A variable star is one whose brightness (or other physical property such as radius or spectral type) changes over time. Intrinsic variables are those in which the change in brightness is due to some change within the star itself such as in pulsating stars like the Cepheids. Extrinsic variables are those in which the light output changes due to some process external to the star itself. The most common example of these are the eclipsing binaries.