M100 Cepheid

This Hubble image of a region of M100 shows a type of pulsating star called a Cepheid variable. These stars are reliable distance indicators to galaxies. The top three frames, taken on three different dates in May 1994, reveal that the star (in the center of each box) changes brightness. Cepheids go through these changes rhythmically over a few weeks. The interval it takes for the Cepheid to complete one pulsation is a direct indication of the star’s intrinsic brightness. Astronomers can use this brightness to precisely determine the galaxy’s distance.