1936

Elliptical Galaxies

Edwin Hubble (1889–1953)

The work of the astronomers Harlow Shapley, Vesto Slipher, Edwin Hubble, and others on determining the scale of the galaxy and collecting spectroscopic data for large numbers of spiral nebulae during the first few decades of the twentieth century eventually led to the realization that they are other galaxies—other Milky Ways—each harboring hundreds of billions of stars of their own. As more galaxies were identified and studied, and as it became clear that they were not all the same, astronomers naturally sought to classify them into distinct categories, as they had done for the stars.

As a leading observer of galaxies and with access to some of the best telescopic facilities in the world, Hubble was in a particularly unique position to take the lead on galaxy classification. And lead he did. In a series of papers and lectures, eventually compiled into a landmark 1936 book called The Realm of the Nebulae, Hubble outlined a scheme for the morphologic (shape, size, brightness) classification of extragalactic nebulae, now called the Hubble sequence.

On one end of the Hubble sequence were the elliptical nebulae, now known as elliptical galaxies. Ellipticals are one of three main classes, the others being spiral galaxies, like our own Milky Way, and lenticular (lens-shaped) galaxies, intermediate in form between ellipticals and spirals.

Edwin Hubble’s original “tuning fork” galaxy classification scheme diagram, from The Realm of the Nebulae.

Elliptical galaxies, as the name implies, are ellipsoidal to spherical in shape, and vary smoothly in brightness from a bright central core to diffuse outer edges. Modern surveys reveal that roughly 10–15 percent of galaxies in the local neighborhood consist of elliptical galaxies, but that fewer existed in the early universe. Ellipticals consist mostly of older, lower-mass stars, and are mostly devoid of the gas and dust needed for new star formation. The origin of elliptical galaxies is controversial, but some astronomers hypothesize that elliptical galaxies might be the end result of ancient mergers and collisions between former spiral galaxies.

SEE ALSO Cepheid Variables and Standard Candles (1908), Size of the Milky Way (1918), Hubble’s Law (1929), Spiral Galaxies (1959).

Hubble Space Telescope image of the massive elliptical galaxy M87—home to trillions of stars, 15,000 globular star clusters, and a massive central black hole.