GLOSSARY 625Numbers in parentheses refer to the page where
the term is fi rst discussed in the text.
absolute age (445) An age determined in years, as
from radioactive dating (see also relative age).
absolute zero (127) The lowest possible tempera-
ture; the temperature at which the particles in a
material, atoms or molecules, contain no energy
of motion that can be extracted from the body.
absorption line (132) A dark line in a spectrum;
produced by the absence of photons absorbed by
atoms or molecules.
absorption (dark-line) spectrum (132) A spec-
trum that contains absorption lines.
acceleration (79) A change in a velocity; a change
in either speed or direction. (See velocity.)
acceleration of gravity (77) A measure of the
strength of gravity at a planet’s surface.
accretion (409) The sticking together of solid
particles to produce a larger particle.
achondrite (553) Stony meteorite containing no
chondrules or volatiles.
achromatic lens (102) A telescope lens composed
of two lenses ground from different kinds of glass
and designed to bring two selected colors to the
same focus and correct for chromatic aberration.
active optics (111) Optical elements whose posi-
tion or shape is continuously controlled by com-
puters.
active region (155) An area on the sun where
sunspots, prominences, fl ares, and the like occur.
adaptive optics (109) Computer-controlled tele-
scope mirrors that can at least partially compensate
for seeing.
albedo (437) The fraction of the light hitting an
object that is refl ected.
alt-azimuth mounting (111) A telescope mount-
ing capable of motion parallel to and perpendicu-
lar to the horizon.
Amazonian period (488) On Mars, the
geological era from about 3 billion years ago to
the present marked by low-level cratering, wind
erosion, and small amounts of water seeping
from subsurface ice.
amino acid (584) One of the carbon-chain
molecules that are the building blocks of protein.
Angstrom (Å) (100) A unit of distance; 1 Å = 10-10
m; often used to measure the wavelength of light.
angular diameter (19) A measure of the size of an
object in the sky; numerically equal to the angle
in degrees between two lines extending from the
observer’s eye to opposite edges of the object.
angular distance (19) A measure of the separation
between two objects in the sky; numerically equal
to the angle in degrees between two lines extend-
ing from the observer’s eye to the two objects.
angular momentum (83) The tendency of a
rotating body to continue rotating; mathemati-
cally, the product of mass, velocity, and radius.
Babcock model (158) A model of the sun’s mag-
netic cycle in which the differential rotation of the
sun winds up and tangles the solar magnetic fi eld
in a 22-year cycle. This is thought to be respon-
sible for the 11-year sunspot cycle.
Balmer series (133) Spectral lines in the visible and
near-ultraviolet spectrum of hydrogen produced
by transitions whose lowest orbit is the second.
barred spiral galaxy (178, 334) A spiral galaxy
with an elongated nucleus resembling a bar from
which the arms originate.
basalt (434) Dark, igneous rock characteristic of
solidifi ed lava.
belt–zone circulation (495) The atmospheric
circulation typical of Jovian planets. Dark belts
and bright zones encircle the planet parallel to its
equator.
big bang (181, 373) The theory that the universe
began with a violent explosion from which the
expanding universe of galaxies eventually formed.
binding energy (124) The energy needed to pull
an electron away from its atom.
biological evolution (586) The combined effect
of variation and natural selection resulting in new
species arising and existing species adapting to
the environment or becoming extinct. Also called
Darwinian evolution.
blackbody radiation (127) Radiation emit-
ted by a hypothetical perfect radiator; the spec-
trum is continuous, and the wavelength of
maximum emission depends only on the body’s
temperature.
black holes (173) A mass that has collapsed to
such a small volume that its gravity prevents the
escape of all radiation; also, the volume of space
from which radiation may not escape.
blueshift (138) The shortening of the wavelengths
of light observed when the source and observer
are approaching each other.
Bok globules (170) Small, dark, and dense
interstellar cloud only about 1 ly in diameter that
contains 10 to 100 M of gas and dust; thought
to be related to star formation.
bow shock (431) The boundary between the
undisturbed solar wind and the region being
defl ected around a planet or comet.
breccia (451) A rock composed of fragments of
earlier rocks bonded together.
bright-line spectrum (132) See emission spectrum.
butterfl y diagram (155) See Maunder butterfl y
diagram.
CAI (555) Calcium–aluminum-rich inclusions
found in some meteorites.
Cambrian explosion (589) The sudden appear-
ance of complex life forms at the beginning of
the Cambrian period 0.6 to 0.5 billion years ago.
Cambrian rocks contain the oldest easily identifi -
able fossils.angular momentum problem (399) An objec-
tion to Laplace’s nebular hypothesis that cited the
slow rotation of the sun.
annular eclipse (39) A solar eclipse in which
the solar photosphere appears around the edge
of the moon in a bright ring, or annulus. The
corona, chromosphere, and prominences cannot
be seen.
anorthosite (451) Rock of aluminum and calcium
silicates found in the lunar highlands.
anticoded (595) Message designed to be under-
stood by a recipient about whom the sender
knows little or nothing, for example an interstel-
lar broadcast aimed at possible inhabitants of
another planet.
aphelion (23) The orbital point of greatest dis-
tance from the sun.
apogee (39) The orbital point of greatest distance
from Earth.
Apollo–Amor object (562) Asteroid whose orbit
crosses that of Earth (Apollo) and Mars (Amor).
apparent visual magnitude (mv) (15) The bright-
ness of a star as seen by human eyes on Earth.
archaea (591) A biological kingdom of micro-
organisms similar to, but distinct from, bacteria,
with characteristics most closely resembling those
inferred for the common ancestor of all present-
day Earth life.
arc minute (19) An angular measure; each degree
is divided into 60 arc minutes.
arc second (19) An angular measure; each arc
minute is divided into 60 arc seconds.
archaeoastronomy (50) The study of the
astronomy of ancient cultures.
asterism (13) A named group of stars not
identifi ed as a constellation, e.g., the Big Dipper.
asteroid (401) Small rocky world; most asteroids
lie between Mars and Jupiter in the asteroid
belt.
astrobiology (582) The fi eld of study involving
searches for life on other worlds and investigation
of possible habitats for such life. Also known as
“exobiology.”
astronomical unit (AU) (4) Average distance from
Earth to the sun; 1.5 × 10^8 km, or 93 × 10^6 miles.
atmospheric window (101) Wavelength region
in which Earth’s atmosphere is transparent—at
visual, infrared, and radio wavelengths.
aurora (163) The glowing light display that results
when a planet’s magnetic fi eld guides charged par-
ticles toward the north and south magnetic poles,
where they strike the upper atmosphere and excite
atoms to emit photons.
autumnal equinox (22) The point on the ce-
lestial sphere where the sun crosses the celestial
equator going southward. Also, the time when the
sun reaches this point and autumn begins in the
Northern Hemisphere—about September 22.Glossary