Stars Variable stars are stars which vary in brightness. Stars vary in brightness because of pulsations, because of eruptions, or because of geometric factors such as eclipses in a binary star system. Observations of this brightness variation may be used to determine the type of variable star. There are more than a dozen different types of variable stars.
Some stars are periodic with periods ranging from several hours to several decades.
Some variables, however, have no well defined periods. The amplitude of the light variation can be as small as a few tenths or hundredths of a magnitude, or can be as large as 18 or 20 magnitudes. As a general rule, the greater the amplitude, the longer the period. It turns out that variable stars are often stars in very special phases of their evolution. A very simple method requires the availability of a bright and a faint comparison star.