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Variable stars are stars that vary in brightness over time. In most cases, these changes occur very slowly over a period of months or even years. In some cases, however, the changes take place in a matter of hours.

The category variable stars encompasses several different types of stars that vary in brightness for entirely different reasons. Examples include red giants, eclipsing binaries, Cepheid variables, and RR Lyrae.

The most common variables, with the longest bright-dim cycles, are red giants. Red giants are stars of average size (like the Sun) in the final stages of life. During the last several million years of its multibillion-year lifetime, a red giant will puff up and shrink many times. It becomes al ternately brighter and dimmer, generally spending about one year in each phase until it completely runs out of fuel to burn.

The apparent variable...