Types of Galaxies Explained: Spiral, Elliptical, and Irregular

When you look up at the night sky, every faint fuzzy blob you see through a telescope is a galaxy, a vast collection of stars, gas, dust, and dark matter held together by gravity. But not all galaxies look alike. Some are elegant spirals with sweeping arms, others are smooth, featureless ellipses, and some are chaotic tangles of stars with no discernible structure at all.

Understanding galaxy types is fundamental to understanding the universe. The shape of a galaxy tells you about its history, its age, how it formed, and what's happening inside it right now.

The Hubble Tuning Fork

In 1926, Edwin Hubble created a classification system for galaxies that astronomers still use today, with modifications. It's called the "Hubble Tuning Fork" because of its shape when drawn as a diagram:

• On the left: Elliptical galaxies, ranging from nearly spherical (E0) to highly elongated (E7)
• On the right, splitting into two branches: Spiral galaxies (top branch: normal spirals, bottom branch: barred spirals)
• Between them: Lenticular galaxies (S0), which have a disk but no spiral arms

Hubble originally thought this diagram represented an evolutionary sequence (ellipticals evolving into spirals or vice versa), but we now know that's not the case. Galaxy shapes are determined by formation history, mergers, and environment, not a simple aging process.

Spiral Galaxies

Spiral galaxies are the most visually dramatic type and what most people picture when they hear the word "galaxy." They feature a rotating disk with prominent spiral arms winding outward from a central bulge.

Key Characteristics

• Flat, rotating disk: The main body is a thin disk of stars, gas, and dust rotating around the center
• Spiral arms: Density waves create the spiral pattern, where gas is compressed and triggers new star formation. The arms are bright because they're full of young, blue stars.
• Central bulge: A dense concentration of older, yellowish stars at the center
• Dust lanes: Dark ribbons of interstellar dust wind through the disk and arms
• Active star formation: Spiral galaxies are still actively making new stars, especially in their arms
• Supermassive black hole: At the very center of the bulge, hidden from direct view

Barred vs. Unbarred

Some spiral galaxies have a linear structure of stars cutting through the center, called a bar. The spiral arms emerge from the ends of this bar rather than from the center directly. Our own Milky Way is a barred spiral, as is the Andromeda Galaxy.

Current research suggests that most spiral galaxies have bars to some degree, and that bars may form and dissolve over a galaxy's lifetime. The bar funnels gas toward the center, potentially feeding the central black hole and triggering bursts of star formation.

Famous Examples

• The Whirlpool Galaxy (M51): A stunning face-on spiral interacting with a smaller companion galaxy
• The Milky Way: Our home galaxy, a barred spiral roughly 100,000 light-years across
• The Pinwheel Galaxy (M101): A beautifully symmetric face-on spiral in Ursa Major
• Bode's Galaxy (M81): A classic grand design spiral visible through amateur telescopes
• M83 (Southern Pinwheel): A vibrant spiral galaxy visible from southern latitudes

Elliptical Galaxies

Elliptical galaxies are the smooth, featureless counterparts to spirals. They range from nearly spherical to highly elongated and lack the dramatic structure of spiral galaxies.

Key Characteristics

• Smooth, rounded shape: No disk, no spiral arms, no prominent dust lanes. Just a smooth distribution of stars.
• Old, red/yellow stars: Ellipticals are dominated by older stellar populations. Most of their star-forming gas has been used up or expelled.
• Little ongoing star formation: Without significant gas reserves, ellipticals produce very few new stars
• Size range: From tiny dwarf ellipticals (a few thousand light-years across) to supergiant ellipticals (several hundred thousand light-years across)
• Random stellar orbits: Unlike the orderly rotation of spiral galaxies, stars in ellipticals orbit in random directions, giving the galaxy its puffy shape

How Elliptical Galaxies Form

Current evidence suggests that many (perhaps most) large elliptical galaxies are the product of galaxy mergers. When two spiral galaxies collide and merge, the orderly rotation is disrupted, gas is consumed in a burst of star formation, and the result is a smooth, elliptical system. This process has been observed in various stages across the universe.

The Antennae Galaxies are an example of two spirals currently in the process of merging, and they're expected to eventually settle into an elliptical galaxy.

Famous Examples

• M87: The giant elliptical galaxy in the Virgo Cluster, famous for its enormous jet of material and its supermassive black hole (the first ever directly imaged)
• M32: A compact elliptical galaxy orbiting the Andromeda Galaxy, visible through binoculars
• NGC 1132: A "fossil group" elliptical that may have swallowed all its companion galaxies

Irregular Galaxies

Irregular galaxies are exactly what they sound like: galaxies that don't fit neatly into the spiral or elliptical categories. They have no particular symmetry, no defined center, and often look chaotic.

Key Characteristics

• No regular structure: No spiral arms, no smooth elliptical shape. They look messy.
• Active star formation: Many irregulars are rich in gas and are actively forming stars, sometimes at very high rates
• Smaller size: Most irregular galaxies are smaller than spirals or large ellipticals
• Often disturbed: Many irregulars got their shape from gravitational interactions or collisions with other galaxies

Two Types of Irregulars

Irr I (Magellanic type): Show some structure but not enough to classify as spiral. Named after the Large Magellanic Cloud, which has a bar-like structure and a single spiral arm but is too disorganized to be called a true spiral.

Irr II: Completely chaotic with no discernible structure. Often the result of recent collisions or strong tidal interactions.

Famous Examples

• The Large Magellanic Cloud: Visible from the Southern Hemisphere as a prominent, cloud-like patch in the sky. It orbits the Milky Way.
• The Small Magellanic Cloud: A smaller companion to the LMC, also visible from the Southern Hemisphere
• NGC 1427A: An irregular galaxy plunging through the Fornax Cluster, its shape distorted by ram pressure

What Determines a Galaxy's Shape?

Galaxy morphology isn't random. Several factors influence what shape a galaxy takes:

• Formation conditions: How quickly gas collapsed and how much angular momentum it had
• Merger history: Galaxies that have experienced major mergers tend toward elliptical shapes
• Environment: Galaxies in dense clusters are more likely to be elliptical (due to more frequent interactions and gas stripping)
• Gas content: Galaxies with abundant gas can maintain spiral structure and ongoing star formation
• Dark matter halo: The invisible dark matter surrounding a galaxy influences its structure and rotation

Lenticular Galaxies: The In-Between

There's a fourth category that bridges spirals and ellipticals: lenticular galaxies (type S0). These have a flat disk like a spiral galaxy but lack prominent spiral arms. They've typically used up their gas and stopped forming new stars, but they retain the disk shape from their spiral past.

Lenticular galaxies are thought to be "faded spirals," galaxies that exhausted their gas supply or had it stripped away by interactions with other galaxies or the intergalactic medium.

Observing Different Galaxy Types

If you want to see examples of each galaxy type through your own telescope, here are accessible targets:

• Spiral: M31 (Andromeda), M51 (Whirlpool), M81 (Bode's Galaxy)
• Elliptical: M32 (companion to Andromeda), M49 and M87 (in the Virgo Cluster)
• Irregular: Large and Small Magellanic Clouds (Southern Hemisphere only)
• Lenticular: NGC 3115 (Spindle Galaxy) in Sextans

For a complete list of the best deep-sky objects for beginners, including galaxies, nebulae, and star clusters, see our Messier objects guide. And if you're curious about what happens when different galaxy types smash into each other, read our galaxy collision guide.