background radiation: See primordial background radiation.
Bailyís beads: Beads of light visible around the rim of the Moon at the beginning and end of a total solar eclipse.† They result from the solar photosphere shining through valleys at the edge of the Moon.
Balmer series: The set of spectral absorption or emission lines resulting from a transition down to or up from the second energy level (first excited level) of hydrogen.
bar: The straight structure across the center of some spiral galaxies, from which the arms unwind.
baryons: Nuclear particles (protons, neutrons, etc.) subject to the strong nuclear force; made of quarks.
basalt: The cone within which a radio telescope is sensitive to radiation.
baseline: The distance between points of observation when it determines the accuracy of some measurement.
beam: The cone within which a radio telescope is sensitive to radiation.
Becklin-Neugebauer object: An object visible only in the infrared in the Orion Molecular Cloud, apparently a very young star.
belts: Dark bands around certain planets, notably Jupiter.
Big Bang theory: A cosmological model, based on Einsteinís general theory of relativity, in which the Universe was once compressed to infinite density and has been expanding ever since.
binary pulsar: A pulsar in a binary system.
binary star: Two stars revolving around each other.
bipolar flow: A phenomenon in young or forming stars in which streams of matter are ejected from the poles.
black body: An object that absorbs all radiation that hits it and emits radiation that exactly follows Planckís law.
black-body curve: A graph of brightness vs. wavelength that follows Planckís law; each such curve corresponds to a given temperature.† Also called a Planck curve.
black-body radiation: Radiation whose distribution in wavelength follows Planckís law, the black-body curve.
black dwarf: A non-radiating ball of gas that results when a white dwarf radiates all its energy.
black hole: A region of space from which, according to the general theory of relativity, neither radiation nor matter can escape.
black hole era: The future era, following the degenerate era, when the only objects (besides photons and subatomic particles) the Universe will contain will be black holes.
blueshifted: A shift of optical wavelengths toward the blue or in all cases towards shorter wavelengths; when the shift is caused by motion, from a velocity of approach.
B-N object: See Becklin-Neugebauer object.
Bohr atom: Niels Bohrís model of the hydrogen atom, in which the energy levels are depicted as concentric circles which the energy levels are depicted as concentric circles of radii that increase as (level number) 2.
bound-fee transition: An atomic transition in which an electron starts bound to the atom and winds up free from it.
brown dwarf: A self-gravitating, self-luminous object, not a satellite and insufficiently massive for nuclear fusion to being (less than 0.08 solar mass).