A star whose actual or observed brightness varies periodically. These changes can occur with varying degrees of regularity and intensity, over times ranging from a fraction of a second to many years. Intrinsic variation occurs because of changes of the star itself, often due to internal vibration or eruptions, or to influx of nearby material. Cepheids and novae are examples of intrinsically variable stars. Extrinsic variation in a star's observed brightness that does not reflect physical changes in the star also occur, as when a darker star periodically eclipses a brighter star in an eclipsing binary system, or with the rotation of a star.