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Balmer lines  - Emission or absorption lines in the spectrum of hydrogen that arise from transitions between the second (or first excited) and higher energy states of the hydrogen atom.
bands (in spectra)  - Emission or absorption lines, usually in the spectra of chemical compounds, so numerous and closely spaced that they coalesce into broad emission or absorption bands. 
bar - A force of 100,000 newtons acting on a surface area of 1 square meter is equal to 1 bar. The average pressure of the Earth's atmosphere at sea level is equal to 1.013 bars. 
barred spiral galaxy  - Spiral galaxy in which the spiral arms begin from the ends of a "bar" running through the nucleus rather than from the nucleus itself. 
barycenter - The center of mass of two mutually revolving bodies.
basalt - Igneous rock, composed primarily of silicon, oxygen, iron, aluminum, and magnesium produced by the cooling of lava. Basalts make up most of Earth's oceanic crust and are also found on other planets that have experienced extensive volcanic activity. 
Big Bang theory  - A theory of cosmology in which the expansion of the universe is presumed to have begun with a primeval explosion. 
binary star  - Two stars revolving about each other. 
binding energy  - The energy required to separate completely the constituent parts of an atomic nucleus.
blackbody - A hypothetical perfect radiator, which absorbs and re-emits all radiation incident upon it. 
black dwarf  - A presumed final state of evolution for a low-mass star, in which all of its energy sources are exhausted and it no longer emits significant radiation.
black hole  - A collapsed massive star (or other collapsed body) whose velocity of escape is equal to or greater than the speed of light; thus no radiation can escape from it. 
Bohr atom  - A particular model of an atom, invented by Niels Bohr, in which the electrons are described as revolving about the nucleus in circular orbits. 
brown dwarf  - An object intermediate in size between a planet and a star. The approximate mass range is from about 1/100 of the mass of the Sun up to the lower mass limit for self-sustaining nuclear reactions, which is 0.08 solar mass. 
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