Cepheid variables are very luminous stars that pulsate in a regular cycle, with rapid brightening followed by gradual dimming. They are named after the star delta Cephei, a naked eye star, which was the first of this type to be identified. Cepheids are relatively rare, but their unique properties enable scientists to measure the distance to stars in galaxies more than 10 Mpc away. Since it is very difficult to tell the difference between a light source that is far away and a dimmer source that is nearer to us, measuring the distance to other galaxies is one of the greatest challenges facing astronomers. Cepheid variables are a fantastic tool to help them.
The outer layers of a Cepheid variable star pulsate in a manner that is predictable. The outer layers of the star periodically expanding and contracting cause this pulsation.
Observations of Cepheids with well-known distances showed that a well-defined correlation exists between the average luminosity of a Cepheid star and its pulsation period. If a pulsating star, therefore, is detected in a distant galaxy, and it is identified as a Cepheid from its period and its spectral characteristics, its apparent brightness and its pulsation period can be used to determine its distance, which can also be defined as the distance to the cluster or galaxy in which it is found. Astronomers, therefore, have used the period-luminosity relationship to determine distances to galaxies.
You can find the light curves for these and other Cepheids on the Hipparcos web pages at:
Period - Magnitude Relationship
To determine the average absolute magnitude for Cepheids, the following equation is used:
Where M is the absolute magnitude of the star and P is the period measured in days.