1.3 Galaxies
›
Chapter 1: The Universe and Its Structure
›
Lesson 3: Galaxies
🎯 Learning Objectives
After completing this lesson, you should be able to:
- Define a galaxy and explain its basic composition.
- Understand the major types of galaxies.
- Differentiate between elliptical, spiral and irregular galaxies.
- Understand the basic classification of spiral and elliptical galaxies.
- Explain the structure and important features of the Milky Way.
- Understand the concept of the Local Group.
- Differentiate between a galaxy, Solar System and Universe.
- Understand the basic role of dark matter in galaxies.
- Understand the basic concept of supermassive black holes in galaxies.
- Prepare important galaxy-related concepts for CSS GSA questions.
1.3.1 What Is a Galaxy?
A galaxy is a vast gravitationally bound system containing
stars, gas, dust and other forms of matter. Galaxies are among the major
large-scale structures of the Universe.
The GSA book describes a galaxy as a fundamental unit of the Universe and
emphasizes its composition of stars together with gas and dust.
Galaxies can differ enormously in their size, shape, stellar population,
amount of gas and dust, and rate of star formation.
A galaxy is a gravitationally bound system of stars, gas, dust and
other matter.
1.3.2 Where Does a Galaxy Fit in the Cosmic Scale?
It is important not to confuse a galaxy with the Solar System or the Universe.
They represent very different scales.
Earth
A planet.
Solar System
The Sun and objects gravitationally associated with it.
Galaxy
A huge gravitationally bound system of stars and other matter.
Universe
The totality of cosmic space, matter, energy and physical phenomena.
Earth → Solar System → Milky Way Galaxy → Local Group → Observable Universe
1.3.3 Classification of Galaxies
Galaxies are commonly classified according to their visible morphology, or
overall appearance and structure.
For CSS preparation, the three major categories emphasized in your GSA book
are:
Elliptical Galaxies
Generally smooth and elliptical or oval in appearance. They contain
relatively little cold gas and dust compared with actively star-forming
spiral galaxies.
Spiral Galaxies
Have a central concentration or bulge and a flattened disk. Many contain
spiral arms rich in gas, dust and young stars.
Irregular Galaxies
Do not have the well-defined elliptical or spiral structure characteristic
of the other major morphological categories.
1.3.4 Elliptical Galaxies
Elliptical galaxies have smooth, rounded or elongated
appearances rather than obvious spiral arms.
Your GSA book classifies elliptical galaxies using the sequence
E0 to E7.
| Feature | Elliptical Galaxies |
|---|---|
| Appearance | Smooth and elliptical/oval. |
| Spiral Arms | Absent. |
| Gas & Dust | Generally less cold gas and dust than typical spiral galaxies. |
| Classification | E0–E7 in the traditional Hubble classification. |
CSS Memory Point:
E0 represents a nearly circular elliptical appearance,
while increasing numbers represent progressively more elongated elliptical
shapes.
1.3.5 Spiral Galaxies
Spiral galaxies generally consist of a flattened disk with
a central bulge and spiral arms extending outward from the central region.
The GSA book emphasizes that stars, gas and dust are gathered into spiral
arms extending from the galaxy’s center. It also gives the traditional
Sa, Sb and Sc classification based largely on the appearance
and winding of the arms.
Sa
Tightly wound arms and a relatively prominent central bulge.
Sb
Intermediate spiral structure.
Sc
More loosely wound arms and a relatively smaller central bulge.
1.3.6 Irregular Galaxies
Irregular galaxies lack the clearly defined structure of
elliptical and spiral galaxies.
The GSA book describes them as having no particular regular shape and
emphasizes their gas and dust content and associated star formation.
🌟 Why Can Irregular Galaxies Have Active Star Formation?
Gas and dust provide the raw material from which new stars can form. Some
irregular galaxies contain substantial amounts of gas and can therefore
experience significant star formation.
1.3.7 Comparison of Major Galaxy Types
| Feature | Elliptical | Spiral | Irregular |
|---|---|---|---|
| General Shape | Oval / elliptical | Disk with spiral arms | No regular shape |
| Spiral Arms | No | Yes | No defined arms |
| Gas & Dust | Generally lower | Often substantial | Can be substantial |
| Star Formation | Usually low | Often active | Can be active |
| Book Classification | E0–E7 | Sa–Sc | No regular sequence |
1.3.8 The Milky Way Galaxy
The Milky Way is the galaxy that contains our Solar System.
It is a spiral galaxy, more specifically classified as a
barred spiral galaxy in modern astronomy.
Your GSA book identifies the Milky Way as a spiral galaxy and gives a diameter
of approximately 100,000 light-years. It also explains that
the Milky Way belongs to a collection of galaxies called the
Local Group.
🌌 Milky Way — CSS Facts
- Our home galaxy.
- Contains the Solar System.
- Spiral galaxy; more specifically, a barred spiral galaxy.
- Diameter is roughly 100,000 light-years as a useful approximate value.
- Member of the Local Group.
- Contains enormous numbers of stars.
1.3.9 The Local Group
The Local Group is the gravitationally associated collection
of galaxies that includes the Milky Way and the Andromeda Galaxy, together
with many smaller galaxies.
The GSA book specifically notes that the Milky Way is part of the Local Group
and identifies the Andromeda Galaxy as a nearby major galaxy.
🌌 Milky Way
Our home spiral galaxy.
🌌 Andromeda
A major neighboring galaxy.
✨ Smaller Galaxies
The Local Group also contains numerous smaller galaxies.
1.3.10 Galaxies and Supermassive Black Holes
Many large galaxies contain a supermassive black hole in or
near their central region. These objects can have masses ranging from
millions to billions of times the mass of the Sun.
