11-15-2025, 09:55 AM
Chapter 5 — Galaxies
Galaxies are vast cosmic systems containing billions of stars, planets, gas, dust, and dark matter,
all held together by gravity.
Our home galaxy is the Milky Way, and the universe contains at least hundreds of billions more.
Galaxies are the building blocks of the universe.
Understanding them helps us understand how the cosmos forms, evolves, and grows.
---
5.1 What Is a Galaxy?
A galaxy is a massive collection of:
• stars
• planets and moons
• gas and dust
• star-forming regions
• black holes
• dark matter (a LOT of it)
Galaxies range in size from small dwarf galaxies with a few million stars to giants
with over a trillion stars.
---
5.2 Types of Galaxies
Galaxies fall into three major shapes:
1. Spiral Galaxies
• Flat disks with spiral arms
• A central bulge of older stars
• Lots of gas and active star formation
• Example: The Milky Way
2. Elliptical Galaxies
• Rounded or football-shaped
• Very little gas
• Mostly old stars
• Example: M87 (home of a supermassive black hole)
3. Irregular Galaxies
• No clear shape
• Often formed by collisions or gravitational disturbances
• Example: The Large Magellanic Cloud
---
5.3 What Galaxies Are Made Of
Galaxies contain several key components:
Stars:
Billions or trillions of them.
Nebulae:
Clouds of gas and dust where new stars form.
Star Clusters:
Groups of stars born from the same cloud.
Black Holes:
Almost every large galaxy contains a supermassive black hole at its centre.
Dark Matter:
Invisible matter that makes up most of a galaxy’s total mass.
It holds galaxies together and affects how stars orbit.
---
5.4 The Milky Way Galaxy
Our home galaxy is:
• a barred spiral galaxy
• about 100,000 light-years wide
• containing over 100 billion stars
• rotating once every 200–250 million years
Our Sun is located in one of the spiral arms called the Orion Arm.
At the centre lies a supermassive black hole named Sagittarius A*.
---
5.5 How Galaxies Form
Galaxies formed after the Big Bang as gas cooled and clumped together under gravity.
The steps:
1. Dark matter formed gravitational "halos."
2. Hydrogen gas collected inside these halos.
3. Stars began to form.
4. Small protogalaxies merged into larger ones.
5. Over time, galaxies grew and changed shape.
Galaxy formation is still happening today.
---
5.6 Galaxy Collisions
Galaxies often collide — it is normal in the universe.
When galaxies collide:
• Stars rarely collide (space is huge)
• Gas clouds smash together and trigger star formation
• Dark matter halos merge
• The galaxy’s shape changes
• Black holes may merge as well
In about 4 billion years, the Milky Way will collide with the Andromeda Galaxy.
The result will be a new, larger galaxy.
---
5.7 The Largest Structures in the Universe
Galaxies are not isolated. They form groups, clusters, and superclusters.
Galaxy Group:
• A small collection of galaxies
• The Milky Way is in the Local Group
Galaxy Cluster:
• Hundreds or thousands of galaxies
• Immense gravity
Supercluster:
• Gigantic regions of space containing many clusters
• Our home supercluster is called Laniakea
Galaxies are connected by enormous cosmic filaments — the "cosmic web."
---
5.8 Why Galaxies Matter
Galaxies are essential for understanding the universe because:
• They show how matter is arranged on cosmic scales
• They reveal the effects of dark matter
• They help us study star formation
• Their rotation curves provide evidence for dark matter
• They contain the environments where planets and life can form
The story of galaxies is the story of the universe itself.
---
Chapter Summary
• Galaxies contain billions of stars, gas, dust, and dark matter.
• They come in spiral, elliptical, and irregular shapes.
• The Milky Way is a barred spiral galaxy with a supermassive black hole at its centre.
• Galaxies formed from cooling gas inside dark matter halos.
• Galaxy collisions are common and shape their evolution.
• Galaxies group into clusters, superclusters, and cosmic web structures.
• Studying galaxies helps us understand the structure and evolution of the universe.
---
Practice Questions
1. Name the three main types of galaxies.
2. What holds galaxies together?
3. Why are galaxy collisions important for star formation?
4. Where is the Sun located in the Milky Way?
5. What is a supermassive black hole, and where is it usually found?
---
Written and Compiled by Lee Johnston — Founder of The Lumin Archive
Galaxies are vast cosmic systems containing billions of stars, planets, gas, dust, and dark matter,
all held together by gravity.
