ARM In galaxies, a structure composed of gas, dust, and stars which winds out from near the galaxy's center in a spiral pattern.
ACTIVE GALAXY A galaxy whose center emits large amounts of excess energy, often in the form of radio emissions. Active galaxies are suspected of having massive black holes in their centers into which matter is flowing.
BIG BANG THEORY A theory in which the expansion of the universe is presumed to have begun with an explosion (referred to as the "Big Bang").
BLACK HOLE An object whose gravity is so strong that not even light can escape from it.
BULGE The round or elliptical central region of a galaxy. It is often uniform in brightness.
CORONA The hot, tenuous, outermost region of the Sun and other stars. The Suns corona is visible during a total solar eclipse.
CONSTELLATION A grouping of stars into one of the 88 areas of the sky.
DARK MATTER A form of matter which does not emit light. Its nature is still being investigated.
DISK The flat, circular region of a spiral galaxy extending out from the central bulge. The disk of a spiral galaxy often has distinct arms of stars and bright gas.
DOPPLER SHIFT The apparent change in wavelength of sound or light caused by the motion of the source, observer or both. Waves emitted by a moving object as received by an observer will be blueshifted (compressed) if approaching, redshifted (elongated) if receding.
FINDERSCOPE A small, low-power telescope with a wide field of view, attached to the main telescope.
GALAXY CLUSTER A group of galaxies bound together by gravity.
GLOBULAR CLUSTER A spherical bundle of stars that orbits a galaxy as a satellite. Globular clusters are very tightly gravitationally bound, which gives them their spherical shape, and extremely dense (in relative terms) towards their core.
GRAVITY A mutual physical force attracting two bodies.
LIGHT YEAR The distance light travels in one year, which is approximately 9.46 x 1015 meters.
MORPHOLOGY The study of the shape and structure of galaxies.
NEBULA A diffuse mass of interstellar dust and gas.
NEUTRON STAR The imploded core of a massive star sometimes produced by a supernova explosion. Neutron stars typically have a mass 1.4 times the mass of the Sun, and a radius of about 5 miles. Neutron stars can be observed as pulsars.
OPEN CLUSTER A group of stars that were born at the same time from a molecular cloud, and are still near to each other. They are also called galactic clusters since they exist within the galaxy's disk.
PARSEC A distance equal to 3.26 light years, or 3.1 x 1018 cm. A kiloparsec (kpc) is equal to 1000 parsecs. A megaparsec (Mpc) is equal to a million (106) parsecs.
PLANETARY SYSTEM A star with one or more planets. This system may include moon(s), comet(s), meteoroids, and asteroid belts in addition to planets.
PLANISPHERE A handheld device which shows the appearance of the night sky at any specified time of day and day of the year.
ROTATION CURVE A graph of stellar velocity versus stellar distance from the center of a galaxy.
SEYFERT GALAXY A spiral galaxy whose nucleus shows bright spectral emission lines in all wavelengths; a class of galaxies first described by C. Seyfert.
SPECTRA A plot of the intensity of light at different frequencies.
STELLAR REGION A region in space consisting of hundreds to thousands of stars.
SUPERCLUSTER A collection of clusters of galaxies.
SUPERNOVA The death explosion of a massive star, resulting in a sharp increase in brightness followed by a gradual fading. At peak light output, supernova explosions can outshine a galaxy. The outer layers of the exploding star are blasted out in a radioactive cloud. This expanding cloud, visible long after the initial explosion fades from view forms a supernova remnant (SNR).
TELESCOPE A tool used to make dim objects look brighter and smaller objects look larger.
X-RAY A form of light with a wavelength between that of ultraviolet radiation and gamma rays.
X-RAY BINARY SYSTEM A binary star system contain contains a normal star and a collapsed star. The collapsed star may be a white dwarf, neutron star, or black hole. X-rays are emitted from the region around the collapsed star.