Types of Galaxies: Spiral, Elliptical, Irregular, and More

The observable universe contains an estimated 2 trillion galaxies, each one a gravitationally bound system of stars, gas, dust, and dark matter — and yet astronomers have found that nearly all of them fall into a handful of recognizable shapes. This page covers the major galaxy classification systems, the physical processes that produce each type, and how astronomers decide which category a galaxy belongs to. Whether someone is reading a telescope guide or trying to make sense of a Hubble image, understanding galaxy types is foundational to reading the key dimensions and scopes of astronomy.

Definition and scope

A galaxy is not simply a large cluster of stars. It is a self-contained gravitational system, and its morphology — its shape — reflects its entire history: how it formed, what it has consumed, and what has crashed into it.

The classification system most people encounter traces back to Edwin Hubble's 1926 paper, which organized galaxies along what became known as the Hubble Tuning Fork. Hubble identified three primary morphological categories: elliptical, spiral, and irregular. Later work by Gérard de Vaucouleurs and others extended that framework to include lenticular galaxies (also called S0 galaxies) and subcategories of barred spirals. The revised Hubble sequence remains the working vocabulary of observational astronomy, appearing in databases like the NASA/IPAC Extragalactic Database and catalogues derived from the Sloan Digital Sky Survey.

How it works

Galaxy shape is not cosmetic — it is diagnostic. Each morphological type reflects a different balance of rotation, stellar age, gas content, and merger history.

Elliptical galaxies are the most structurally settled. Classified on a scale from E0 (nearly spherical) to E7 (highly elongated), they are dominated by old, red stars and contain relatively little cold gas or active star formation. The giant elliptical M87, located at the center of the Virgo Cluster roughly 53 million light-years away, is one of the most massive galaxies known — its central black hole was the target of the Event Horizon Telescope's landmark 2019 image.

Spiral galaxies are rotating disk systems with distinct arms winding outward from a central bulge. The arms are sites of active star formation, lit blue by young, hot stars. Spirals are subdivided into Sa (tightly wound, large bulge) through Sd (loosely wound, small bulge). A significant fraction of spirals — including the Milky Way — are barred spirals, designated SB, where the arms originate from the ends of a straight stellar bar rather than directly from the nucleus. The Milky Way is classified as SBbc, a moderately barred spiral with moderately open arms.

Lenticular galaxies (S0) occupy the middle ground. They have a disk like spirals but lack prominent arms and show little active star formation, more closely resembling ellipticals in stellar population. The Sombrero Galaxy (M104) is frequently cited as a near-edge-on lenticular or Sa spiral — its exact classification is still debated in the literature, which is itself a useful reminder that nature does not perfectly obey human categories.

Irregular galaxies carry no clean symmetry. The Large Magellanic Cloud, a satellite galaxy of the Milky Way at approximately 160,000 light-years' distance, is the canonical example. Irregulars are often gas-rich and actively forming stars; their chaotic structure typically reflects gravitational distortion by a neighboring massive galaxy.

Common scenarios

When a beginning observer looks through a telescope or browses processed Hubble images on the astronomy frequently asked questions page, three situations come up repeatedly:

1. Identifying a galaxy from its appearance. A smooth, featureless oval with a bright core and no visible structure is almost certainly elliptical. A disk with visible spiral arms is a spiral. A patchy, asymmetric smear of light without obvious rotation is likely irregular or a merger in progress.

2. Encountering interacting or merging galaxies. Roughly half of large galaxies show signs of past or ongoing interaction. The Antennae Galaxies (NGC 4038 and NGC 4039) are a collision between two spirals that is producing intense starburst activity — a common intermediate stage before the pair settles into a single large elliptical.

3. Reading catalogue entries. Galaxy databases assign morphological codes using the de Vaucouleurs system, where a barred spiral with loosely wound arms and a small central lens is coded SB(s)c. Understanding the notation unlocks the information in professional catalogues.

Decision boundaries

The Hubble sequence works well for isolated, well-resolved galaxies observed face-on or at mild inclination. It runs into limits quickly — and knowing those limits matters.

Comparing ellipticals versus lenticulars requires spectral data, not just imaging. Both appear smooth and featureless in optical light. The diagnostic is the presence of a kinematically cold stellar disk: lenticulars have one; true ellipticals do not. This distinction became clearer after integral-field spectroscopy surveys like ATLAS3D systematically studied hundreds of nearby early-type galaxies.

Comparing spirals versus irregulars in distant galaxies is genuinely difficult. At redshifts above roughly z = 1, galaxies were younger and more chaotic; many look irregular by modern standards but are progenitors of today's spirals. The James Webb Space Telescope, launched in December 2021, is extending morphological classification into epochs where the Hubble sequence was not yet established.

Galaxy type also correlates strongly with environment. Ellipticals dominate the cores of rich galaxy clusters; spirals are more common in lower-density field environments. This density-morphology relation, documented by Alan Dressler in a 1980 paper in The Astrophysical Journal, is one of the foundational empirical patterns in extragalactic astronomy. Anyone moving deeper into the subject will find it discussed across the how it works resources and broader astronomy reference material on this site.

References

• Chandra X-ray Center, Harvard-Smithsonian
• Harvard-Smithsonian Center for Astrophysics, Multiple Star Catalog context
• LASP / University of Colorado, SORCE mission data
• LIGO Scientific Collaboration
• LIGO Scientific Collaboration, 2017 announcement
• LIGO Scientific Collaboration, Technical Overview
• MAST