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Galaxies are gravitationally bound systems of stars, gas, dust, and dark matter ranging from dwarf galaxies with millions of stars to giant ellipticals with trillions. Hubble's tuning-fork diagram classifies galaxies into ellipticals (E0–E7, spherical to flattened), lenticulars (S0), spirals (Sa–Sd, with winding arms), and barred spirals (SBa–SBd). Ellipticals contain mostly old stars and little cold gas; spirals have active star formation in their arms. Irregular galaxies lack symmetry, often due to tidal interactions or mergers. Morphology correlates with environment: ellipticals dominate dense galaxy cluster cores.
Classify a sample of galaxy images from SDSS or the Galaxy Zoo citizen science database using Hubble's scheme. Study real examples of interacting galaxies (Antennae, Mice) to understand how mergers distort morphology.
When you look at images of galaxies, the most immediately striking feature is their shape. Some are smooth, featureless ellipses; others have dramatic spiral arms winding outward from a central bulge; still others are chaotic, irregular smears of light. Galaxy morphology is the systematic classification of these shapes, and the organizing framework that has endured since the 1920s is Edwin Hubble's tuning-fork diagram.
The tuning fork splits galaxies into two main sequences that branch from a common point. On the left sit elliptical galaxies, labeled E0 through E7 based on how elongated they appear (E0 is nearly circular, E7 is highly flattened). Ellipticals are dominated by old, red stars and contain very little cold gas or dust, meaning they have largely stopped forming new stars. At the fork's junction sit lenticular galaxies (S0), which have a central bulge and a disk like spirals but lack prominent spiral arms — they are transitional in appearance. The two prongs of the fork represent normal spirals (Sa through Sd) and barred spirals (SBa through SBd), where a linear bar of stars extends through the center. Moving from "a" to "d" along either branch, the central bulge gets smaller, the spiral arms become more open and prominent, and the fraction of young blue stars increases.
Beyond the tuning fork, irregular galaxies lack the symmetry of any Hubble type. Many irregulars are small, gas-rich systems (like the Magellanic Clouds), while others have been distorted by gravitational interactions — galaxy mergers and tidal encounters can warp spirals into unrecognizable shapes. The Antennae Galaxies, two spirals in the process of colliding, show dramatic tidal tails and triggered bursts of star formation that defy simple classification. Morphology is therefore not static; it evolves as galaxies interact and merge over cosmic time.
An important pattern connects morphology to environment. Galaxy clusters — dense concentrations of hundreds or thousands of galaxies — are dominated by ellipticals and lenticulars in their cores, while spirals are more common in the less crowded outskirts and in the general field. This morphology-density relation suggests that the cluster environment transforms galaxies: tidal stripping, ram-pressure removal of gas, and frequent mergers can convert gas-rich spirals into gas-poor ellipticals. Understanding why galaxies look the way they do is therefore inseparable from understanding the environments they inhabit and the histories they have lived through.