Hubble sorted galaxies with a tuning fork drawn from photographic plates
In 1926 Edwin Hubble sorted galaxies by how they looked on photographic plates: smooth ellipses, flat spirals, and lens-shaped galaxies in between. Drawn as a two-pronged fork, his scheme is still the most widely used system, from professional observatories to backyard telescopes, even though computers now do much of the sorting.
Hubble's ellipticals show smooth, featureless light. Each gets an E followed by a number for how flattened it looks, calculated from the ratio between its short and long axes, so a round one is E0. Spirals, marked S, have a thin disk with arms, usually two, wrapped around a central bulge that resembles a small elliptical. About half have a bar of stars running out from the bulge and get the label SB. Lenticulars, S0, have a bulge and a disk but no visible arms and little new star formation. Irregular galaxies, with no clear shape, were added later.
On the diagram, ellipticals sit on the handle, getting flatter from left to right, with barred and unbarred spirals forming the two prongs and lenticulars at the junction. Even so, citizen scientists working with Galaxy Zoo reported in June 2019 that the assumed link between spiral arms and a galaxy's core may need rethinking.
In 1959 Gérard de Vaucouleurs argued that Hubble's scheme missed important features of spirals and turned the fork into something closer to a three-dimensional shape, tracking how tightly wound the arms are, how strongly barred the galaxy is, and whether it has a ring. A weakly barred galaxy with loose arms and a ring becomes SAB(r)c. He also gave every class a number on a scale running from minus 6 for compact ellipticals to plus 10 for irregulars, which lets researchers treat shape quantitatively.
A third approach, the Yerkes scheme from the American astronomer William Wilson Morgan, weighs the spectra of a galaxy's stars as well as its shape. Under it, our neighbour the Andromeda Galaxy is kS5.
Source: Galaxy morphological classification
