Edwin Hubble’s “tuning fork” diagram classifies galaxies primarily by their visual appearance, organizing them into a sequence of structural categories. Forming the “handle” of the tuning fork are the elliptical galaxies (E), which consist mostly of older, reddish stars and lack both a flattened disk and the cool gas or dust required to form new stars. These are subdivided by their apparent elongation, ranging from the nearly spherical E0 to the highly flattened, cigar-shaped E7. At the junction where the handle splits into two prongs sit the lenticular galaxies, designated S0 (or SB0 if a central bar is present). Lenticulars represent an intermediate shape; they feature a central bulge and a thin stellar disk, but, like ellipticals, they lack distinct spiral arms and star-forming interstellar gas.

The two diverging prongs of the tuning fork represent normal spirals and barred spirals. Normal spiral galaxies are characterized by a central bulge surrounded by a flattened disk containing distinct, pinwheel-like spiral arms. They are subclassified from Sa to Sd: Sa galaxies have the largest central bulges and the most tightly wound arms, while Sb, Sc, and Sd galaxies exhibit progressively smaller bulges, more loosely wrapped arms, and increasingly abundant quantities of gas and dust. The second prong contains the barred-spiral galaxies, which are fundamentally similar to regular spirals but feature an elongated “bar” of stellar and interstellar material passing through their center; their spiral arms trail outward from the ends of this bar rather than directly from the central bulge. Just like normal spirals, these are sequentially categorized from SBa to SBd according to the decreasing relative size of the central bulge and the increasing openness of their spiral arms.

While the modern universe is dominated by the mature, distinct spirals and ellipticals of the Hubble tuning fork, these structures evolved from a much more chaotic early state. Roughly 11 billion years ago, during the peak of the quasar epoch, the cosmos was populated primarily by small, irregularly shaped pregalactic fragments and dwarf galaxies. During this violent era, frequent “major mergers” between comparably sized systems destroyed delicate emerging gas disks to form the first elliptical galaxies. Conversely, “minor mergers” allowed other galaxies to absorb smaller companions while preserving their overall disk structure, seeding the early spiral galaxies. During the cosmic “middle ages” billions of years later, these galaxies took a more defined shape as the expansion of the universe spread galaxy clusters farther apart, steadily reducing the rate of major, destructive collisions. As their central fuel supplies were consumed, the brilliant quasars gradually faded into less energetic active galaxies, such as Seyferts and radio galaxies. Today, these systems are much more “in shape” and have finally calmed down; they have evolved into the quiescent, highly structured normal spirals and ellipticals we observe, leaving behind only dormant supermassive black holes lurking quietly at their cores.