Edwin Hubble’s work on galaxies is legendary. In 1923 the astronomer, working at Mount Wilson Observatory in California, proved that M31, the “spiral nebula” in Andromeda, was another galaxy. In one blow he vastly increased the scale of the universe. If that wasn’t the achievement of a lifetime, he subsequently showed that the farther away a galaxy is, the faster it recedes from us, providing the first direct observational evidence of the Big Bang.
And Hubble made another major contribution for which we deep-sky observers will be forever in his debt: a classification system for galaxies.
The Hubble classification
Hubble published the most widely used version of his system in 1936, a decade after his first draft. The system is simple to learn, following a two-pronged “tuning fork” model. The handle and each prong represent one of three classes of galaxies: ellipticals, spirals, and barred spirals.
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Elliptical galaxies, designated by the letter E, form the handle. As their name suggests, these galaxies are roughly elliptical in shape, but they are further classified by how stretched and squashed they appear. Spherical galaxies are designated E0, slightly elongated ellipticals are E1, and so on, with cigar-shaped ellipticals designated as E7. Elliptical galaxies are featureless patches of light without spiral arms or dust lanes, and they have no active star-forming regions.
Spiral galaxies, designated by the letter S, form the upper prong of the tuning fork. They have two major features: a central bulge containing old stars surrounded by a flat disk with spiral arms actively forming new stars. Hubble further classified them into three groups: Sa, Sb, and Sc, based on the openness of the spiral arms and the relative size of the central bulge. Class Sa shows tightly wound, almost circular spiral arms and a large central bulge that occupies a majority of the disk, whereas class Sc has multiple open spiral arms, and the central bulge is small and inconspicuous. Those that fall between the two extremes are designated as Sb galaxies, which have definite spiral arms and a central bulge that occupies a minor, but still obvious, portion of the disk.
Barred spirals, designated SB, form the lower prong of the tuning fork. They have a bar of stars crossing the central bulge. Spiral arms extend from each end of the bar like streamers from the ends of a bicycle handlebar. Barred spirals also have three subclasses, SBa, SBb, and SBc, using the same scheme as the classic spirals. (Our own Milky Way is a barred spiral, probably an SBb in Hubble’s scheme.)
At the dividing point in the tuning fork exists a special class of galaxies known as lenticulars (lens-shaped), designated S0. These galaxies have a large central bulge surrounded by an extended disk-like structure. However, the disk lacks visible spiral arms and is not actively forming any significant quantity of new stars. S0 galaxies are considered a transitional form between the ellipticals and the two classes of spirals. Their addition to the system was one of the revisions Hubble made in 1936. Hubble postulated their existence prior to that, but the overexposed bright centers of lenticular galaxies on earlier, small-scale glass photographic plates overwhelmed their extended disks, rendering them indistinguishable from ellipticals.