Types of Galaxies
There are four distinct types of galaxies in the universe: elliptical, spiral, barred spiral, and irregular. Although these are the four main types, there are various types of galaxies, and they are classified by shape.
A galaxy is a cluster of stars, gas, and dust that is kept together by the force of gravity. Within galaxies, there are also planets, black holes, moons, comets, and dark matter.
For example, the Milky Way, the solar system in which we reside, is classified as a barred spiral galaxy because of its general spiral shape, accompanied by a bar and a disk of stars at its center.
The other general classifications of galaxies include spiral, elliptical, and irregular galaxies. This classification of galaxy shapes was created by Edwin Hubble in 1926 and laid out in a diagram called the Hubble Tuning Fork, because of its resemblance to a tuning fork.
If you want to see some of the most incredible galaxy photos ever taken, be sure to visit the Hubble Space Telescope Image Archives: Galaxies.
Jump to a Section (Table of Contents)
As a full-time astrophotographer with over 15 years of experience, I have photographed dozens of galaxies over the course of my career. From elegant spirals like Andromeda Galaxy and Bode’s Galaxy to chaotic irregulars and bright ellipticals, each one my image reveals a different stage in galactic evolution.
To learn more about the science and art of deep-sky astrophotography, be sure to visit my getting started guide. For a better idea of how I capture different galaxy types from my backyard in the city, you can also view the astrophotography equipment needed for this type of astronomy.
Galaxy Photography by Trevor Jones. Pictured: Messier 106, Messier 33, and Messier 82.
Recent Galaxy Discoveries & News
The field of galaxy research is evolving rapidly, and several exciting findings highlight how much is still to learn about galaxy formation and evolution. Here are a few recent highlights:
- Astronomers using the James Webb Space Telescope (JWST) discovered a galaxy at redshift 14.2 (about 300 million years after the Big Bang), showing surprisingly advanced chemical complexity for its age.
- Researchers found a colossal bridge of neutral hydrogen gas stretching 185,000 light-years between two dwarf galaxies (NGC 4532 and DDO 137), underscoring the role of interactions even among small galaxies.
- A newly identified isolated early-type dwarf galaxy, apparently “runaway” and far from any group, may challenge conventional ideas of how such galaxies form and evolve in clusters versus isolation.
This data continues to reshape our understanding of how galaxies grow, merge and evolve across cosmic time.
Spiral Galaxies
Named for their shape, spiral galaxies are the most common type of galaxy in the universe. Of all the known galaxies in the universe, 77% of them are classified as spiral galaxies.
These types of galaxies are classified by their 3 main components: a disk, a bulge, and a halo. The bulge is housed at the center of the galaxy. It is spherical and contains only old stars.
The disk component is both the shape of the galaxy and the region where new stars form. The halo is located on the outer edge of the galaxy. Similar to the bulge, the halo is composed of old star clusters, known as globular clusters.
The Triangulum Galaxy with annotated nebula regions. Photo by Trevor Jones.
Typically, the new stars form at the “arms” of the disk; however, when this type of galaxy does not have arms, it is referred to as lenticular, or S0.
Regular or standard spiral galaxies are typically notated as S or SA. For both spiral and barred spiral galaxies, after the initial notation of the broad classification, there is a more specific classification noting the tightness of the spiral.
This more specific notation is the letters ‘a’, ‘b’, ‘c’, ‘d’, and so on where ‘a’ represents the tightest spiral.
Galaxy Type Chart (Tuning-fork-style diagram of the Hubble sequence) (Wikipedia)
There are many beautiful examples of spiral galaxies in the night sky. One of the best examples of a spiral galaxy is the Andromeda Galaxy.
Other than the Large and Small Magellanic clouds, which are only visible from the southern hemisphere, the Andromeda Galaxy is the brightest galaxy you can see with your naked eye. To the unaided eye, this spiral galaxy appears as a smudge of light in the Andromeda constellation.
Through astrophotography, we can reveal much more of the galaxy’s structure, including the outer arms containing bright nebula regions. A camera’s sensor can record much more detail in a long-exposure image than our eyes could ever see visually.
The Andromeda Galaxy. Photo by Trevor Jones.
Lenticular Galaxy
Briefly elaborating, S0 galaxies are like a combination between a spiral galaxy and elliptical galaxy because of the bulge and disk that they do have, like spiral galaxies, but they do not have a spiral-like structure.
A lenticular galaxy is a type of galaxy intermediate between an elliptical galaxy and a spiral galaxy. This designation is according to the galaxy morphological classification scheme.
Lenticular galaxies are large-scale disc galaxies that do not have large-scale spiral arms. They have minimal ongoing star formation, as they have used up or lost their interstellar matter. As shown in the Sombrero Galaxy image below, they often retain a significant amount of dust in their disks.