A black hole is an object or region where gravity is sufficiently strong that
light cannot escape once it passes inside the event horizon.
🕳️ Black Hole — Important Terms
- Event Horizon: Boundary beyond which escape is impossible
according to classical general relativity. - Singularity: The central region predicted by classical
general relativity where the theory encounters extreme conditions. - Supermassive Black Hole: A black hole with a mass of millions
or billions of Suns, commonly associated with galaxy centers.
1.3.11 Dark Matter and Galaxies
One of the important concepts associated with galaxies is
dark matter.
Dark matter does not appear to emit, absorb or reflect electromagnetic
radiation in the way ordinary matter does. Its presence is inferred mainly
from its gravitational effects.
The GSA book explains that the observed motions of stars and galaxies cannot
be fully explained by the gravity of visible matter alone, leading scientists
to infer the presence of unseen matter.
🌑 Dark Matter — Remember
Not directly visible
Its presence is inferred through effects such as gravity.
Gravitational effect
It contributes to the gravitational behavior of galaxies and larger systems.
Different from dark energy
Dark matter and dark energy are different concepts.
1.3.12 Dark Energy
Dark energy is a separate concept from dark matter. It is
used in modern cosmology to describe the unknown component associated with
the accelerated expansion of the Universe.
Your GSA book connects dark energy with the accelerating expansion of the
Universe and gives an approximate contribution of 68 percent.
Do Not Confuse:
Dark Matter → associated with gravitational effects and
structure formation.
Dark Energy → associated with the accelerated expansion of
the Universe.
| Dark Matter | Dark Energy |
|---|---|
| Inferred mainly through gravitational effects. | Associated with accelerated cosmic expansion. |
| Important for understanding galaxy dynamics and structure. | Important for understanding the large-scale evolution of the Universe. |
| Does not interact with electromagnetic radiation in the ordinary way. | Its physical nature remains unknown. |
1.3.13 Galaxies as Stellar Systems
Galaxies are not simply collections of isolated stars. Their gas, dust, stars,
dark matter and other components interact through gravity and other physical
processes.
Gas clouds within galaxies can collapse under gravity and form new stars.
Stars can then enrich their surroundings with heavier elements through
stellar evolution and stellar explosions. Over cosmic time, these processes
contribute to the continuing evolution of galaxies.
🌟 Simplified Galactic Cycle
Gas + Dust
→
Clouds
→
Star Formation
→
Stellar Evolution
→
Material Returned to Space
→
Future Stars & Planets
📝 CSS Exam Focus
- Galaxy: A huge gravitationally bound system of stars, gas,
dust and other matter. - Major types: Elliptical, spiral and irregular.
- Elliptical classification: E0–E7 in the traditional Hubble
classification. - Spiral classification: Sa, Sb and Sc in the traditional
sequence. - Milky Way: Our home galaxy and a barred spiral galaxy.
- Local Group: Gravitationally associated collection containing
the Milky Way, Andromeda and many smaller galaxies. - Dark Matter: Inferred mainly from gravitational effects.
- Dark Energy: Associated with the accelerated expansion of
the Universe. - Supermassive Black Hole: Very massive black holes associated
with the centers of many large galaxies.
⚡ Quick Revision
Galaxy
A gravitationally bound cosmic system containing stars and other matter.
Spiral
Disk-shaped galaxy with spiral arms; Milky Way is a barred spiral galaxy.
Elliptical
Smooth elliptical or oval morphology; traditional classification E0–E7.
Irregular
Galaxy without a regular elliptical or spiral structure.
Local Group
Our local collection of gravitationally associated galaxies.
Dark Matter
Invisible matter inferred from its gravitational influence.
🚀 CSS GSA One-Liners
- A galaxy is a gravitationally bound system of stars and other matter.
- The three major morphological categories are elliptical, spiral and irregular.
- Elliptical galaxies are traditionally classified from E0 to E7.
- Spiral galaxies are traditionally classified as Sa, Sb and Sc in the basic sequence.
- The Milky Way is a barred spiral galaxy.
- The Solar System is located inside the Milky Way Galaxy.
- The Milky Way belongs to the Local Group.
- Andromeda is a major neighboring galaxy in the Local Group.
- Dark matter is inferred mainly through gravitational effects.
- Dark energy is associated with the accelerated expansion of the Universe.
🧠 Check Your Knowledge
- What is a galaxy?
- What are the three major types of galaxies?
- What is an elliptical galaxy?
- What does the E0–E7 classification represent?
- What is a spiral galaxy?
- What are spiral arms?
- What do Sa, Sb and Sc represent?
- What is an irregular galaxy?
- Which type of galaxy is the Milky Way?
- What is the Local Group?
- Which major neighboring galaxy is associated with the Local Group?
- What is dark matter?
- How do scientists infer the presence of dark matter?
- What is dark energy?
- What is the difference between dark matter and dark energy?
- What is a supermassive black hole?
- Where are supermassive black holes commonly found?
📚 Lesson Summary
Galaxies are enormous gravitationally bound systems containing stars, gas,
dust and other matter. Their visible structures allow astronomers to classify
them into major categories such as elliptical, spiral and irregular galaxies.
The Milky Way is the spiral galaxy that contains our Solar System. It belongs
to the Local Group, which also contains the Andromeda Galaxy and numerous
smaller galaxies.
The study of galaxies also introduces important modern cosmological concepts
such as dark matter, dark energy and supermassive black holes. Together these
concepts help us understand the structure and evolution of the Universe.
1.4 The Solar System
Explore the Sun, planets, moons, asteroids, comets, meteoroids and the
position of Earth within our Solar System.
Course Reference
This lesson incorporates the galaxy-related concepts and terminology from
the LearnCraftLab GSA reference book, including galaxy classification,
Milky Way and Local Group.