baryons Heavy subatomic particles, such as protons and neutrons, that make up the nuclei of atoms.
Big Bang The event that most cosmologists consider to have been the beginning of the universe, in which space-time originated in a state of enormously high temperature and density and subsequently expanded and cooled.
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. [More Info: Field Guide]
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. [More Info: Field Guide]