Introduction
variable star, star that varies, either periodically or irregularly, in the intensity of the light it emits. Other physical changes are usually correlated with the fluctuations in brightness, such as pulsations in size, ejection of matter, and changes in spectral type, color, or temperature. The class to which a variable star belongs is determined by a plot of its light curve, which is a graph of the star's apparent brightness versus time. The light curve reveals its maximum and minimum brightness and gives evidence for periodicity, if any exists. The approximately 30,000 known variable stars are grouped into three broad classes: the pulsating variables and the eruptive variables (both of which are intrinsic variables, because the variation is caused by a physical change within the star) and the eclipsing variables (which are extrinsic variables, because the variation is caused by two or more bodies eclipsing one another).
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