M100, a majestic spiral in the Virgo Cluster of Galaxies, contains a rare class of pulsating stars called Cepheid variable stars. Cepheids, which Hubble detected within M100's spiral arms, are reliable cosmic distance mileposts. This is demonstrated in the top three frames of this image. The star in the center of each box changes brightness. The interval it takes for the Cepheid to complete one pulsation is a direct indication of the star's intrinsic brightness. This value can then be used to make a precise measurement of the galaxy's distance, which turns out to be 56 million light-years, making M100 the most distant galaxy in which Cepheids have been measured accurately. By understanding M100's distance from Earth, astronomers can calculate how fast the universe is expanding, which is crucial for understanding the age and size of the universe.