B ring One of three Saturnian rings visible from Earth. The B ring is the brightest of the three, and lies just within the Cassini division, closer to the planet than the A ring.

barred-spiral galaxy Spiral galaxy in which a bar of material passes through the center of the galaxy, with the spiral arms beginning near the ends of the bar.

baseline The distance between two observing locations used for the purposes of triangulation measurements. The larger the baseline, the better the resolution attainable.

belt Dark, low-pressure region in the atmosphere of a jovian planet where gas flows downward.

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 in orbit about their common center of mass, held together by their mutual gravitational attraction. Most stars are found in binary-star systems.

blackbody curve The characteristic way in which the intensity of radiation emitted by a hot object 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 dwarf The end-point of the evolution of an isolated, low-mass star. After the white dwarf stage, the star cools to the point where it is a dark clinker in interstellar space.

black hole A region of space where the pull of gravity is so great that nothing-not even light-can escape. A possible outcome of the evolution of a very massive star.

blue giant Large, hot, bright star at the upper left end of the main sequence on the Hertzsprung–Russell diagram. Its name comes from its color and size.

blue shift Motion-induced changed 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 at the uppermost left end of the main sequence on the Hertzsprung–Russell diagram.

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 for the electron, 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 Remnant of a fragment of collapsing gas and dust that did not contain enough mass to initiate core nuclear fusion. Such objects are frozen somewhere along their pre-main-sequence contraction phase, continually cooling into compact dark objects. Because of their small sizes and low temperatures they are extremely difficult to detect observationally.

brown oval Feature of Jupiter's atmosphere that appears only at latitudes near 20 degrees N, this structure is a long-lived hole in the clouds that allows us to look down into Jupiter's lower atmosphere.