Cepheid 3 NGC 4603

Harvard Observatory astronomer Henrietta S. Leavitt makes the first crucial step in establishing the distances to nearby "spiral nebulae." Studying variable stars in the Magellanic Clouds, she discovers the presence of rhythmically pulsating stars, known as Cepheid variables, which brighten and dim over a period of days. By observing the relationship between a Cepheid's brightness and its pulsation rate, astronomers can calculate how much light it emits and then use that number to estimate its distance.