Cepheids are a type of pulsating variable star of great use in astronomy for determining distances. The reason is that the
period of light
variation of a "
classical" Cepheid correlates strongly with its luminosity, or brightness. The longer the period of variation is, the brighter the star is. Once the intrinsic luminosity of a classical Cepheid is establishedÑas it has been for some nearby onesÑthe luminosity of a Cepheid in another galaxy can be determined. Then, from the difference between the Cepheid's apparent and absolute magnitude, the galaxy's distance can be calculated. This method was first applied to the Magellanic Clouds. Later, Edwin Hubble used it to prove that the Andromeda "nebula" was indeed another galaxy beyond our own.