Our home galaxy is the Milky Way, and the universe contains at least hundreds of billions more.
Galaxies are the building blocks of the universe.
Understanding them helps us understand how the cosmos forms, evolves, and grows.
---
5.1 What Is a Galaxy?
A galaxy is a massive collection of:
• stars
• planets and moons
• gas and dust
• star-forming regions
• black holes
• dark matter (a LOT of it)
Galaxies range in size from small dwarf galaxies with a few million stars to giants
with over a trillion stars.
---
5.2 Types of Galaxies
Galaxies fall into three major shapes:
1. Spiral Galaxies
• Flat disks with spiral arms
• A central bulge of older stars
• Lots of gas and active star formation
• Example: The Milky Way
2. Elliptical Galaxies
• Rounded or football-shaped
• Very little gas
• Mostly old stars
• Example: M87 (home of a supermassive black hole)
3. Irregular Galaxies
• No clear shape
• Often formed by collisions or gravitational disturbances
• Example: The Large Magellanic Cloud
---
5.3 What Galaxies Are Made Of
Galaxies contain several key components:
Stars:
Billions or trillions of them.
Nebulae:
Clouds of gas and dust where new stars form.
Star Clusters:
Groups of stars born from the same cloud.
Black Holes:
Almost every large galaxy contains a supermassive black hole at its centre.
Dark Matter:
Invisible matter that makes up most of a galaxy’s total mass.
It holds galaxies together and affects how stars orbit.
---
5.4 The Milky Way Galaxy
Our home galaxy is:
• a barred spiral galaxy
• about 100,000 light-years wide
• containing over 100 billion stars
• rotating once every 200–250 million years
Our Sun is located in one of the spiral arms called the Orion Arm.
At the centre lies a supermassive black hole named Sagittarius A*.
---
5.5 How Galaxies Form
Galaxies formed after the Big Bang as gas cooled and clumped together under gravity.
The steps:
1. Dark matter formed gravitational "halos."
2. Hydrogen gas collected inside these halos.
3. Stars began to form.
4. Small protogalaxies merged into larger ones.
5. Over time, galaxies grew and changed shape.
Galaxy formation is still happening today.
---
5.6 Galaxy Collisions
Galaxies often collide — it is normal in the universe.
When galaxies collide:
• Stars rarely collide (space is huge)
• Gas clouds smash together and trigger star formation
• Dark matter halos merge
• The galaxy’s shape changes
• Black holes may merge as well
In about 4 billion years, the Milky Way will collide with the Andromeda Galaxy.
The result will be a new, larger galaxy.
---
5.7 The Largest Structures in the Universe
Galaxies are not isolated. They form groups, clusters, and superclusters.
Galaxy Group:
• A small collection of galaxies
• The Milky Way is in the Local Group
Galaxy Cluster:
• Hundreds or thousands of galaxies
• Immense gravity
Supercluster:
• Gigantic regions of space containing many clusters
• Our home supercluster is called Laniakea
Galaxies are connected by enormous cosmic filaments — the "cosmic web."
---
5.8 Why Galaxies Matter
Galaxies are essential for understanding the universe because:
• They show how matter is arranged on cosmic scales
• They reveal the effects of dark matter
• They help us study star formation
• Their rotation curves provide evidence for dark matter
• They contain the environments where planets and life can form
The story of galaxies is the story of the universe itself.
---
Chapter Summary
• Galaxies contain billions of stars, gas, dust, and dark matter.
• They come in spiral, elliptical, and irregular shapes.
• The Milky Way is a barred spiral galaxy with a supermassive black hole at its centre.
• Galaxies formed from cooling gas inside dark matter halos.
• Galaxy collisions are common and shape their evolution.
• Galaxies group into clusters, superclusters, and cosmic web structures.
• Studying galaxies helps us understand the structure and evolution of the universe.
---
Practice Questions
1. Name the three main types of galaxies.
2. What holds galaxies together?
3. Why are galaxy collisions important for star formation?
4. Where is the Sun located in the Milky Way?
5. What is a supermassive black hole, and where is it usually found?
---
Written and Compiled by Lee Johnston — Founder of The Lumin Archive