A relatively flat, rapidly rotating disk of gas surrounding
a black hole, a newborn star, or any massive object that attracts and swallows
matter. Accretion disks around stars are expected to contain dust particles
and may show evidence of active planet formation. Beta Pictoris is an example
of a star known to have an accretion disk.
Binary Star System
A system of two stars orbiting around a common center
of mass that are bound together by their mutual gravitational attraction.
A massive, hot star that appears blue in color. Spica
in the constellation Virgo is an example of a blue star.
An object too small to be an ordinary star because it
cannot produce enough energy by fusion in its core to compensate for the radiative
energy it loses from its surface. A brown dwarf has a mass less than 0.08 times
that of the Sun.
A type of pulsating star whose light and energy output
vary noticeably over a set period of time. The time period over which the star
varies is directly related to its light output or luminosity, making these stars
useful standard candles for measuring intergalactic distances.
Dark Dust Cloud
A region of interstellar space that contains a rich concentration
of gas and dust. Such a cloud is often irregular in shape but sometimes has
a well-defined edge. Visible light cannot pass through these clouds, so they
obscure the light from stars beyond them.
A glowing cloud of gas in interstellar space. The cloud
of gas may be either an emission nebula, which absorbs ultraviolet light from
nearby stars and re-radiates visible light, or a reflection nebula, which reflects
light off of its dust particles.
A dying star that has used up the hydrogen fuel in its
core and has begun to expand. Giant stars are generally larger than our Sun.
A collection of hundreds of thousands of old stars held
together by gravity. Globular clusters are usually spherically shaped and are
often found in the halos of galaxies. Each star belonging to a cluster revolves
around the cluster’s common center of mass.
A plot showing the relationship between the brightness
(luminosity) and the surface temperatures of many stars. Often the spectral
class, which is based on the temperature of the star, is used as a label.
Small particles of solid matter, similar to smoke, in
the space between stars.
Interstellar Medium (ISM)
The sparse gas and dust located between the stars of a
The dark regions of space located between the stars.
A plot showing how the light output of a star (or other
variable astronomical object) changes with time.
A relatively dense, cold region of interstellar matter
where hydrogen gas is primarily in molecular form. Stars generally form in molecular
clouds. Molecular clouds appear as dark blotches in the sky because they block
all the light behind them.
An extremely compact ball of neutrons created from the
central core of a star that collapsed under gravity during a supernova explosion.
Neutron stars are extremely dense: they are only 10 kilometers or so in size,
but have the mass of an average star (usually about 1.5 times more massive than
our Sun). A neutron star that regularly emits pulses of radiation is known as
A binary star system (consisting of a white dwarf and
a companion star) that rapidly brightens, then slowly fades back to normal.
A relationship that describes how the luminosity or absolute
brightness of a Cepheid variable star depends on the period of time over which
that brightness varies.
An expanding shell of glowing gas expelled by a star late
in its life. Our Sun will create a planetary nebula at the end of its life.
A collection of interstellar gas and dust whose gravitational
pull is causing it to collapse on itself and form a star.
A neutron star that emits rapid and periodic pulses of
Red Giant Star
An old, bright star, much larger and cooler than the Sun.
Betelgeuse (alpha Orionis) is an example of a red giant.
Spectral Class (Spectral
A classification scheme that groups stars according to
their surface temperatures and spectral features.
In a spectrum, an emission (bright) or absorption (dark)
at a specific frequency or wavelength.
A huge ball of gas held together by gravity. The central core of a star is extremely hot and produces energy. Some of this energy is released as visible light, which makes the star glow. Stars come in different sizes, colors, and temperatures. Our Sun, the center of our solar system, is a yellow star of average temperature and size.
A group of stars born at almost the same time and place,
capable of remaining together for billions of years because of their mutual
A galaxy undergoing an extremely high rate of star formation.
Starburst galaxies contain massive, deeply embedded stars that are among the
youngest stars observed.
The process of change that occurs during a star’s
lifetime from its birth to its death.
The explosive death of a massive star whose energy output causes its expanding gases to glow brightly for weeks or months. A supernova remnant is the glowing, expanding gaseous remains of a supernova explosion.
The glowing, expanding gaseous remains of a supernova
A class of very young, flaring stars on the verge of becoming
normal stars fueled by nuclear fusion.
A star whose luminosity (brightness) changes with time.
White Dwarf Star
The hot, compact remains of a low-mass star like our Sun
that has exhausted its sources of fuel for thermonuclear fusion. White dwarf
stars are generally about the size of the Earth.