Harlow Shapley and the Scale of the Universe

(Picture Essay of the Day)

Since the time of ancient Greek philosophers such as Aristotle, scientists have stared up at the heavens and wondered: How big is the universe? And where does our planet rest within the spiraling band of stars above? Many astronomers, including Sir William Herschel and his sister Caroline, attempted to determine the shape of our galaxy and to count the number of stars it contained. But it was not until the early 20th century, when astronomer Harlow Shapley began to look at variable stars as measures of distance that answers to these questions began to emerge.

Shapley was born on Nov. 2, 1885, in Nashville, Missouri, and was set on a career in journalism until the program he was scheduled to attend at the University of Missouri was postponed a year. In lieu of journalism, Shapley studied astronomy, earning a bachelor’s degree in 1910. During a fellowship at Princeton, he learned about Cepheid variables, stars whose pulsation period is closely related to luminosity, making them useful for measuring interstellar and intergalactic distances.

In 1914 Shapley took a research post at Mount Wilson Observatory in Pasadena, California, where, using the observatory’s 1.5-meter (60-inch) reflecting telescope, he undertook studies of the distribution of globular star clusters in the Milky Way Galaxy. He discovered that of the then 100 known globular clusters, about one-third were within the Sagittarius constellation, with the clusters forming a sphere with Sagittarius in the center. Shapley determined the clusters’ distribution using the variable stars within them as distance markers.
In 1920 Shapley was engaged in a debate with fellow American astronomer Heber D. Curtis on the size of the universe and whether spiral nebulae were their own galaxies or were gas clouds within a single galaxy. Known as the Great Debate, Shapley argued that the universe consisted of just a single galaxy—the Milky Way—in which the Sun was relatively far from the Galaxy’s center and the spiral nebulae were gas clouds. Curtis, on the other hand, placed the Sun at the center of the universe and argued that spiral nebulae were independent galaxies.

Of course, today it is known that there exists more than one galaxy in the universe and that the Sun is in fact distant from the Milky Way Galaxy’s center. Thus, Shapley was correct in his placement of the Sun in the center of Galaxy, but incorrect in his assumption that the universe consisted of a single galaxy. Shapley’s estimate that the Sun lies 50,000 light-years from the center of the Milky Way was also later corrected to 30,000 light-years.Considering the technological challenges and gaps in knowledge facing astronomers nearly a century ago, Shapley’s observations in his time were extraordinary. Thanks to his work, scientists were able to achieve a much deeper understanding of our place in the universe.