Galaxies and Cosmology
1. Galaxies
Learning outcomes
- I can classify galaxies by shape.
- I can compare spiral, elliptical, and irregular galaxies.
- I can describe the structure of the Milky Way.
- I can identify the location of our Solar System within the Milky Way.
- I can compare galaxies of different sizes.
Introduction
When we look into the night sky, we see thousands of stars. However, those stars are only a tiny fraction of the billions that make up our own galaxy.
A galaxy is a vast collection of stars, planets, gas, dust, and dark matter, all held together by gravity. Galaxies are the building blocks of the Universe, and there are thought to be hundreds of billions of galaxies in the observable Universe.
Studying galaxies helps astronomers understand how the Universe formed, how stars evolve, and where our own Solar System fits into the cosmos.
What Is a Galaxy?
A galaxy is an enormous system containing:
- Billions of stars
- Planetary systems
- Gas clouds
- Dust
- Dark matter
All of these objects are held together by gravity.
Galaxies vary enormously in:
- Size
- Shape
- Mass
- Number of stars
Some contain only a few million stars, while others contain more than a trillion.
Classifying Galaxies
Astronomers classify galaxies according to their shape.
The three main types are:
- Spiral galaxies
- Elliptical galaxies
- Irregular galaxies
Each type has different characteristics.
Spiral Galaxies
Spiral galaxies have:
- A bright central bulge.
- Curved spiral arms.
- Large amounts of gas and dust.
- Active regions where new stars are forming.
Most spiral galaxies rotate around their centres.
Examples include:
- The Milky Way
- The Andromeda Galaxy
Spiral arms contain many young, bright stars.
Elliptical Galaxies
Elliptical galaxies are:
- Rounded or oval in shape.
- Smooth in appearance.
- Mostly made of older stars.
- Low in gas and dust.
Because they contain little gas, very few new stars form within them.
Elliptical galaxies range from:
- Small dwarf ellipticals
- Giant elliptical galaxies
Some giant ellipticals contain trillions of stars.
Irregular Galaxies
Irregular galaxies have:
- No definite shape.
- Uneven appearance.
- Large amounts of gas and dust.
- Many regions of active star formation.
Irregular galaxies often form after gravitational interactions or collisions between galaxies.
Examples include the Large Magellanic Cloud and the Small Magellanic Cloud.
Comparing Galaxy Types
Each type tells astronomers something about a galaxy's history and evolution.
The Structure of the Milky Way
The Milky Way is a barred spiral galaxy.
Its main parts include:
- Central bulge
- Central bar
- Spiral arms
- Halo
- Globular star clusters
The Milky Way contains approximately:
- 100–400 billion stars
- Billions of planets
- Vast clouds of gas and dust
It has a diameter of about 100,000 light-years.
Where Is Our Solar System?
Our Solar System is not near the centre of the Milky Way.
Instead, it is located:
- In the Orion Arm (Orion Spur).
- About 26,000 light-years from the galactic centre.
The Sun takes about 225–250 million years to complete one orbit around the centre of the Milky Way.
This journey is called a galactic year.
Comparing Galaxy Sizes
Galaxies come in many different sizes.
| Galaxy Type | Approximate Number of Stars |
|---|---|
| Dwarf Galaxy | Millions to billions |
| Milky Way | 100–400 billion |
| Giant Elliptical Galaxy | Up to trillions |
Although the Milky Way is enormous, it is not among the largest galaxies in the Universe.
Galaxies in Motion
Galaxies are constantly moving.
Gravity causes galaxies to:
- Orbit one another.
- Form groups and clusters.
- Occasionally collide and merge.
In about 4–5 billion years, the Milky Way and the Andromeda Galaxy are expected to collide and gradually merge into a single, larger galaxy.
Real-World Applications
Studying galaxies helps astronomers:
- Understand how the Universe formed.
- Investigate dark matter.
- Study stellar evolution.
- Measure cosmic distances.
- Predict the future of the Milky Way.
- Explore the large-scale structure of the Universe.
Worked Examples
Example 1
Which type of galaxy has spiral arms?
Answer:
A spiral galaxy.
Example 2
Which type of galaxy contains mostly older stars and very little gas?
Answer:
An elliptical galaxy.
Example 3
Where is our Solar System located?
Answer:
In the Orion Arm (Orion Spur) of the Milky Way Galaxy, about 26,000 light-years from the galactic centre.
Example 4
Which type of galaxy has no regular shape?
Answer:
An irregular galaxy.
Example 5
Which is generally larger?
A dwarf galaxy or a giant elliptical galaxy?
Answer:
A giant elliptical galaxy.
Did You Know?
The Messier 87 contains trillions of stars, making it many times larger than the Milky Way. At its centre lies one of the largest known supermassive black holes, whose image was captured by the Event Horizon Telescope Collaboration in 2019.
Key Terms
| Term | Definition |
|---|---|
| Galaxy | A massive system of stars, gas, dust, planets, and dark matter held together by gravity. |
| Spiral Galaxy | A galaxy with a central bulge and spiral arms containing many young stars. |
| Elliptical Galaxy | A rounded galaxy made mostly of older stars with little gas or dust. |
| Irregular Galaxy | A galaxy with no definite shape, often rich in gas and active star formation. |
| Milky Way | The barred spiral galaxy that contains our Solar System. |
| Spiral Arm | A curved region of a spiral galaxy where many young stars are found. |
| Galactic Year | The time taken for the Solar System to orbit once around the centre of the Milky Way. |
Key Takeaways
- Galaxies are enormous collections of stars, gas, dust, planets, and dark matter held together by gravity.
- The three main galaxy types are spiral, elliptical, and irregular.
- The Milky Way is a barred spiral galaxy containing hundreds of billions of stars.
- Our Solar System is located in the Orion Arm, far from the centre of the Milky Way.
- Galaxies vary greatly in size, from dwarf galaxies with millions of stars to giant elliptical galaxies with trillions.
- Studying galaxies helps astronomers understand the evolution and large-scale structure of the Universe.