Theories of gravity
in which the equations of motion contain
higher than second derivatives of the basic variables
(the potentials) of the theory.
highlands, lunar See lunar highlands.
high-pressure and high-temperature experiment
An experiment to measure density of
rocks and minerals, velocity of elastic waves and
elastic modulus, and to investigate materials,
phases, and melting, realizing the high-pressure
and high-temperature state of the Earth’s interior.
There are two kinds of high-pressure apparatuses:
a dynamic high-pressure apparatus
utilizing shock waves and a static high-pressure
apparatus applying constant load. For the former,
it is possible to produce pressure of several
hundred gigaPascals (GPa) instantaneously, corresponding
to the pressure at the Earth’s core.
However, because of the short reaction time
and inaccuracies of the estimate of temperature,
other approaches are being used, such as piston
cylinder apparatus which is able to produce pressure
and temperature characteristic of the uppermost
mantle (up to 6 GPa and 2,800 K), multianvil
type apparatus which can produce pres-
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