Hubble sequence

If you sort galaxies by their shapes — the way you might sort leaves or seashells — they fall into a surprisingly tidy family portrait. Edwin Hubble drew this portrait in 1926 as a diagram shaped like a tuning fork: smooth round blobs at the handle, and two prongs of pinwheel-shaped spirals branching off. Almost every bright galaxy in the sky fits somewhere on this chart, which is called the Hubble sequence.

On the left sit the ellipticals (labelled E0 to E7, from nearly round to quite flattened), featureless egg-shaped clouds of stars. At the fork's joint sit the lenticulars (S0), which have a disk but no spiral arms. Then the two prongs: ordinary spirals (Sa, Sb, Sc) on one prong and barred spirals (SBa, SBb, SBc) on the other, where a straight bar of stars crosses the center. Along each prong, going from a to c, the central bulge shrinks and the arms grow looser and more knotted with bright young stars. Galaxies that fit no neat shape are simply called irregulars. Hubble once called ellipticals 'early-type' and spirals 'late-type', a naming that stuck even though it does not mean ellipticals are younger.

The sequence matters because shape encodes history and physics, not just looks. Moving from spirals toward ellipticals, galaxies tend to hold less cold gas, form fewer new stars, and shift from blue (young stars) to red (old stars). So a galaxy's spot on the tuning fork hints at its star-forming life story, its gas content, and the violence of its past mergers. It remains the everyday language astronomers use to describe galaxy morphology, even though we now know shape is shaped by environment and collisions, not a fixed evolutionary track.