11-15-2025, 10:02 AM
Chapter 8 — Galaxies: Structure, Types & Evolution
Galaxies are the building blocks of the universe.
Each one contains billions or even trillions of stars, plus planets, gas, dust, dark matter,
and supermassive black holes at their centres.
Understanding galaxies helps us understand the universe itself.
---
8.1 What Is a Galaxy?
A galaxy is a massive gravitational system made of:
• stars
• planets
• nebulae
• star clusters
• gas and dust
• dark matter
• a central supermassive black hole
Galaxies are held together by gravity and shaped by the invisible influence of dark matter.
Galaxies range in size from:
• dwarf galaxies: ~10 million stars
• giant galaxies: several trillion stars
The Milky Way contains around 100–400 billion stars.
---
8.2 The Three Main Types of Galaxies
Galaxies come in three primary shapes:
1. Spiral Galaxies
• Disc-shaped
• Central bulge
• Rotating spiral arms
• Lots of gas and dust → active star formation
Examples:
• The Milky Way
• Andromeda
2. Elliptical Galaxies
• Round or oval shape
• Little gas or dust
• Mostly old stars
• Very little new star formation
They can be small (dwarf elliptical) or enormous (giant elliptical).
3. Irregular Galaxies
• No fixed shape
• Often formed by collisions
• Very active star-forming regions
Examples:
• The Large Magellanic Cloud
• The Small Magellanic Cloud
---
8.3 The Milky Way Galaxy
Our galaxy is a barred spiral galaxy.
Key features:
• Diameter: ~100,000 light-years
• Thickness: ~1,000 light-years
• Four major spiral arms
• A bar-shaped centre
• Contains a supermassive black hole (Sagittarius A*)
• Sun orbits the centre every ~225 million years
The Sun is located about 27,000 light-years from the centre, in the Orion Arm.
---
8.4 Galactic Rotation & Dark Matter
Stars orbit the centre of a galaxy, but something strange happens:
Stars far from the centre orbit too fast to be held in place by visible matter.
This led to the discovery of:
Dark Matter
• Invisible
• Does not emit light
• Exerts gravitational influence
• Makes up ~85% of a galaxy’s total mass
Dark matter forms vast haloes around galaxies, holding them together.
Without dark matter, galaxies would fly apart.
---
8.5 How Galaxies Form
Galaxies began forming after the Big Bang from tiny density fluctuations.
Galactic formation steps:
1. Gas clouds collapse under gravity
2. Small protogalaxies merge
3. Over billions of years, these grow into the galaxies we see today
Mergers shape a galaxy’s final structure:
• Merging spirals can form an elliptical galaxy
• Small dwarfs are often absorbed by bigger galaxies
The Milky Way is currently absorbing several dwarf galaxies.
---
8.6 Supermassive Black Holes
Almost every large galaxy has a supermassive black hole at its centre.
Mass ranges from:
• millions of times the Sun’s mass
to
• tens of billions of times the Sun’s mass
These black holes influence:
• galactic rotation
• galaxy growth
• star formation rates
• energy output (e.g., quasars)
Quasars are galaxies with extremely active central black holes, some of the brightest objects in the universe.
---
8.7 Interactions and Collisions
Galaxies frequently collide — but space is so vast that stars rarely crash into each other.
Collisions cause:
• tidal distortions
• bursts of star formation
• black hole activity
• merging into a new galaxy
The Milky Way and Andromeda will collide in ~4.5 billion years, eventually forming a giant elliptical galaxy.
---
8.8 Large-Scale Structure of the Universe
Galaxies are not scattered randomly.
They form:
• groups
• clusters
• superclusters
• filaments and walls
These structures create the “cosmic web” — vast networks of galaxies separated by immense voids.
---
Chapter Summary
• Galaxies contain stars, planets, gas, dust, dark matter, and black holes.
• Spiral, elliptical, and irregular are the main types.
• Dark matter holds galaxies together and shapes their motion.
• Supermassive black holes play a key role in galaxy evolution.
• Galaxies grow through mergers and interactions.
• On the largest scale, galaxies form a cosmic web across the universe.
---
Practice Questions
1. What are the three main types of galaxies, and how do they differ?
2. What evidence supports the existence of dark matter in galaxies?
3. Why do galaxies often collide, even though stars rarely do?
4. What role do supermassive black holes play in galaxy formation?
5. What is the cosmic web?
---
Written and Compiled by Lee Johnston — Founder of The Lumin Archive
Galaxies are the building blocks of the universe.
