Cosmic yardsticks
The debate about the physical nature of the Milky Way continued into the early 20th century. Two new technologies helped charge the discussion: spectroscopy and photography. The ability to analyze starlight gave astronomers a powerful new way of understanding the chemistry of stars, while photography augmented the limited light-gathering ability of the human eye.
Armed with these tools, astronomers Henrietta Leavitt, Edward C. Pickering, and Ejnar Hertzsprung discovered and defined a relationship between the period of dimming and brightening of a class of stars called Cepheid variables. In 1908, Leavitt was studying variable stars in photographs of the Large and Small Magellanic Clouds sent to the Harvard College Observatory, where she worked, from Harvard’s observatory in Peru. She noticed a rhythmic and predictable variation in brightness of these stars in the Large and Small Magellanic Clouds, which might last from a single day to more than a month before repeating.
Furthermore, she discovered, the longer the period of variation, the brighter the star appeared to be. Since all the stars in the Small Magellanic Cloud are at roughly the same distance, she reasoned that the period of a Cepheid variable was related to its true, intrinsic brightness.
Pickering, the observatory director, suggested this period-luminosity relation could be useful to determine the distribution of star clusters and nebulae. And Hertzsprung was able to calibrate this technique by making independent distance measurements to Cepheids using the parallax method, seeing how much they shifted against background stars as Earth orbited the Sun.
Thus, by measuring the period of a Cepheid, astronomers could know its true brightness — and by comparing that to its apparent brightness, calculate how far away it was. Astronomers finally had a reliable cosmic yardstick.
Around the same time, the young astronomer Harlow Shapley began measuring the distribution of globular clusters — compact and dense spheres of stars. By 1918, he had found that the clusters centered around the constellation Sagittarius, forming a halo around the Milky Way. He also made improved parallax measurements of Cepheid variables, which in turn improved the calibration of Leavitt’s relation.
Using this data, Shapley not only located the center of our galaxy — in Sagittarius — but also showed that the Milky Way was 10 times the size of previous estimates. His observations also placed our solar system far from the center of the galaxy. Given the size of our galaxy, Shapley was convinced that spiral nebulae, like globular clusters, were all part of the Milky Way.