626 GLOSSARY
carbonaceous chondrite (552) Stony meteor-
ite that contains both chondrules and volatiles.
These may be the least altered remains of the solar
nebula still present in the solar system.
Cassegrain focus (110) The optical design of a
refl ecting telescope in which the secondary mirror
refl ects light back down the tube through a hole
in the center of the objective mirror.
catastrophic hypothesis (398) Explanation for
natural processes that depends on dramatic and
unlikely events, such as the collision of two stars
to produce our solar system.
celestial equator (17) The imaginary line around
the sky directly above Earth’s equator.
celestial sphere (18) An imaginary sphere of very
large radius surrounding Earth to which the plan-
ets, stars, sun, and moon seem to be attached.
centaur (563) An outer solar system body with
an orbit entirely within the region of the Jovian
planets, for example Chiron, that orbits between
Saturn and Uranus.
center of mass (85) The balance point of a body
or system of bodies.
charge-coupled device (CCD) (113) An electron-
ic device consisting of a large array of light-sensitive
elements used to record very faint images.
chemical evolution (589) The chemical process
that led to the growth of complex molecules on
the primitive Earth. This did not involve the
reproduction of molecules.
Chicxulub (576) The buried crater associated
with the mass extinction event at the end of the
age of dinosaurs, named after the town in the
coastal region of Mexico’s Yucatan peninsula near
the center of the crater.
chondrite (553) A stony meteorite that contains
chondrules.
chondrule (553) Round, glassy body in some
stony meteorites; thought to have solidifi ed very
quickly from molten drops of silicate material.
chromatic aberration (102) A distortion found
in refracting telescopes because lenses focus dif-
ferent colors at slightly different distances. Images
are consequently surrounded by color fringes.
chromosome (585) One of the bodies in a cell that
contains the DNA carrying genetic information.
chromosphere (41) Bright gases just above the
photosphere of the sun.
circular velocity (84) The velocity required to
remain in a circular orbit about a body.
circumpolar constellation (19) Any of the con-
stellations so close to the celestial pole that they
never set (or never rise) as seen from a given lati-
tude.
closed orbit (85) An orbit that returns to its start-
ing point; a circular or elliptical orbit. (See open
orbit.)
coma (568) The glowing head of a comet.
comet (404) One of the small, icy bodies that or-
bit the sun and produce tails of gas and dust when
they near the sun.
comparative planetology (425) The study
of planets by comparing the characteristics of
different examples.
comparison spectrum (113) A spectrum of
known spectral lines used to identify unknown
wavelengths in an object’s spectrum.
composite volcano (472) A volcano built up of lay-
ers of lava fl ows and ash falls. These are steep sided
and typically associated with subduction zones.
condensation (409) The growth of a particle by
addition of material from surrounding gas, one
atom or molecule at a time.
condensation sequence (408) The sequence in
which different materials condense from the solar
nebula at increasing distances from the sun.
constellation (12) One of the stellar patterns
identifi ed by name, usually of mythological gods,
people, animals, or objects; also, the region of the
sky containing that star pattern.
continuous spectrum (132) A spectrum in which
there are no absorption or emission lines.
convection (145) Circulation in a fl uid driven by
heat; hot material rises, and cool material sinks.
convective zone (152) The region inside a star
where energy is carried outward as rising hot gas
and sinking cool gas.
corona (41) The faint outer atmosphere of the
sun; composed of low-density, very hot, ionized
gas. On Venus, round networks of fractures and
ridges up to 1000 km in diameter.
coronae (471) On Venus, large, round geologi-
cal faults in the crust caused by the intrusion of
magma below the crust.
coronagraph (146) A telescope designed to pho-
tograph the inner corona of the sun.
coronal hole (163) An area of the solar surface
that is dark at X-ray wavelengths; thought to be
associated with divergent magnetic fi elds and the
source of the solar wind.
coronal mass ejection (CME) (163) Gas trapped
in the sun’s magnetic fi eld.
cosmic microwave background radiation (182,
375) Radiation from the hot clouds of the big
bang explosion. Because of its large redshift, it ap-
pears to come from a body whose temperature is
only 2.7 K.
cosmic ray (119) A subatomic particle traveling
at tremendous velocity that strikes Earth’s atmo-
sphere from space.
Coulomb barrier (152) The electrostatic force
of repulsion between bodies of like charge; com-
monly applied to atomic nuclei.
Coulomb force (124) The repulsive force between
particles with like electrostatic charge.
critical point (496) The temperature and pressure
at which the vapor and liquid phases of a material
have the same density.
C-type asteroid (564) A type of asteroid common
in the outer asteroid belt, with very low refl ectiv-
ity and grayish color, probably composed of car-
bonaceous material.dark-line spectrum (132) See absorption spectrum.
dark nebula (195) A nonluminous cloud of gas
and dust visible because it blocks light from more
distant stars and nebulae.
debris disk (415) A disk of dust found by infrared
observations around some stars. The dust is de-
bris from collisions among asteroids, comets, and
Kuiper belt objects.
deferent (57) In the Ptolemaic theory, the large
circle around Earth along which the center of the
epicycle moved.
density (137) The amount of matter per unit vol-
ume in a material; measured in grams per cubic
centimeter, for example.
deuterium (151) An isotope of hydrogen in which
the nucleus contains a proton and a neutron.
diamond ring effect (41) A momentary phenom-
enon seen during some total solar eclipses when the
ring of the corona and a bright spot of photosphere
resemble a large diamond set in a silvery ring.
differential rotation (158) The rotation of a body
in which different parts of the body have differ-
ent periods of rotation; this is true of the sun, the
Jovian planets, and the disk of the galaxy.
differentiation (410) The separation of planetary
material according to density.
diffraction fringe (104) Blurred fringe
surrounding any image caused by the wave prop-
erties of light. Because of this, no image detail
smaller than the fringe can be seen.
direct collapse (412) The hypothetical process by
which a Jovian planet might skip the accretion of
a solid core, instead forming quickly and directly
from the gases of the solar nebula.
distance modulus (mV – MV) (172) The difference
between the apparent and absolute magnitude of a
star; a measure of how far away the star is.
DNA (deoxyribonucleic acid) (584) The long
carbon-chain molecule that records information
to govern the biological activity of the organism.
DNA carries the genetic data passed to offspring.
Doppler effect (137) The change in the
wavelength of radiation due to relative radial mo-
tion of source and observer.
Drake equation (596) A formula for the number
of communicative civilizations in our galaxy.
dust (type II) tail (568) The tail of a comet
formed of dust blown outward by the pressure of
sunlight. (See gas tail.)
dwarf planet (546) An object that orbits the sun
and has pulled itself into a spherical shape but has
not cleared its orbital lane of other objects. Pluto
is a dwarf planet.
dynamo effect (158) The process by which a rotat-
ing, convecting body of conducting matter, such
as Earth’s core, can generate a magnetic fi eld.
east point (18) The point on the eastern horizon
exactly halfway between the north point and the
south point; exactly east.
eccentric (55) (noun) An off-center circular path.
(Note that the adjective “eccentric” refers instead
to an ellipse that is not a perfect circle.)