10.1.4.3. Galaxy Classification

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.

Rendering scientific sketch…

Basic galaxy shape classes

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.

Rendering scientific sketch…

Simplified Hubble tuning fork

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.

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