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Absolute magnitude A measure
of the intrinsic brightness of a
star. It is defined as the apparent
magnitude of the star from a distance
of 10 parsecs (32.6 light-years).
Accretion The process by
which smaller particles or
bodies collide and join together
to form larger bodies.
Aphelion The point on its elliptical
orbit around the sun at which a
planet, asteroid, or comet is
farthest from the sun.
Apparent magnitude A measure
of the brightness of a star as seen
from Earth. The fainter the object,
the higher the value of its apparent
magnitude. The faintest stars
visible to the naked eye are of
a magnitude 6.
Armillary sphere An instrument
that models the celestial sphere.
At its center is Earth or the sun,
around which is a framework of
rings representing lines of celestial
longitude and latitude.
Asteroid A small body that
orbits the sun independently.
Asteroids are found throughout
the solar system, with the greatest
concentration in the asteroid belt
between the orbits of Mars and
Jupiter. Their diameters range from
a few yards to 600 miles (1,000 km).
Astronomical unit (AU)
A distance equal to the average
distance between Earth and the
sun. 1 AU = 92,956,000 miles
(149,598,000 km).
Big Bang The event with which
the universe is thought to have
begun, at a particular time in the
past, from a hot, dense initial state.Black body A theoretical, idealized
body that absorbs all the radiation
that falls on it, reflecting nothing.
A black body would emit a
spectrum of radiation with
a peak at a particular wavelength,
depending on its temperature.Black hole A region of spacetime
surrounding a mass that is so
dense that its gravitational pull
allows no mass or radiation to
escape from it.Blueshift A shift in a spectrum
of light or other radiation toward
shorter wavelengths that occurs
when the source of the light is
moving toward the observer.Bok globule Small, dark clouds of
cold gas and dust, within which it is
thought that new stars are forming.Brown dwarf A starlike ball
of gas that is not massive enough
to sustain nuclear fusion in its core.Celestial sphere An imaginary
sphere surrounding Earth. The
positions of stars and other celestial
bodies can be defined by their
places on this sphere if they were
imagined to be attached to it.Cepheid variable A pulsating
star whose brightness increases
and decreases over a regular
period. The more luminous it is, the
longer the period of its variation.Comet A small, icy body in orbit
around the sun. When a comet
approaches the sun, gas and dust
evaporate from its nucleus (solid
core) to produce a cloud called
a coma and one or more tails.Constellation One of 88 named
regions on the celestial sphere,
containing an identifiable pattern
of naked-eye stars.Cosmic Microwave Background
(CMB) Faint microwave radiation
that is detectable from all directions.
The CMB is the oldest radiation
in the universe, emitted when the
universe was 380,000 years old.
Its existence was predicted by the
Big Bang theory, and it was first
detected in 1964.Cosmic rays Highly energetic
particles, such as electrons and
protons, that travel through space
at close to the speed of light.Cosmological constant A term
that Albert Einstein added to
his general relativity equations,
which may correspond to the dark
energy that is accelerating the
expansion of the universe.Dark energy A little-understood
form of energy that exerts a repulsive
force, causing the expansion of the
universe to accelerate.Dark matter A form of matter that
does not emit radiation or interact
with other matter in any way other
than through the effect of its gravity.
It comprises 85 percent of all mass
in the universe.GLOSSARY