Antiparticle: A particle of opposite charge but otherwise identical to its partner. Most of the observable universe consists of particles and matter, as opposed to antiparticles and antimatter.
Asteroids: Small planetlike bodies of the solar system.
Atom: The basic building block of matter. Each atom consists of a nucleus with positive electric charge and a surrounding cloud of electrons with negative charge.
Baryon: Type of hadron, consisting of proton, neutron, and the unstable hyperons (and their antiparticles).
Binary System: A system of two objects orbiting around a common center. The objects may be stars or black holes or a star and a black hole.
Black Hole: A region in which matter has collapsed to such an extend that light can no longer escape from it.
Boson: A class of particles with integer units of the basic unit of spin h/2.
Cluster of Galaxies: An aggregate of galaxies. Clusters may range in richness from loose groups, such as the Local Group, with 10 to 100 members, to great clusters of over 1000 galaxies.
Cluster of Stars (Globular Star Cluster): An aggregate of stars. Globular star clusters contain the oldest stars in the galaxy and high-velocity stars.
Comet: A diffuse body of gas and solid particles that orbit the Sun in a highly eccentric trajectory.
Cosmic Microwave Background Radiation: Diffuse isotrope radiation whose spectrum is that of a blackbody at 3 degrees kelvin and consequently is most intense in the microwave region of the spectrum.
Cosmic String: A hypothetical one-dimensional, string-like object that is made from a curvature of space.
Cosmogony: The study of the origin of celestial systems, ranging from the solar system to stars, galaxies, and clusters of galaxies.
Cosmology: The study of the large-scale structure and evolution of the universe.
Dark Matter: Matter whose presence is inferred from dynamical measurements but which has no optical counterpart.
Electron: A particle of matter with negative electric charge. All chemical properties of atoms and molecules are determined by the electrical interactions of electrons with each other and with the atomic nuclei.
Fermion: A particle with half integral units of the basic unit of spin, h/2.
Galactic Nucleus: Innermost region of a galaxy exhibiting a concentration of stars and gas.
Galaxy: A large gravitationally bound cluster of stars that all orbit around a common center. Galaxies are basic building blocks of the universe.
Graviton: The particle which, according to wave-particle duality, is associated with gravitational waves.
Group of Galaxies: Gravitationally bound system of few galaxies.
Group of Stars: Gravitationally bound system of few stars (multiple stars).
HI Cloud: Cloud of cool, neutral hydrogen.
HII Region: Cloud of hot, ionized hydrogen, usually heated by a massive young hot star.
Hadron: Particle (protons, neutrons, mesons) which takes part in strong nuclear reactions.
Halo: The diffuse, nearly spherical cloud of old stars and globular clusters that surrounds a spiral galaxy.
Hertzsprung-Russell Diagram: Plot of stellar luminosity (or absolute magnitude) against effective temperature (or color), in which the evolution of stars of different masses may be followed.
Hubble Classification of Galaxies: Organizes galaxies according to shape. They range from amorphous, relatively uniform elliptical systems to highly flattened spiral disks with prominent nuclei. It is not a classification based on evolution but one of different rates of star formation.
Hypergalaxy: A system consisting of a dominant spiral galaxy surrounded by a cloud of dwarf satellite galaxies, often ellipticals. The Milky Way and the Andromeda galaxy are hypergalaxies.
Intergalactic Gas: Matter that is present in the region between galaxies. It has been detected in considerable amounts in great clusters of galaxies, where the intergalactic gas is so hot that it emits copious amounts of x rays.
Interstellar Grains: Small needle-shaped particles in the interstellar gas with dimensions from 10-6 to 10-5 cm. They are primarily composed of silicates and strongly absorb, scatter, and polarize visible light in the far-infrared region of the spectrum.
Irregular Galaxy: A galaxy without spiral structure or smooth, spheroidal shape, often filamentary or very clumpy, and generally of low mass [(107 to 1010)M].
Lepton: Particles (neutrinos, electrons, muons) which do not take part in strong interactions.
Local Group: Small group of 30 or so galaxies of which the Milky Way and the Andromeda galaxy are the two dominant members.
Meson: A class of strongly interacting particles with zero baryon number, among them the pi meson.
Meteorites: A solid portion of a meteoroid that has reached the Earth's surface.
Molecular Cloud: An interstellar cloud consisting predominantly of molecular hydrogen, with trace amounts of other molecules such as carbon monoxide and ammonia.
Molecule: An entity made of several atoms that share their electron clouds with each other.
Neutrino: A particle that resembles the photon, except that it interacts weakly with matter. Neutrinos come in at least three varieties, known as electron-type, muon-type, and tauon-type.
Neutron: The uncharged particle found along with protons in atomic nuclei.
Neutron Star: Cold, degenerate, compact star in which nuclear fuels have been exhausted and pressure support against gravity is provided be the pressure of neutrons.
Nucleon: Nuclear particles, i.e. neutrons and protons.
Observable Universe: The extend of the universe that we can see with the aid of large telescopes. Its ultimate boundary is determined by the horizon size.
Photon: A discrete unit of electromagnetic energy. The particle which, according to wave-particle duality, is associated with electromagnetic energy.
Population I: Young stars with relatively high abundances of metals, found in the disk of a galaxy, especially the spinal arms and in dense regions of interstellar gas. The Sun is a Population I star.
Population II: Old stars with relatively low abundances of metals, found in the nuclear bulge of a galaxy or in globular clusters.
Positron: The antiparticle of an electron, having positive charge but being otherwise similar.
Proton: The positively charged particle found along with neutrons in atomic nuclei.
Pulsar: A magnetized, spinning neutron star that emits a beam of radiation; radio waves and sometimes also light and x rays).
Quark: Particles of which all hadrons are supposed to be composed.
Quasar: Luminous and compact quasi-stellar radio source related to violent events in the nuclei of a galaxy, believed to be powered by a massive black hole.
Red Giant: Phase in star's evolution after completion of hydrogen burning when outer layers become very extended.
Spiral Galaxy: A galaxy with a prominent nuclear bulge and luminous spiral arms of gas, dust, and young stars that wind out from the nucleus.
Star: Stars are basic building blocks of the universe.
Supercluster: A cluster of clusters of galaxies.
Universe: Our universe.
White Dwarf: Cool, degenerate, compact star, in which nuclear fuels are exhausted and pressure support against gravity is provided by the degeneracy pressure of electrons.