Yardsticks of the Universe.

As the result of her work in the Harvard Observatory on Cepheid variable stars in the Magellanic Cloud, Henrietta Leavitt in 1912 made an important contribution to astronomers' attempts to understand the size of the universe. (Cepheids are the class of variable stars that brighten and dim with constant periods.) Leavitt discovered that the period of time it takes for a Cepheid variable to complete its bright-dim cycle is related to the star's luminosity. In 1914 Harlow Shapley used Leavitt's work in his detailed explanation of the correlation between luminosity and distance in Cepheid variable stars. Such insights led astronomers to develop the means of gauging large interstellar distances that could not be calculated by parallax, the method for measuring planetary and stellar distances by triangulation. Appropriately, Cepheid variables have been called the "yardsticks of the universe." It is now understood that Cepheid...