Types of Galaxies: Ellipticals, Spirals and Irregulars

Galaxies come in a wide range of shapes, from smooth glowing spheres to thin rotating disks with graceful arms. Astronomers sort them into broad families according to appearance, and that appearance turns out to reflect what the galaxy is made of, how fast it forms new stars and how much it has been disturbed by neighbours. The classification introduced by Edwin Hubble in the 1920s and 1930s is still the starting point for most galaxy studies.

The Hubble classification

Edwin Hubble arranged galaxies into a diagram that looks like a tuning fork. Ellipticals sit on the handle, and spirals split into two branches, one with a plain disk and one with a central bar. Lenticular galaxies sit at the junction between the two. Irregular galaxies do not fit the diagram at all.

It is important to know what the diagram does and does not mean. Hubble's sequence is a morphological description, not an evolutionary timeline. Early-type galaxies were so named because Hubble mistakenly thought they came first. Modern work shows that galaxies change type through mergers and gas loss, so the sequence describes shape and not age.

Elliptical galaxies

Elliptical galaxies are smooth, rounded systems with little visible structure. Their stars move on random orbits, so they are supported mainly by velocity dispersion rather than by rotation. Ellipticals are labelled E0 to E7, where the number reflects how flattened the galaxy looks from our viewpoint. E0 is nearly round and E7 is strongly elongated.

Most ellipticals contain old, red stars. Star formation has largely stopped in them, because the gas that once fed star formation has been used up, heated or removed. Giant ellipticals are among the most massive galaxies known, and they often sit at the centres of galaxy clusters. Dwarf ellipticals are much fainter and far more common by number.

Spiral galaxies

Spiral galaxies have a flat disk, a central bulge and arms that wind outward. Their disks are rich in gas and dust, and that gas fuels ongoing star formation. The arms contain bright, young, blue stars and glowing clouds of ionised hydrogen that trace regions where new stars are being born.

Spirals are divided into two main groups. Normal spirals (S) have arms that spring directly from the nucleus. Barred spirals (SB) have a central bar of stars, and their arms usually emerge from the ends of the bar. Within each group, the letters a, b and c describe how tightly wound the arms are and how large the bulge is. Sa galaxies have large bulges and tight arms, while Sc galaxies have small bulges and loose, open arms.

The Milky Way is a barred spiral, usually classed as SBbc. Andromeda, our nearest large neighbour, is a large spiral of type Sb.

Our own galaxy shows the structure that spirals share. Its thin disk holds most of the young stars and gas, and the Sun sits in that disk. A thicker, older disk surrounds it, and a roughly spherical halo contains old stars and about 150 globular clusters. The halo is so sparse that its stars are hard to see, which is a reminder that most of the galaxy's mass is not in visible stars. The dark matter halo extends far beyond it.

Lenticular galaxies

Lenticular galaxies, labelled S0, have a disk and a bulge like spirals, but no obvious arms. They contain little gas and little ongoing star formation, which makes them look more like ellipticals in their stellar population. Their disk structure suggests that they once had arms that were later removed or faded.

The leading explanation for lenticulars is environmental. In dense regions such as galaxy clusters, interactions with hot gas and with other galaxies can strip away gas that would otherwise form stars. Without that gas, a spiral can fade into a lenticular over time.

Irregular galaxies

Irregular galaxies have no regular shape. They tend to be gas-rich, and many show active star formation scattered in clumps. Their irregularity often comes from gravitational disturbance by a neighbour or from the lack of a strong rotating structure. The Large and Small Magellanic Clouds, which orbit the Milky Way, are the best-known examples.

Comparing the types

The table below gives rough, representative values for each family. The numbers are approximate and are meant to show trends rather than exact averages.

Type	Typical shape	Approximate stars	Gas and star formation	Example
Giant elliptical	Smooth, round	hundreds of billions to trillions	Little gas, very low	M87
Dwarf elliptical	Faint, compact	millions to billions	Little gas, low	Sagittarius dwarf (satellite)
Lenticular (S0)	Disk, no arms	tens to hundreds of billions	Little gas, low	NGC 2787
Spiral (Sa to Sc)	Disk with arms	tens to hundreds of billions	Gas-rich, active	Andromeda (M31)
Barred spiral (SB)	Disk with central bar	tens to hundreds of billions	Gas-rich, active	Milky Way
Irregular	No regular form	billions to tens of billions	Gas-rich, clumpy	Large Magellanic Cloud

The pattern is clear. Ellipticals and lenticulars are gas-poor and have mostly old stars, while spirals and irregulars keep forming stars. Elliptical galaxies tend to be the most massive, though spirals such as the Milky Way are also very large.

Colour is one of the quickest clues to a galaxy's type. Light from a galaxy is the sum of all its stars, so a galaxy full of old stars looks red, while one with many young, hot stars looks blue. Astronomers therefore use colour as a rapid stand-in for star formation rate when they survey thousands of galaxies at once. Spectroscopy gives a more detailed picture by revealing emission lines from glowing gas, which mark the places where new stars are forming.

Size and number also differ. Dwarf galaxies are by far the most numerous type in the universe, yet they contribute only a small share of the total starlight. Most of a galaxy's light and mass is concentrated in the few giant systems that are found in clusters and in the largest spirals and ellipticals. Counting galaxies in deep surveys shows that the faint, low-mass population dominates by number, while the bright, massive population dominates by light.

Dark matter and rotation

Spiral galaxies reveal something important about their hidden mass. In the 1970s, Vera Rubin and collaborators measured how fast stars and gas orbit at different distances from the centre. Those speeds stay roughly flat far beyond the visible disk. Visible matter alone cannot produce that pattern, so the galaxies must be embedded in a halo of dark matter that extends well beyond the stars.

Ellipticals and irregulars also contain dark matter, but the evidence there comes from different methods, such as the speeds of stars in ellipticals and the hot gas around giant ellipticals in clusters.

Mergers and how types change

Galaxies do change over time. When two spirals collide, their gas can be driven into the centre, triggering intense bursts of star formation. If the merger is violent enough, the disks can be destroyed and the remnant can look like an elliptical. The Milky Way and Andromeda are on a collision course, expected in roughly 4.5 billion years. That merger is likely to produce a large elliptical-like galaxy, although the details are still being modelled.

Gas removal in clusters can also change a spiral into a lenticular, as described above. So the Hubble classification captures a snapshot of shape, and the shape can shift as the galaxy interacts with its surroundings.

Key takeaways

Hubble's classification sorts galaxies into ellipticals, spirals, lenticulars and irregulars. The sorting is based on shape, but shape tracks stellar age, gas content and current star formation, which makes it a useful first clue about a galaxy's history.

Ellipticals are old and smooth, spirals are disks with active star formation, lenticulars are disks that have lost their gas, and irregulars are disturbed and clumpy. Dark matter and mergers add the rest of the story, shaping how galaxies rotate, grow and change over billions of years.