Another way to find the distances to star clusters is by watching the pulsations of Cepheid variable stars within the cluster. Cepheids, named after the star Delta Cephei, are yellow supergiant stars that pulsate. They are more massive than the sun and about 20,000 times more luminous. Their pulsation periods can be as short as 1 day or as long as 70 days. By watching clusters that have Cepheids, astronomers can calculate a distance based on the magitudes of the stars.
Astronomers observe the apparent magnitude of the stars.
This is how bright the stars look to be. By observing the pulsation period, the absolute magnitude can be determined. This is how bright the star would be at a standard distance. Cepheids are used because they are extremely bright and therefore easily seen at great distances. They are not only used in determining the distances to clusters of stars, but also to nearby galaxies.