1.4 Types of Galaxies
› Chapter 1: The Universe and Its Structure
› Lesson 1.4: Types of Galaxies
🎯 What You Will Learn
In this lesson, we will study the major types of galaxies and understand how
astronomers classify them according to their observable structure and shape.
- What galaxy classification means
- Elliptical galaxies and their E0–E7 classification
- Spiral galaxies and their Sa, Sb and Sc classification
- Barred spiral galaxies
- Irregular galaxies
- Major differences between galaxy types
- The relationship between galaxy structure, gas, dust and star formation
- The Milky Way’s galaxy type
- Important CSS examination points
1.4.1 What Are the Major Types of Galaxies?
Galaxies have different shapes and structures. Astronomers classify them
primarily according to their visible morphology, meaning their overall
observable form and structure.
For CSS GSA preparation, the three major galaxy categories emphasized in
your reference book are:
1. Elliptical
Smooth, rounded or elongated galaxies without prominent spiral arms.
2. Spiral
Disk-shaped galaxies with a central region and spiral arms.
3. Irregular
Galaxies without a well-defined regular elliptical or spiral structure.
The reference book identifies elliptical, spiral and irregular galaxies as
the principal classifications discussed in this section.
1.4.2 Elliptical Galaxies
Elliptical galaxies have a generally smooth appearance and
range from nearly spherical forms to significantly elongated ellipsoidal
shapes.
Unlike spiral galaxies, they do not have prominent spiral arms. Many
elliptical galaxies contain older stellar populations and comparatively
little cold gas and dust available for new star formation.
Your GSA book describes elliptical galaxies as an important galaxy category
and gives the traditional classification range of E0 to E7.
🔭 E0–E7 Classification
The traditional Hubble elliptical classification uses numbers from
E0 to E7.
Nearly circular
Most elongated
CSS Memory Point
E0 → nearly circular
E7 → more elongated
1.4.3 Spiral Galaxies
Spiral galaxies are characterized by a flattened disk
containing stars, gas and dust, together with a central concentration of
stars. Many spiral galaxies have distinctive spiral arms.
The GSA book explains that stars, gas and dust are gathered in spiral arms
that spread outward from the center. It also gives the basic
Sa, Sb and Sc classification based on how tightly the arms
are wound.
🌀 Basic Spiral Classification
Sa
Generally tighter spiral arms and a more prominent central bulge.
Sb
Intermediate spiral structure between Sa and Sc.
Sc
Generally more loosely wound arms and a smaller central bulge.
Main Parts of a Spiral Galaxy
Central Bulge
A dense central concentration of stars.
Disk
A flattened region containing stars, gas and dust.
Spiral Arms
Regions containing substantial gas, dust and many young stars.
Halo
A more extended region surrounding the main disk.
1.4.4 Barred Spiral Galaxies
A barred spiral galaxy is a type of spiral galaxy in which
the central bulge is crossed by an elongated bar-shaped structure of stars.
The spiral arms generally emerge from the ends of this bar.
⭐ CSS Important Example
The Milky Way is classified as a
barred spiral galaxy.
1.4.5 Irregular Galaxies
Irregular galaxies do not possess the well-defined elliptical
or spiral structure seen in the other major categories.
The reference book describes irregular galaxies as having no particular
shape and emphasizes their gas and dust content and associated star
formation.
No Regular Shape
Their structure does not fit the typical elliptical or spiral pattern.
Gas & Dust
Many irregular galaxies contain significant gas and dust.
Star Formation
Gas and dust provide material from which new stars can form.
1.4.6 Why Do Galaxies Have Different Shapes?
Galaxy structure is influenced by several physical and evolutionary
processes. These include gravitational interactions, mergers, the amount and
distribution of gas, angular momentum and the history of star formation.
Galaxies can also interact with neighboring galaxies. Gravitational
interactions can distort their structures and, in some cases, contribute to
changes in their morphology.
🌌 Factors Affecting Galaxy Structure
Controls the large-scale organization of matter.
Important raw material for star formation.
Can distort or transform galactic structures.
Galaxies change over cosmic time.
1.4.7 Comparison of Galaxy Types
| Feature | Elliptical | Spiral | Irregular |
|---|---|---|---|
| Shape | Rounded to elongated | Disk with spiral arms | No regular shape |
| Spiral Arms | Absent | Present | No defined spiral structure |
| Gas & Dust | Generally less cold gas and dust | Often substantial | Can be substantial |
| Star Formation | Usually relatively low | Often active | Can be active |
| Traditional Classification | E0–E7 | Sa–Sc | No regular sequence |
1.4.8 The Milky Way — What Type Is It?
The Milky Way is the galaxy containing our Solar System.
