DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

horse latitudes
from the loci of the stars in an HR diagram,
which is roughly horizontal, at least for some
choicesoftemperatureindicator. RRLyraevariables,
one of the common distance indicators,
are horizontal branch stars. The corresponding
phase for stars of higher metallicity is called
the clump phase, while more massive stars pass
through the phase without abrupt changes in
their locations on the HR diagram. See clump
star, CNO cycle, helium burning, HR diagram,
metallicity.
horse latitudes The latitudes approximately
30 ◦ N and 30 ◦ S which are regions of calm winds
and generally high pressure.
horst An elevated block of crustal material
forming a ridge or plateau, bounded by parallel
normal or reverse faults. A horst is usually
produced by compressional tectonic forces.
hot dark matter See dark matter, hot.
hot spot (1.) A bright, compact component
observed in the radio lobes of powerful radio
sources, such as radio galaxies and quasars.
Hot spots are frequently found in radio source
of Fanaroff–Riley class II, which includes the
brightest lobe dominated radio source. Hot
spots are ∼ 1 kpc in size, and they appear unresolved
when observed at moderate resolution.
Their radio spectrum is described by a powerlaw
over the frequency and a spectral index
≈−0.5, suggesting that radiation is emitted via
a synchrotron process. Their location, often at
the outer end of the jet, suggests that hot spots
are the site of impact between the high speed
particles of the jet and the lobes. See Fanaroff–
Riley.
(2.) In geophysics, areas of enhanced volcanic
activity not directly associated with plate
boundary volcanism, either ocean ridges or the
volcanic lines behind subduction zones. Hot
spots represent areas where magma from the
mantle erupts directly onto the surface, creating
features associated with fluid lavas such as
flood basalts and shield volcanos. The Hawaiian
islands show the passage of the Pacific Plate
over a hot spot. Iceland is considered to be a hot
spot because of its large volcanic flux. Hot spot
volcanism is a common process on other plan-
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ets, particularly Mars and Venus where many of
the large volcanos are the result of this process.
hot towers The cores of large cumulonimbus
clouds in the intertropical convergence zone.
See intertropical convergence zone.
hour angle The angular distance on the celestial
sphere measured westward along the celestial
equator from the vernal equinox to the
hour circle passing through the celestial object
in question. It is equal to 360 ◦ minus the right
ascension.
HR (Hertzsprung–Russell) or color-magnitude
diagram A plot of brightness or luminosity
vs. some indicator of effective temperature
for observed stars, calculated models of
stars, or a comparison between the two. Such
diagrams have been a primary tool for displaying
and testing ideas of stellar structure and evolution
since the early 20th century. Such a plot
was first attempted for stars in the Pleiades in
1910 by Hans Rosenberg, following advice from
Karl Schwarzschild, while Einar Hertzsprung
and Henry Norris Russell soon after considered
stars of known distances in the solar neighborhood.
The vertical axis can be luminosity in solar
units or absolute magnitude or even apparent
magnitude for a cluster of stars known to be
all the same distance from us, either over a full
range of wavelengths (bolometric luminosity) or
over some restricted range of wavelengths (U, B,
V ,R, I, etc., UBV colors). The horizontal axis
can be effective temperature (especially in the
case of models), a color index, or spectral type.
Real stars do not populate such a diagram at
random. Ninety percent (in a distance-limited
sample) will fall on a diagonal from Hot-Bright
(upper left) to Cool-Faint (lower right) called the
main sequence. The scattering of stars above the
main sequence are called red giants and supergiants;
those fainter are sub-dwarfs and white
dwarfs. A cluster of stars all with the same age
and initial chemical composition has a characteristic
HR diagram with signatures for age and
metallicity. An HR diagram representing observed
stars normally shows individual symbols
for each star.