628 GLOSSARY
hypothesis (66) A conjecture, subject to further
tests, that accounts for a set of facts.
ice line (408) In the solar nebula, the boundary
beyond which water vapor and other compounds
could form ice particles.
infrared radiation (100) Electromagnetic
radiation with wavelengths intermediate between
visible light and radio waves.
intercrater plain (459) The relatively smooth ter-
rain on Mercury.
interferometry (112) The observing technique in
which separated telescopes combine to produce a
virtual telescope with the resolution of a much-
larger-diameter telescope.
interstellar medium (ISM) (168, 193) The gas
and dust distributed between the stars.
interstellar reddening (196) The process in
which dust scatters blue light out of starlight and
makes the stars look redder.
intrinsic brightness (171) The true brightness
of an object independent of its distance. Also re-
ferred to as luminosity.
inverse square law (81) The rule that the strength
of an effect (such as gravity) decreases in propor-
tion as the distance squared increases.
Io fl ux tube (498) A tube of magnetic lines and
electric currents connecting Io and Jupiter.
Io plasma torus (498) The doughnut-shaped
cloud of ionized gas that encloses the orbit of Ju-
piter’s moon Io.
ion (124) An atom that has lost or gained one or
more electrons.
ionization (124) The process in which atoms lose
or gain electrons.
iron meteorite (552) A meteorite composed
mainly of iron–nickel alloy.
irregular galaxy (179, 335) A galaxy with a cha-
otic appearance, large clouds of gas and dust, and
both population I and population II stars, but
without spiral arms.
irregular satellite (495) A moon with an orbit that
has large eccentricity and/or high inclination to the
equator of its parent planet and/or is retrograde. Ir-
regular moons are thought to been captured.
isotopes (124) Atoms that have the same number
of protons but a different number of neutrons.
joule (J) (86) A unit of energy equivalent to a
force of 1 newton acting over a distance of 1 me-
ter; 1 joule per second equals 1 watt of power.
Jovian planet (402) Jupiter-like planet with large
diameter and low density.
jumbled terrain (453) Disturbed regions of the
moon’s surface opposite the locations of the Im-
brium Basin and Mare Orientale, possibly due to
focusing of seismic waves from the large impacts
that formed those basins.
Kelvin temperature scale (127) The tempera-
ture, in Celsius (centigrade) degrees, measured
above absolute zero.
kinetic energy (86) Energy of motion. Depends
on mass and velocity of a moving body.
Kirchhoff ’s laws (132) A set of laws that describe
the absorption and emission of light by matter.
Kirkwood’s gaps (562) Regions in the asteroid
belt in which there are very few asteroids; caused
by orbital resonances with Jupiter.
Kuiper belt (404) The collection of icy planetesi-
mals that orbit in a region from just beyond Nep-
tune out to about 50 AU.
large-impact hypothesis (454) The hypoth-
esis that the moon formed from debris ejected
during a collision between Earth and a large
planetesimal.
late heavy bombardment (452) The surge in cra-
tering impacts in the solar system that occurred
about 3.8 billion years ago.
L dwarf (135) A type of star that is even cooler
than the M stars.
light curve (183) A graph of brightness versus
time commonly used in analyzing variable stars
and eclipsing binaries.
light-gathering power (103) The ability of a tele-
scope to collect light; proportional to the area of
the telescope objective lens or mirror.
light pollution (106) The illumination of the night
sky by waste light from cities and outdoor lighting,
which prevents the observation of faint objects.
light-year (ly) (4) The distance light travels in
one year.
limb (145, 444) The edge of the apparent disk of
a body, as in “the limb of the moon.”
limb darkening (145) The decrease in bright-
ness of the sun or other body from its center to
its limb.
line of nodes (44) The line across an orbit con-
necting the nodes; commonly applied to the orbit
of the moon.
liquid metallic hydrogen (496) A form of hydro-
gen under high pressure that is a good electrical
conductor.
lobate scarp (457) A curved cliff such as those
found on Mercury.
lunar eclipse (33) The darkening of the moon
when it moves through Earth’s shadow.
Lyman series (133) Spectral lines in the ultravio-
let spectrum of hydrogen produced by transitions
whose lowest orbit is the ground state.
magnetic carpet (147) The widely distributed,
low-level magnetic fi eld extending up through the
sun’s visible surface.
magnetosphere (431) The volume of space
around a planet within which the motion of
charged particles is dominated by the planetary
magnetic fi eld rather than the solar wind.
magnifying power (105) The ability of a tele-
scope to make an image larger.
magnitude scale (14) The astronomical bright-
ness scale; the larger the number, the fainter the
star.
main sequence (169) The region of the H–R
diagram running from upper left to lower right,
which includes roughly 90 percent of all stars.mantle (426) The layer of dense rock and metal
oxides that lies between the molten core and Earth’s
surface; also, similar layers in other planets.
mare(mä ́ra ̄)(442) (plural: maria) One of the
lunar lowlands fi lled by successive fl ows of dark
lava; from the Latin word for sea.
mass (79) A measure of the amount of matter
making up an object.
Maunder butterfl y diagram (155) A graph
showing the latitude of sunspots versus time; fi rst
plotted by W. W. Maunder in 1904.
Maunder minimum (155) A period of less nu-
merous sunspots and other solar activity from
1645 to 1715.
meteor (404) A small bit of matter heated by fric-
tion to incandescent vapor as it falls into Earth’s
atmosphere.
meteorite (405) A meteor that has survived its
passage through the atmosphere and strikes the
ground.
meteoroid (405) A meteor in space before it
enters Earth’s atmosphere.
meteor shower (556) An event lasting for hours
or days in which the number of meteors entering
Earth’s atmosphere suddenly increases. The me-
teors in a shower have a common origin and are
traveling through space on nearly parallel paths.
microlensing (418) Brightening of a background
star due to focusing of its light by the gravity of
a foreground extrasolar planet, allowing the plan-
et to be detected and some of its characteristics
measured.
micrometeorite (447) Meteorite of microscopic
size.
midocean rise (434) One of the undersea moun-
tain ranges that push up from the seafl oor in the
center of the oceans.
Milankovitch hypothesis (26) The hypothesis
that small changes in Earth’s orbital and rotation-
al motions cause the ice ages.
Milky Way (6, 176) The hazy band of light that
circles the sky, produced by the combined light of
billions of stars in our Milky Way Galaxy.
Milky Way Galaxy (6, 176) The spiral galaxy con-
taining the sun; visible at night as the Milky Way.
Miller experiment (587) An experiment that re-
produced the conditions under which life began
on Earth and amino acids and other organic com-
pounds were manufactured.
molecule (124) Two or more atoms bonded
together.
momentum (79) The tendency of a moving
object to continue moving; mathematically, the
product of mass and velocity.
morning star (24) Any planet visible in the sky
just before sunrise.
M-type asteroid (564) A type of asteroid with
relatively high refl ectivity and grayish color, prob-
ably composed primarily of metal.
multicellular (589) An organism composed of
many cells.