What Defines a Galaxy?
A galaxy is a massive system containing:
Stars
Planets
Gas and dust
Dark matter
All of these components are bound together by gravity.
Galaxies can contain millions to trillions of stars, and range from a few thousand light-years to hundreds of thousands of light-years across.
Types of Galaxies
Astronomers classify galaxies into several main types:
Spiral galaxies
Flat disks with spiral arms
Actively forming stars
Example: The Milky Way
Elliptical galaxies
Round or oval shapes
Older stars, little gas
Very large or very small
Irregular galaxies
No clear shape
Often disturbed by interactions
The Milky Way
We live inside the Milky Way, a barred spiral galaxy about 100,000 light-years across.
Key features:
Hundreds of billions of stars
A supermassive black hole at its centre
Spiral arms rich in star-forming regions
Our Sun is located in a spiral arm called the Orion Arm, far from the galaxy’s centre.
Galaxies Are Not Isolated
Galaxies interact through gravity:
They pass near one another
Pull stars and gas apart
Sometimes collide and merge
The Milky Way is on a slow collision course with the Andromeda Galaxy, expected to occur in around 4–5 billion years.
Key Takeaways
Galaxies are gravitationally bound systems
They come in different shapes and sizes
We live inside the Milky Way
Galaxies evolve and interact