The reference book identifies it as a spiral galaxy and gives
an approximate diameter of 100,000 light-years.
Modern astronomical classification identifies the Milky Way more specifically
as a barred spiral galaxy.
🌌 Milky Way — Quick Facts
- Our home galaxy.
- Contains the Solar System.
- Spiral galaxy.
- More specifically, a barred spiral galaxy.
- Approximately 100,000 light-years across as a useful approximate value.
- Part of the Local Group.
- Andromeda is a major neighboring galaxy.
1.4.9 Galaxy Classification at a Glance
ELLIPTICAL → E0 – E7
SPIRAL →Sa – Sb – Sc
IRREGULAR →No regular shape/class sequence
📝 CSS Exam Focus
- Three major galaxy types: Elliptical, Spiral and Irregular.
- Elliptical galaxies: Traditionally classified E0–E7.
- E0: Nearly circular elliptical appearance.
- E7: Most elongated end of the traditional elliptical sequence.
- Spiral galaxies: Characterized by a disk and spiral arms.
- Sa, Sb, Sc: Traditional basic spiral classifications.
- Irregular galaxies: Lack a well-defined regular shape.
- Milky Way: Spiral galaxy, more specifically a barred spiral.
- Gas and dust: Important for star formation, particularly in
star-forming regions.
⚡ Important One-Liners
- The major galaxy types are elliptical, spiral and irregular.
- Elliptical galaxies are traditionally classified from E0 to E7.
- E0 represents the nearly circular end of the traditional elliptical sequence.
- E7 represents the more elongated end of the sequence.
- Spiral galaxies contain a flattened disk and spiral arms.
- Traditional basic spiral classification includes Sa, Sb and Sc.
- Irregular galaxies do not have a well-defined regular shape.
- The Milky Way is a spiral galaxy.
- The Milky Way is more specifically classified as a barred spiral galaxy.
- The Solar System is located within the Milky Way.
🔁 Quick Revision
Elliptical
Smooth shape; traditional classification E0–E7.
Spiral
Disk and spiral arms; basic sequence Sa–Sc.
Irregular
No well-defined regular structure.
Milky Way
Our barred spiral galaxy.
🧠 Check Your Knowledge
- What are the three major types of galaxies?
- What is an elliptical galaxy?
- What does the E0–E7 classification represent?
- Which end of the elliptical sequence is more circular?
- Which end is more elongated?
- What is a spiral galaxy?
- What are spiral arms?
- What do Sa, Sb and Sc represent?
- What is a barred spiral galaxy?
- What type of galaxy is the Milky Way?
- What is an irregular galaxy?
- Why can gas and dust be important for star formation?
- How are elliptical and spiral galaxies structurally different?
- How are irregular galaxies different from elliptical and spiral galaxies?
🎯 CSS-Style Practice MCQs
1. Which classification is traditionally associated with elliptical galaxies?
A) Sa–Sc
B) E0–E7
C) O–B–A–F
D) I–IV
Answer: B) E0–E7
2. Which type of galaxy has prominent spiral arms?
A) Elliptical
B) Spiral
C) Irregular
D) None
Answer: B) Spiral
3. The Milky Way is best classified as a:
A) Elliptical galaxy
B) Irregular galaxy
C) Barred spiral galaxy
D) Ring nebula
Answer: C) Barred spiral galaxy
📚 Lesson Summary
Galaxies display a wide variety of shapes and structures. For basic CSS GSA
preparation, the three major categories are elliptical, spiral and
irregular galaxies.
Elliptical galaxies have smooth, rounded or elongated shapes and are
traditionally classified from E0 to E7. Spiral galaxies
contain a flattened disk and spiral arms, with the basic traditional sequence
Sa, Sb and Sc. Irregular galaxies lack a clearly defined
regular structure.
The Milky Way, which contains our Solar System, is a spiral
galaxy and is more specifically classified as a barred spiral
galaxy.
Understanding these classifications is important because questions about
galaxy types, the Milky Way, galaxy structure and astronomical classification
can be tested directly or indirectly in CSS General Science and Ability.
Reference Note
The core classification terminology in this lesson follows the provided
GSA reference book, including elliptical galaxies (E0–E7), spiral galaxies
(Sa, Sb, Sc), irregular galaxies, and the Milky Way’s classification.
1.5 Stars and Their Life Cycle
Learn about stars, their formation, evolution, classification, nuclear
fusion, supernovae, neutron stars and black holes.