Most stars spend at least some time as *Cepheid Variables.
Cepheid variables are stars which oscillate in regular rhythms, alternately expanding and contracting to as much as 30 per cent of their normal size. These regular rhythmic changes in size are accompanied by changes in brightness, temperature, and spectral type.
Since the periods of these stars are proportional to their brightness, astronomers have been able to calculate their luminosity, and consequently to more accurately predict the distances and ages of the various galaxies in which they are observed.
* (pronounced ‘seph-id’, with the accent on the first syllable)