A term used to describe a very common phenomenon whereby a star'sbrightness, as measured by its magnitude, varies periodically. This might be the result of numerous possible causes, including internal forces within the star itself, or the presence of one or more invisible companion objects.
Delta Cephei, in the northern constellation of Cepheus, is an important example of a variable star. Over a period of five days and nine hours, physical processes in the star's outer layers cause its light to pulsate, changing its brightness by almost a full degree of magnitude. The pulsation periods in stars like this (named Cepheid variables after this prototype star) are dependent on their luminosity, which makes it possible to calculate the distance to a Cepheid variable with extreme precision. Delta Cephei itself is some 887 light years from the Sun, making it one of the closest examples of a variable of this kind. Imagery provided by Aladin sky atlas
