KAHIBARO
Table of Contents
Why galaxies are classified
Astronomers see enormous numbers of galaxies, and they do not all look the same. Some are smooth and round, some are thin and rotating disks, and some are messy and distorted. Galaxy classification is the system used to group galaxies by their visible structure and appearance. This helps astronomers compare galaxies, describe them clearly, and study how different kinds of galaxies form and evolve.
Classification is mainly based on morphology, which means shape and visible features. It is not only about size. Two galaxies can have similar sizes but belong to different classes because their structures are different.
Galaxy classification is primarily a classification by appearance, especially shape and structure, not simply by mass, brightness, or distance.
The main galaxy classes
The most widely used basic system divides galaxies into elliptical galaxies, spiral galaxies, barred spiral galaxies, lenticular galaxies, and irregular galaxies. These groups are often introduced using the Hubble classification system.
| Galaxy type | Main appearance | Typical features |
|---|---|---|
| Elliptical | Round or oval, smooth | Little visible structure, few dust lanes |
| Spiral | Flat disk with spiral arms | Central bulge, disk, gas and dust |
| Barred spiral | Spiral galaxy with a central bar | Arms begin from ends of bar |
| Lenticular | Disk-like but no clear spiral arms | Bulge plus disk, smoother than spirals |
| Irregular | No clear regular shape | Distorted or chaotic appearance |
Elliptical galaxies
Elliptical galaxies look smooth and featureless compared with spirals. They range from nearly spherical to very stretched ovals. They are labeled from $E0$ to $E7$, where the number indicates how elongated the galaxy appears.
A simple way to describe the ellipticity is
$$
n = 10\left(1 - \frac{b}{a}\right)
$$
where $a$ is the apparent long axis and $b$ is the apparent short axis. If $b \approx a$, the galaxy looks round and is near $E0$. If $b$ is much smaller than $a$, it appears more elongated.
Elliptical galaxies do not show spiral arms. Their light distribution is generally smooth, and they often contain much less visible gas and dust than spiral galaxies.
In the elliptical class, $E0$ means nearly circular in appearance, and larger numbers such as $E7$ mean more elongated appearance.
Spiral galaxies
Spiral galaxies have a flat rotating disk and curved spiral arms. They usually also have a brighter central bulge. The arms often stand out because they contain gas, dust, and many bright young stars.
Spiral galaxies are commonly divided into subtypes such as Sa, Sb, and Sc. These subtypes describe how tightly wound the arms are and how large the central bulge appears.
| Spiral subtype | Bulge size | Spiral arms |
|---|---|---|
| Sa | Large bulge | Tightly wound |
| Sb | Medium bulge | Moderately wound |
| Sc | Smaller bulge | More open and loosely wound |
This sequence is a way of describing structure. It does not simply mean that one type is older and another is younger.
Barred spiral galaxies
Barred spiral galaxies are similar to spiral galaxies, but they have a straight bar-like structure crossing the central region. The spiral arms begin at the ends of this bar. These galaxies are classified as SBa, SBb, and SBc, with meanings similar to those for ordinary spirals.
The bar is an important visible feature, so barred spirals are placed in their own category rather than being treated as ordinary spirals.
Lenticular galaxies
Lenticular galaxies are written as $S0$. They look like a middle case between elliptical and spiral galaxies. They have a disk and a central bulge, but they do not show obvious spiral arms. Their appearance is smoother than that of spirals.
Because they have a disk but lack prominent arms, they are placed in a separate class. They are useful in classification because they show that galaxies do not all fit only into the two simple groups of elliptical and spiral.
Irregular galaxies
Irregular galaxies do not have a simple symmetric shape like spirals or ellipticals. They may look patchy, twisted, or asymmetric. Some are irregular because of their internal properties, and others may have been disturbed by gravitational interactions with other galaxies.
In a basic classification, irregular galaxies are simply those that do not fit neatly into the regular classes.
The Hubble sequence
A famous way to arrange galaxy classes is the Hubble sequence, sometimes drawn as a tuning fork diagram. Elliptical galaxies lie on one side, lenticular galaxies near the center, and spiral and barred spiral galaxies branch into two related sequences.
The sequence is useful because it gives a simple visual map of galaxy morphology.
The Hubble sequence is a classification scheme based on observed form. It is not a strict timeline that says every galaxy moves along the sequence in one simple direction.
How astronomers classify galaxies
Originally, astronomers classified galaxies by looking at telescope images and describing their shapes. Even now, visual inspection remains important. Modern classification also uses digital imaging and computer analysis to measure properties such as brightness profile, symmetry, bulge size, and arm structure.
A galaxy can look different depending on the angle from which we view it. For example, a spiral galaxy seen face-on clearly shows its arms, while the same galaxy seen edge-on may look like a thin line with a bulge. This makes classification harder.
| Viewing angle | Possible effect on appearance |
|---|---|
| Face-on spiral | Arms easy to see |
| Edge-on spiral | Disk appears thin, arms hard to identify |
| Elliptical from different angles | Shape may appear more or less round |
So classification is based on observed appearance, but astronomers must remember that appearance depends partly on orientation.
Why classification matters
Galaxy classification gives a common language for astronomy. If one astronomer says a galaxy is an $E2$ elliptical or an $SBb$ barred spiral, another astronomer immediately knows its basic structure. This makes it easier to compare observations and organize large surveys of galaxies.
Classification also helps connect visible structure with physical properties. For example, galaxies with disks, bars, or smooth light distributions often behave differently. The classification itself is morphological, but it becomes a starting point for deeper physical study.
Limits of simple classification
Real galaxies are not always easy to place into one box. Some show weak bars, faint arms, rings, tidal distortions, or signs of interaction with nearby galaxies. Others may be in the process of merging. Because of this, basic categories are useful but not perfect.
For beginners, the key idea is that galaxy classification is a structured way to describe what galaxies look like. The main categories are elliptical, spiral, barred spiral, lenticular, and irregular, and the Hubble sequence is the classic framework for arranging them.
A good beginner summary is this: ellipticals are smooth, spirals have arms, barred spirals have a central bar, lenticulars have a disk without clear arms, and irregulars lack a regular overall shape.