The Sombrero Galaxy. Photo by Trevor Jones.
Barred Spiral Galaxies
Although similar to spiral galaxies, barred spiral galaxies differ slightly in their shape. Rather than just being spiraled from the nucleus like ordinary spiral galaxies, these have a ‘bar’ of matter running through them. From the ends of this ‘bar’, the arms emerge.
This middle bar, with the two arms extended from the ends of the bar, causes barred galaxies to be shaped similarly to the letter ‘S’. Other than this major difference, barred spiral galaxies and typical spiral galaxies are essentially the same.
Just like typical spiral galaxies, barred spiral galaxies contain a disk, bulge, and halo.
NGC 1300. NASA APOD June 11, 2020.
According to Hubble Tuning Fork, barred spiral galaxies are notated as SB rather than just S or SA as typical spiral galaxies are. Additionally, as noted earlier, the Milky Way is a barred galaxy!
But this went unnoticed by astronomers for a while, compared to other galaxies, because the Milky Way’s bar is smaller than those of other barred galaxies.
Some other examples of barred galaxies include NGC 1300 and NGC 1365. NGC 1300 and NGC 1365 are only 60 million light-years from Earth, which is close enough to be seen by a telescope.
Because of their close proximity to Earth, both of these galaxies are pieces of two different constellations. NGC 1300 is a part of the constellation Eridanus, and NGC 1365 is a part of Fornax.
Elliptical Galaxies
These elliptical galaxies are characterized by their oval shapes and lack of central bulges. In contrast to their name, however, sometimes these galaxies are rather circular.
The stars and gas in these types of galaxies are spread fairly evenly throughout; however, the center remains the brightest region. Additionally, elliptical galaxies do not contain many new stars. Instead, these galaxies are comprised of old, low-mass stars.
At the center of an elliptical galaxy is typically a black hole. Compared to a barred spiral or typical spiral galaxy, in an elliptical galaxy, there is a black hole where the bulge of old stars should be.
This black hole may be part of the reason new stars do not often form in elliptical galaxies.
NGC 3610. ESA/Hubble & NASA, Acknowledgement: Judy Schmidt
Because the central black hole of elliptical galaxies likely consumes the dust and gas necessary for new star formation, the materials necessary to make these new stars are not abundant in elliptical galaxies.
The notation for elliptical galaxies is E. Unlike the additional spiral tightness classification of barred and typical spiral galaxies, elliptical galaxies have a more specific classification of the numbers 0 through 7.
The 0 represents the most round elliptical galaxy, and 7 means that the galaxy is very oval.
Irregular Galaxies
As the name implies, irregular galaxies are all quite unique. Each irregular galaxy does not have a size or shape that is what we know as ‘regular’ or ‘normal’. These do not have any previously discussed components, such as a bulge, disk, or arms, that other galaxies are known to have.
According to NASA, the uniqueness of irregular galaxies may be because of interactions they could have had in the past with other neighboring galaxies.
Irregular Galaxy NGC 55. NASA APOD, September 21, 2018. Martin Pugh.
Although there is nothing typical, symmetrical, or ordinary about irregular galaxies, there are two distinct types: Irr I and Irr II. Irr I tend to have regions with an abundance of hydrogen gas as well as young stars.
In contrast, Irr II is extremely dust-rich, making it incredibly difficult to see stars in this type of galaxy.
Colliding Galaxies
Galactic collisions are among the most spectacular events in the universe. When two galaxies pass close enough, their mutual gravity distorts their shapes, pulling out vast tidal tails of stars, gas, and dust.
While the individual stars within galaxies almost never collide, their orbits are dramatically altered, and massive bursts of star formation often occur as gas clouds are compressed.
Over millions of years, two spirals can merge into a single, larger elliptical galaxy, a process that completely reshapes their structure. Famous examples include the Antennae Galaxies and the Whirlpool Galaxy, both of which show the telltale signs of tidal interaction.
Even our own Milky Way is on a collision course with the Andromeda Galaxy; in about 4-5 billion years, the two will merge to form a new, more massive system. Galactic collisions are not rare; they’re a natural part of how galaxies grow and evolve.
Formation
Clusters and clouds of stars and dust moving through the universe are thought to be the very beginnings of galaxy formation. As these groupings move closer together, gravity begins to play a role.
It is thought that once close enough, gravity will force together groups of stars and dust into a much larger entity.
Galaxy Formation and Evolution. Houjun Mo, Frank van den Bosch, and Simon White.
The way in which different types of galaxies are formed is dependent upon the interactions of each component during the original formation of the galaxy.
For example, collisions of space masses are likely to occur, which may then force certain pieces to the outer edges of the galaxy. This interaction would then potentially form the arms of a spiral galaxy.
How Galaxies Get Their Shape
Galaxies do not simply “appear” in a fixed form. Their shapes result from complex processes that combine gravity, angular momentum, star formation, gas dynamics, and sometimes dramatic collisions.
