Big Bang : Event that cosmologists consider the beginning of the universe, in which all matter and radiation in the entire universe came into being.
Binary-star System : A system which consists of two stars orbit about their common center of mass, held together by their mutual gravitational attraction. Most stars are found in binary star systems.
Black-body Radiation : The characteristic way in which the intensity of radiation emitted by a hot object in thermal equilibrium depends on frequency. The frequency at which the emitted intensity is highest is an indication of the temperature of the radiating object. Also referred to as the Planck curve.
Black-holes : A dense, compact object whose gravitational pull is so strong that - within a certain distance of it - nothing can escape, not even light. Black holes are thought to result from the collapse of certain very massive stars at the ends of their evolution.
Blazars : a class of active galaxies that exhibit rapidly variable emission from the radio through gamma-ray band. The radiation is predominantly from jets moving near the speed of light. Blazars are thought to be radio galaxies with their jets oriented toward Earth.
Blue-giant : Large, hot, bright star at the upper left end of the main sequence on the H-R diagram. Its name comes from its color and size.
Blue-shift : Motion-induced change in the observed wavelength from a source that is moving toward us. Relative approaching motion between the object and the observer causes the wavelength to appear shorter (and hence bluer) than if there were no motion at all.
Blue Supergiant : The very largest of the large, hot, bright, stars.
Bohr model : First theory of the hydrogen atom to explain the observed spectral lines. This model rests on three ideas: that there is a state of lowest energy, that there is a maximum energy, beyond which the electron is no longer bound to the nucleus, and that within these two energies the electron can only exist in certain energy levels.
Brown Dwarf : Clouds of collapsing gas and dust that did not contain enough mass to initiate core nuclear fusion. Such objects are then frozen somewhere along their pre-main sequence contraction phase, continually cooling into compact dark objects. Because of their small size and low temperature, they are extremely difficult to detect observationally.