A star with a detectable change in brightness that is often accompanied by other physical changes. Every star varies in brightness sometime during its life, usually in the early stages (while it is forming) or in the late stages (close to its death). Therefore, variability provides important clues about the evolution and nature of stars. Depending upon the type of star, variability in brightness can provide information about its size, radius, mass, temperature, luminosity, internal and external structure, composition, and distance from the Earth.