At the most basic level, a rotating cloud of gas and dust will, under gravity and conservation of angular momentum, settle into a flattened disk: the gas contracts, spins faster, and the stars form preferentially in the plane of rotation.
Conversely, when galaxies merge or interact strongly, those orderly disks can be disturbed and converted into thicker, more spheroidal or irregular shapes.
The nuclear bulges of some spiral galaxies form from the collapse of a single protogalactic cloud, while others grow over time through mergers with smaller galaxies. Lumen Learning Astronomy Course.
In other words, collisions and tidal forces scramble the orbits of stars and gas, often reducing rotation‐dominated structure and leaving more random motions, a hallmark of many elliptical or irregular galaxies.
Finally, internal factors also contribute: the amount of gas and dust available, the rate of star formation, and whether a galaxy has a central bar or bulge all influence morphology.
These give rise to the variety of forms we see, spiral arms, bars, lenticular disks, or chaotic irregulars.
The Role of the Galactic Bulge
At the heart of most galaxies lies a dense, central region of stars known as the bulge. Its origin remains one of astronomy’s enduring puzzles.
In spiral galaxies like the Milky Way, the bulge is a bright, rounded core surrounded by a rotating disk of stars and gas. Some bulges are smooth and spheroidal, resembling miniature elliptical galaxies, while others are elongated and “boxy,” often shaped by bars and dynamic instabilities within the disk. Astronomers are still debating how bulges form.
Examples of pseudobulges and classical bulges in spiral galaxies. NASA/IPAC Extragalactic Database.
One theory suggests they are remnants of ancient mergers, where smaller galaxies combined and sank toward the center, building up dense stellar populations over time. Another proposes that bulges can grow internally, as bars funnel gas inward, triggering waves of star formation near the galactic core.
The composition and shape of a galaxy’s bulge reveal key clues about its past, whether it grew peacefully through slow evolution or violently through collisions.
With instruments like the James Webb Space Telescope now peering deep into galactic centers, scientists hope to finally unravel how these mysterious central structures formed and what they reveal about the life stories of galaxies.
Naked-Eye Galaxies You Can See
Certain galaxies are so bright that you can observe them visually with the unaided eye. This means that you do not need a telescope or binoculars to see the galaxy; you just need to look up at the right time and place.
Light pollution can spoil the view, so be sure to try to view these naked-eye galaxies from a dark-sky location. Here is a list of the brightest galaxies you can observe with the naked eye under clear, moonless conditions.
Naked-Eye Galaxies by Visual Magnitude
| Galaxy Name | Apparent Magnitude | Notes |
|---|---|---|
| Large Magellanic Cloud | 0.9 | Visible from the Southern Hemisphere; satellite of the Milky Way |
| Small Magellanic Cloud | 2.7 | Another Milky Way companion, near the Tucana constellation |
| Andromeda Galaxy (M31) | 3.4 | Nearest major galaxy; visible under dark skies |
| Triangulum Galaxy (M33) | 5.7 | Faint but visible in pristine skies with averted vision |
| Centaurus A (NGC 5128) | 6.8 | Bright elliptical galaxy with dust lane; southern skies only |
| Bode’s Galaxy (M81) | 6.9 | Large spiral in Ursa Major; often paired with M82 |
| Sculptor Galaxy (NGC 253) | 7.2 | Bright starburst galaxy in the Sculptor constellation |
I have personally observed the two brightest northern-hemisphere galaxies from a dark-sky location (Bortle Scale class 2). The Andromeda Galaxy and Triangulum Galaxy were noticeable “smudges” in the night sky.
In the picture below, you’ll notice the Andromeda Galaxy is visible to the upper right of the image. Although this is a long-exposure photograph, it is a useful (exaggerated) representation of what you can expect to see under pristine skies.
The Andromeda Galaxy is visible to the naked eye from a dark sky location.
Specific constellations, such as Virgo, contain galaxy clusters. The Virgo Cluster is a galaxy cluster found in the constellations Coma Berenices and Virgo.
The cluster’s center is located approximately 53.8 million light-years away from the solar system, at the center of the Virgo Supercluster, the larger cluster of galaxies that also contains the Local Group, which in turn includes the Andromeda Galaxy and the Milky Way.
The Virgo Cluster contains about 1300 galaxies, possibly even up to 2000. The image below highlights a particularly handsome cluster of galaxies, known as Markarian’s Chain.
Markarian’s Chain (Trevor Jones) The Virgo Cluster of Galaxies.
Types of Galaxies (Video)
In the following video, the author explains how galaxies are categorized by their apparent shapes. The shape of the galaxy gives astronomers clues about the age and types of stars within it.
This is an excellent summary of the different types of galaxies in the universe.