The high resolution of the HST pinpoints a Cepheid variable star, located in a star-forming region in one of the spiral arms of M100. The top three boxes, from left to right, were taken on May 9, May 4 and May 31 in 1994 and show how the star varies in brightness.
A Cepheid variable is a pulsating star that changes rhythmically in brightness. Times of peak brightness are separated by equal intervals, known as the 'period' of the star. Periods for different Cepheids range in length from hours to days. The brighter the Cepheid, the longer its period. These rare stars act as cosmic 'mileposts', making it possible to deduce the distance to any galaxy in which they can be identified. By discovering Cepheids and measuring their periods, astronomers can determine their true brightnesses. Investigators can then determine the distance of a Cepheid star by comparing its true brightness with its apparent brightness as seen from Earth (or from the HST).
It is difficult to detect individual Cepheids in distant galaxies with groundbased telescopes but, in twelve one-hour exposures over a two-month period, 20 Cepheids in M100 were discovered with the HST.
Camera: WFPC2
Technical Information: Black and white
image taken at visible wavelengths.
Credit: W. L. Freedman (Observatories
of the Carnegie Institution of Washington), and NASA
