Types of Galaxies

Core Concepts

The Hubble tuning fork

Hubble's diagram arranges galaxies by shape. The handle holds ellipticals, labeled E0 through E7 by apparent flattening: E0 is round, E7 is the most elongated (the number is roughly 10 × (1 − b/a), where a and b are the semimajor and semiminor axes). The upper tine holds spirals (Sa, Sb, Sc), the lower tine barred spirals (SBa, SBb, SBc), and at the junction sit lenticulars (S0). Within each spiral class, the letters run from large, tightly wound arms and big bulges (Sa) to small bulges and loose, patchy arms (Sc).

Elliptical galaxies

Ellipticals look like smooth, featureless ellipses of light — no disk, no arms, no dust lanes. They are made mostly of old, red stars (Population II), contain very little gas or dust, and show essentially no current star formation. Their sizes range from tiny dwarf ellipticals — the most numerous galaxies in many clusters — to giant ellipticals in the cores of rich clusters, some of which hold over a trillion stars and are among the most massive galaxies known. Ellipticals typically host many globular clusters, and the biggest ones can grow by swallowing smaller galaxies.

Spiral galaxies

Spirals are flattened disks of gas, dust, and stars with a central bulge and a surrounding halo. Their defining feature is the set of spiral arms, traced by young, blue stars, glowing clouds of ionized hydrogen (H II regions), and dark dust lanes — all signposts of active star formation. The arms are not solid objects; they are density waves that compress gas and trigger star formation as material flows through them (see Chapter 25's spiral-structure topic). Classification runs by bulge size and arm tightness: an Sa has a large bulge and tightly wound, smooth arms; an Sc has a small bulge and loose, clumpy arms. In a barred spiral, a bar of stars runs through the center, with arms emerging from the bar's ends. Our own Milky Way is a barred spiral. Spirals are rich in gas and continue forming stars today.

Lenticular galaxies

Lenticulars (S0) sit at the fork between ellipticals and spirals: they have a disk and a bulge but no spiral arms, and like ellipticals they are gas-poor, red, and no longer forming many stars. They look like spirals that have used up or lost their star-forming gas. Their existence is a reminder that the morphological classes form a continuum, not watertight boxes.

Irregular galaxies

Irregulars have no regular or symmetric shape — they are patchy, clumpy collections of stars and gas. The best-known examples are the Large and Small Magellanic Clouds, the Milky Way's small companions, visible from the Southern Hemisphere. Irregulars tend to be gas-rich, with lots of young stars and active star formation, and many are small dwarf irregulars. They are common, even though they rarely appear in pretty textbook pictures.

Morphology, environment, and evolution

Galaxy type is not random. In the cores of rich clusters, ellipticals and lenticulars dominate; in the outskirts and in low-density "field" regions, spirals are common. This pattern is thought to reflect history: galaxies in dense environments lose their gas (through collisions, stripping, and gravitational harassment) and stop forming stars, turning spiral-like systems into S0s and ellipticals. Dwarf galaxies are by far the most numerous type in the universe. Crucially, the tuning fork is not an evolutionary track — Hubble himself once speculated that ellipticals evolved into spirals or vice versa, but modern understanding is that a galaxy's type reflects its formation history and environment, not its age. All galaxy types are roughly the same cosmic age; they differ in what has happened to their gas.

Common Confusions