- Variable star type which in the early 20th century was used to determine the size of the universe, especially the distance of other galaxies from our own. Cepheids exhibit a constant relationship between their period of variation and their absolute magnitude. So finding them in other galaxies and observing their apparent magnitude and their period of variation allows their distances to be calculated. This method allowed the size of our own galaxy to be found as well as the distance to other nearby galaxies including the Magellanic Clouds. Fortunately Cepheids are inherently bright enough to be seen in other galaxies at a distance at which other types of star would not be discernible. Delta Cephei is the Cepheid from which the others take the name.