Each one contains billions or even trillions of stars, plus planets, gas, dust, dark matter,
and supermassive black holes at their centres.
Understanding galaxies helps us understand the universe itself.
---
8.1 What Is a Galaxy?
A galaxy is a massive gravitational system made of:
• stars
• planets
• nebulae
• star clusters
• gas and dust
• dark matter
• a central supermassive black hole
Galaxies are held together by gravity and shaped by the invisible influence of dark matter.
Galaxies range in size from:
• dwarf galaxies: ~10 million stars
• giant galaxies: several trillion stars
The Milky Way contains around 100–400 billion stars.
---
8.2 The Three Main Types of Galaxies
Galaxies come in three primary shapes:
1. Spiral Galaxies
• Disc-shaped
• Central bulge
• Rotating spiral arms
• Lots of gas and dust → active star formation
Examples:
• The Milky Way
• Andromeda
2. Elliptical Galaxies
• Round or oval shape
• Little gas or dust
• Mostly old stars
• Very little new star formation
They can be small (dwarf elliptical) or enormous (giant elliptical).
3. Irregular Galaxies
• No fixed shape
• Often formed by collisions
• Very active star-forming regions
Examples:
• The Large Magellanic Cloud
• The Small Magellanic Cloud
---
8.3 The Milky Way Galaxy
Our galaxy is a barred spiral galaxy.
Key features:
• Diameter: ~100,000 light-years
• Thickness: ~1,000 light-years
• Four major spiral arms
• A bar-shaped centre
• Contains a supermassive black hole (Sagittarius A*)
• Sun orbits the centre every ~225 million years
The Sun is located about 27,000 light-years from the centre, in the Orion Arm.
---
8.4 Galactic Rotation & Dark Matter
Stars orbit the centre of a galaxy, but something strange happens:
Stars far from the centre orbit too fast to be held in place by visible matter.
This led to the discovery of:
Dark Matter
• Invisible
• Does not emit light
• Exerts gravitational influence
• Makes up ~85% of a galaxy’s total mass
Dark matter forms vast haloes around galaxies, holding them together.
Without dark matter, galaxies would fly apart.
---
8.5 How Galaxies Form
Galaxies began forming after the Big Bang from tiny density fluctuations.
Galactic formation steps:
1. Gas clouds collapse under gravity
2. Small protogalaxies merge
3. Over billions of years, these grow into the galaxies we see today
Mergers shape a galaxy’s final structure:
• Merging spirals can form an elliptical galaxy
• Small dwarfs are often absorbed by bigger galaxies
The Milky Way is currently absorbing several dwarf galaxies.
---
8.6 Supermassive Black Holes
Almost every large galaxy has a supermassive black hole at its centre.
Mass ranges from:
• millions of times the Sun’s mass
to
• tens of billions of times the Sun’s mass
These black holes influence:
• galactic rotation
• galaxy growth
• star formation rates
• energy output (e.g., quasars)
Quasars are galaxies with extremely active central black holes, some of the brightest objects in the universe.
---
8.7 Interactions and Collisions
Galaxies frequently collide — but space is so vast that stars rarely crash into each other.
Collisions cause:
• tidal distortions
• bursts of star formation
• black hole activity
• merging into a new galaxy
The Milky Way and Andromeda will collide in ~4.5 billion years, eventually forming a giant elliptical galaxy.
---
8.8 Large-Scale Structure of the Universe
Galaxies are not scattered randomly.
They form:
• groups
• clusters
• superclusters
• filaments and walls
These structures create the “cosmic web” — vast networks of galaxies separated by immense voids.
---
Chapter Summary
• Galaxies contain stars, planets, gas, dust, dark matter, and black holes.
• Spiral, elliptical, and irregular are the main types.
• Dark matter holds galaxies together and shapes their motion.
• Supermassive black holes play a key role in galaxy evolution.
• Galaxies grow through mergers and interactions.
• On the largest scale, galaxies form a cosmic web across the universe.
---
Practice Questions
1. What are the three main types of galaxies, and how do they differ?
2. What evidence supports the existence of dark matter in galaxies?
3. Why do galaxies often collide, even though stars rarely do?
4. What role do supermassive black holes play in galaxy formation?
5. What is the cosmic web?
---
Written and Compiled by Lee Johnston — Founder of The Lumin Archive