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Published online by Cambridge University Press: 30 March 2016
Be stars are B stars in which emission has been observed in at least one hydrogen line on at least one occasion. Some Be stars are pre-main-sequence stars, mass-transfer binaries, or supergiant stars with extended atmospheres. The majority, however, are classical Be stars: single stars on or near the main sequence. An important characteristic of these stars seems to be their rapid rotation – close to but not at the “critical” velocity at which the effective gravity vanishes at the equator.