1959

Spiral Galaxies

Harlow Shapley (1885–1972), Edwin Hubble (1889–1953), Fritz Zwicky (1898–1974), Vera Rubin (1928–2016)

In the galaxy classification scheme defined by Edwin Hubble in the 1920s and 1930s, spiral galaxies—immense numbers of stars bound together and sculpted by gravity, with two or more arms slowly spinning around a common center of mass—represent an extreme shape (the others being elliptical and lenticular galaxy forms). Some spiral galaxies are seen face-on, and others are seen edge-on. Edge-on galaxies reveal that the arms are confined to a wide, flat disk with a central bulge, all of which is surrounded by a halo of distant stars and globular clusters. As worked out by Harlow Shapley and others, our own galaxy—the Milky Way—is more than 100,000 light-years across.

Around 1959, radio astronomers developed techniques to use observations of the strong (21-centimeter) emission line of hydrogen detected by spectroscopy to map out the rotational velocities of the arms of face-on spiral galaxies. The expectation was that stars farther from the galactic center would rotate more slowly, following Kepler’s Laws, like planets orbiting a central star. But the observations revealed that the velocities of stars remain roughly constant with distance from the galactic center. This “galaxy rotation problem” was confirmed by more and more spiral galaxy observations.

In the mid-1970s, the American astronomer Vera Rubin proposed a solution: if most of the mass in spiral galaxies consisted not of the kind of matter that we could see through telescopes but instead of the same kind of unseen dark matter that Fritz Zwicky had proposed in 1933, then the observed non-Keplerian motions of spiral galaxies could make sense. Most astrophysicists today accept the existence of dark matter based on evidence presented by Zwicky, Rubin, and others.

Spiral galaxies are beautiful and ancient structures that are believed to have formed in the early universe and that continue to evolve today. Enshrouded in halos of mysterious dark matter and hosting million-solar-mass black holes in their cores, they are like enormous pinwheels in the cosmos, spinning along for billions of years on the virtual winds of the Big Bang.

M51, the Whirlpool galaxy, photographed by the Hubble Space Telescope, shows luminous hydrogen emission (red) in the strongest regions of new star formation.