1927

The Milky Way Rotates

Bertil Lindblad (1895–1965), Jan Oort (1900–1992)

In 1918 the American astronomer Harlow Shapley had obtained the first quantitative estimate of the Size of the Milky Way galaxy by measuring the distances and directions to the halo of globular clusters that surround the galaxy’s disk. Shapley’s study also allowed him to estimate the position of the approximate center of the galaxy, which he placed in the brightest part of the Milky Way’s vivid band of stars, in the direction of the constellation Sagittarius.

As it became more clear that we were embedded inside a spiral galaxy much like others that were now being studied in detail with astrophotography and spectroscopy, it dawned on some astronomers that, presumably like the other spiral galaxies, individual stars in the Milky Way could be spinning around a common galactic gravitational center. In the 1920s the Swedish astronomer Bertil Lindblad was the first to work out this hypothesis in detail.

In 1927 the Dutch astronomer Jan Oort provided the first observational proof of Lindblad’s hypothesis by carefully measuring the movements of many hundreds of individual stars. He confirmed that the Milky Way rotates, and further, that the rotation is differential—that is, stars at different distances from the rotation axis orbit the center at different speeds, with farther-away stars lagging behind the closer-in ones. The Sun, about halfway out from the galactic center, takes about 250 million years to orbit the galaxy’s center.

Oort’s and Lindblad’s work helped to refine the exact rotational center of the galaxy based on Shapley’s earlier estimate. It was difficult for astronomers at the time to learn much more from visual observations, however, because much of the galactic center is obscured by the dark lanes of dusty nebulosity that E. E. Barnard and Max Wolf had studied in the late nineteenth century. Later astronomers would use X-ray, infrared, and radio telescopes to study this region intensely, eventually learning that an enormous energy source—called Sagittarius A* (pronounced “Sagittarius A-star”)—probably powered by a 4-million-solar-mass black hole, lurks at the center of our galaxy.

A cluster of stars, gas, and dust surrounding the center of our Milky Way galaxy. This photo is a composite of infrared images from the ground-based Two Micron All Sky Survey (2MASS) program; infrared images like this allow astronomers to peer deeper into this dusty region.