This sequence of images taken with NASA's Hubble Space Telescope chronicles the rhythmic changes in a rare class of variable star (located in the center of each image) in the spiral galaxy M100. This class of pulsating star is called a Cepheid Variable. The Cepheid in this Hubble picture doubles in brightness (24.5 to 25.3 apparent magnitude) over a period of 51.3 days.

The interval it takes for the Cepheid to complete one pulsation is a direct indication of the stars's intrinsic brightness. This value can be used to make a precise measurement of the galaxy's distance, which turns out to be 56 million light-years from Earth.

Cosmic distance measurements as accurate as this are needed to calculate the rate at which the universe is expanding. This value, called the Hubble Constant, is used to estimate the age and size of the universe.

Though M100 is the most distant galaxy in which Cepheid variables have been discovered, HST must find Cepheids in a larger sample of galaxies before a definitive number can be agreed upon for the size and age of the universe

Hubble Space Telescope was used to image repeatedly a region of M100 in order to pick out the flickering Cepheid candidates from normal stars. Twelve one-hour exposures, timed carefully in a two-month observing window, resulted in the discovery of 20 Cepheid variable stars.

Technical Information: The Hubble Space Telescope images were taken on (from left to right) April 23, May 4, 9, 16, 20, 31, 1994 with the Wide Field Planetary Camera 2 (WFPC 2) This black and white picture was take at visible light wavelengths.

Credit: Dr. Wendy L. Freedman, Observatories of the Carnegie Institution of Washington, and NASA