Galaxy types: spiral, elliptical, irregular
Three main shapes, and a classification diagram that implies an evolution running the wrong way.
4 min read · Published 27 August 2026
The short answer
Galaxies fall into three broad classes. Spirals have a flattened rotating disc with arms and ongoing star formation. Ellipticals are smooth, rounded, and dominated by old stars with little new formation. Irregulars have no clear structure, often through gravitational disturbance. Hubble arranged these on a tuning fork diagram which is still used for classification, though the evolutionary sequence it implies is now known to run backwards.
The three classes
| Type | Shape | Star formation | Contents |
|---|---|---|---|
| Spiral | Flattened rotating disc with arms and a central bulge | Ongoing | Mix of young blue and old red stars; substantial gas and dust |
| Barred spiral | As above with a central bar structure | Ongoing | Similar; the Milky Way is one |
| Elliptical | Smooth ellipsoid, from nearly spherical to elongated | Very little | Predominantly old red stars; little gas or dust |
| Lenticular | A disc without spiral arms — intermediate | Little | Old stars, disc structure, depleted gas |
| Irregular | No regular structure | Often vigorous | Frequently gas-rich; commonly disturbed by interaction |
The Milky Way is a barred spiral. Andromeda is a spiral. The largest galaxies known are giant ellipticals at the centres of galaxy clusters, and the smallest are dwarf irregulars and dwarf spheroidals, which are far more numerous than large galaxies.
The tuning fork, and what it got wrong
Hubble arranged galaxies with ellipticals on the handle, splitting into two prongs for normal and barred spirals. Ellipticals were labelled 'early type' and spirals 'late type'.
That terminology survives and is misleading, because it implies ellipticals evolve into spirals. The current understanding is the reverse: ellipticals form from mergers of spirals.
When two spirals collide, the ordered rotation of their discs is destroyed, stars are scattered into more random orbits, and a burst of star formation consumes the available gas. What remains is a smooth structure of old stars with little gas left — an elliptical.
So the sequence runs from spiral to elliptical, and the 'early' and 'late' labels are a fossil of an incorrect interpretation, retained because the classification itself remains useful.
Why spirals have arms
Not because they are structures rotating with the galaxy. If they were, the differential rotation of the disc — inner regions orbiting faster than outer — would wind them up tight within a few rotations. They plainly are not wound up, which is called the winding problem.
The leading explanation is density waves: patterns of compression moving through the disc, rather than fixed collections of stars. Stars pass through an arm and out again, like cars moving through a traffic jam that itself moves slowly.
As gas passes through the compressed region it collapses and forms stars. The most massive of those are hot, blue and short-lived, so they die before leaving the arm — which is why arms appear bright and blue while the disc between them is redder. The arms are not more massive; they are more actively forming luminous stars.
Observing them
Andromeda (M31) is the only large galaxy visible to the naked eye from the northern hemisphere, as a faint elongated smudge under dark skies. It is roughly 2.5 million light years away and is approaching us.
The Magellanic Clouds are irregular satellite galaxies of the Milky Way, prominent to the naked eye from the southern hemisphere.
In amateur telescopes galaxies appear as grey smudges, and the spiral structure so familiar from photographs is generally invisible to the eye. The colour and detail come from long exposures.
The realisation that these are separate galaxies rather than nebulae within the Milky Way came in the 1920s, using Cepheid variables to measure their distances. Before that, whether they were external systems was a genuine open question.
Common questions
- What are the main types of galaxy?
- Spirals, with a rotating disc, arms and ongoing star formation; ellipticals, smooth and dominated by old stars with little new formation; and irregulars with no clear structure. Lenticulars are an intermediate disc type without arms.
- What type of galaxy is the Milky Way?
- A barred spiral — a rotating disc with spiral arms and a bar-shaped structure through the central region.
- Do ellipticals evolve into spirals?
- No, the reverse. Hubble's tuning fork diagram labels ellipticals 'early type' and spirals 'late type', which implies the wrong direction. Ellipticals are now understood to form from mergers of spirals, which destroy disc rotation and consume the available gas.
- Why do spiral arms not wind up?
- Because they are density waves rather than fixed structures of stars. Stars pass through an arm and out again, like cars through a moving traffic jam. Gas compressed in the arm forms bright short-lived blue stars, which is why arms appear luminous.