Galaxies
Physics: Cambridge International GCSE Supplement, topic 6.2.2
Galaxies
What is a galaxy?
- A galaxy is a huge collection of stars, dust, and gas, all held together by gravity.
- Each galaxy contains billions of stars, and often billions of planets too.
- The Sun is one star among billions in our galaxy, the Milky Way.
The Milky Way
- The Milky Way is a spiral-shaped galaxy containing about 100–400 billion stars.
- Other stars in the Milky Way look much dimmer than the Sun because they are much further away from Earth.
Light-years
- Distances in space are so large that we use light-years instead of kilometres.
- One light-year is the distance light travels in one year in a vacuum.
- One light-year is equal to 9.5 × 1012 km.
- Light from nearby stars takes years to reach us, while light from galaxies beyond the Milky Way takes millions or even billions of years.
Test yourself on Galaxies
Why do other stars in the Milky Way look much dimmer than the Sun?
What holds the stars, dust, and gas together in a galaxy?
What type of galaxy is the Milky Way?