DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

color excess
color excess A measure of the reddening
of starlight due to small intervening interstellar
dust grains. It has been found for most interstellar
dust clouds that the extinction of light is
proportional to the amount of reddening that a
given interstellar cloud causes.
AV
E
= 3.2 .
AV is the visual extinction (obscuration) of light
in magnitudes, and E is the color excess of B and
V magnitudes, given by the intrinsic color index
subtracted from the measured color index
E=(B −V)−(B −V)o
where (B−V) and (B−V)o are the measured and
expected color indices, respectively, of a star of
a known temperature (see color index). Since
the ratio of extinction to reddening is known,
then the luminosity of the star can be derived
and hence the distance to the star.
color index The difference between a star’s
apparentmagnitudeatsomegivenwavelengthto
that at another (longer) wavelength. A typical
color index used is B–V, where B and V are the
apparent blue and visible magnitudes measured
with standard filters at 4200 and 5400, respectively.
The color index is independent of distance
and gives a measure of a star’s color temperature,
that temperature which approximates
the radiation distribution of the star as a black
body(seecolortemperature). Foranygivenstellar
type, there is an expected color index, calculated
by assuming a black body temperature for
the star. Any difference between the observed
color index and the measured color index of a
star is due to interstellar reddening. See color
excess.
color-magnitude diagram See HR (Hertzsprung–Russell)
diagram.
color temperature The temperature that describes
a black body radiation distribution based
on the intensity ratios at two or more wavelength
intervals (or “colors”). One reason that the radiation
distribution from stars deviates from that
of a black body is that atoms and molecules in
the stellar atmosphere deplete radiation from the
© 2001 by CRC Press LLC
continuum for spectral line formation. Then a
least squares fit of a Planck curve to the stellar
spectrum will underestimate the radiation temperature
of the star. However, since the shape
of a black body curve for any temperature is the
same, the ratio of the intensity at two wavelength
intervals judiciously chosen to avoid strong absorption
features will give a temperature that
more closely describes the radiation distribution
that the star is actually putting out.
coma In a comet, the region of heated gas
and dust surrounding the nucleus of the comet,
distinct from the tails which are generated by
nuclear and coma material driven off by solar
wind and solar radiation pressure. The gas in
the coma may be ionized, and its composition
can vary with distance from the comet as the
different molecules undergo photo-dissociation
with time. The coma can reach many millions of
kilometers in radius. In optics, an image defect
in which the image of a point consists of an offcenter
bright spot in a larger, fainter “coma”.
comet A small solar system object composed
substantially of volatile material, which is distinctive
because of its rapid motion across the
sky and its long tail(s) of gas and dust emitted
from the body. An accurate description of
a comet is a “dirty snowball”, “dirty iceberg”
since comets contain some rocky material but
are primarily composed of ice. The rocky-icy
center of a comet is called the nucleus. Comets
usually orbit the sun in highly elliptical orbits.
As the comet nears the sun, the increased temperature
causes the ice in the nucleus to sublimate
and form a gaseous halo around the nucleus,
called the coma. Comets often possess
two tails: a dust tail that lies in the orbit behind
the comet generated by surface activity,
and a brighter, ionized gas tail, that points away
from the sun, driven by solar wind. Long period
comets (periods greater than 200 years) are
thought to originate in the Oort cloud, a spherical
shell surrounding the Earth at distances exceeding
50,000 AU. They are perturbed by the planets
(especially Jupiter) to fall in toward the sun.
Their orbits typically have random inclinations
and very large eccentricities; some hyperbolic
orbits have been observed. Short period comets
apparently arise in the Kuiper Belt, in the zone