GLOSSARY 341
Degeneracy pressure An
outward pressure within a
concentrated ball of gas, such
as a collapsed star, that is exerted
due to the principle that no two
particles with mass can exist in
the same quantum state.
Doppler effect The change in
frequency of radiation experienced
by an observer in relative motion
to the source of the radiation.
Dwarf planet An object in orbit
around a star that is large enough to
have formed a spherical shape but
that has not cleared its orbital path of
other material. Examples in the solar
system include Pluto and Ceres.
Dwarf star Also called a main
sequence star, a star that shines by
converting hydrogen to helium. About
90 percent of stars are dwarf stars.
Eclipse The blocking of light
from one celestial body, caused
by another body passing between
it and an observer, or it and a light
source that it reflects.
Ecliptic The apparent path along
which the sun travels across the
celestial sphere. It is equivalent
to the plane of Earth’s orbit.
Electromagnetic radiation
Waves that carry energy through
space in the form of oscillating
electric and magnetic disturbances.
The electromagnetic spectrum
ranges from short, high-energy
gamma rays to long, low-energy
radio waves, and includes the
visible spectrum.
Electron A subatomic particle
with negative charge. In an atom,
a cloud of electrons orbits a central,
positively charged nucleus.
Equinox A twice-yearly occasion
when the sun is directly overhead
at a planet’s equator, meaning that
day and night are of roughly equal
duration across the entire planet.Escape velocity The minimum
velocity an object needs to be
traveling at to escape the
gravitational pull of a larger
body such as a planet.Event horizon A boundary
around a black hole beyond
which no mass or light can
escape its gravity. At this point,
the escape velocity of the black
hole equals the speed of light.Exoplanet A planet that orbits
a star other than the sun.Fraunhofer lines Dark absorption
lines found in the spectrum of the
sun, first identified by German
Joseph von Fraunhofer in the
19th century.Galaxy A large collection of stars
and clouds of gas and dust that is
held together by gravity.Galilean moon One of the four
biggest moons of Jupiter, first
discovered in 1610 by Galileo.General theory of relativity
A theory that describes gravity
as a warping of spacetime by
the presence of mass. Formulated
by Albert Einstein in 1916,
many of its predictions, such as
gravitational waves, have now
been confirmed experimentally.Geocentric Of a system or an orbit,
treated as having Earth at the center.Gnomon The part of a sundial
that casts a shadow.Gravitational wave A
distortion of space that travels
at the speed of light, generated
by the acceleration of mass.Harvard Spectral Classification
A system first devised by the
Harvard Observatory in the late
19th century to classify stars by
the appearance of their spectra.Heliocentric Of a system or an
orbit, treated as having the sun
at the center.Hertzsprung–Russell diagram
A scatter diagram on which stars
are plotted according to their
luminosity and surface temperature.Hubble’s law The observed
relationship between the redshifts
and distances of galaxies, which
shows galaxies receding with
a velocity proportional to their
distance. The number that
quantifies the relationship is
called Hubble’s constant (H 0 ).Inflation A short period of rapid
expansion that the universe
is thought to have undergone
moments after the Big Bang.Ionization The process by which
an atom or molecule gains or loses
electrons to gain a positive or
negative charge. The resultant
charged particles are called ions.Kepler’s laws of planetary
motion Three laws devised by
Johannes Kepler to describe the
shapes and speeds of the orbits
of the planets around the sun.Kuiper belt A region of space
beyond Neptune in which a large
number of comets orbit the sun. It
is the source of short-period comets.