1The Sun is a star
The Sun is a star: a huge ball of very hot gas that gives out its own light. Planets and moons give out none: we see them because they reflect the Sun's light.
star
Lesson 30.4 notes
Stars, galaxies and the light year
The Sun as a star, our galaxy and others, and measuring the vast distances of space in light years.
2Stars are suns
Every star you see at night is a sun, many of them bigger and brighter than ours. They look like tiny points because they're so far away. Many have planets too.
3The Milky Way
The Sun is one of hundreds of billions of stars in our galaxy, the Milky Way. It's so big that light takes about 100 000 years to cross it.
galaxy
4Other galaxies
The Universe holds billions of other galaxies. The nearest big one, Andromeda, is so far away that its light takes about 2.5 million years to reach us.
Universe
5The light year
Light travels 300 000 km every second. A light year is the distance it travels in a year: about 9.5 million million km. It's a distance, not a time.
light year
6Light takes time
Even light takes time to cross space. Light from the Moon takes just over a second to reach us, and sunlight about 8 minutes. From the next nearest star, it takes 4.2 years.
7Looking back in time
We see things as they were when their light set off. We see the Sun as it was 8 minutes ago, and Andromeda as it was 2.5 million years ago.
Explore: Light-speed lab
Send a flash of light from the Earth and race it to the Moon, the Sun, the next nearest star and beyond. See how long it takes, and what a light year really means.
Common mistake: A light year is a distance, not a time
It's how far light travels in a year: about 9.5 million million km. A star 4 light years away is so far that its light takes 4 years to reach us.
Remember
- The Sun is a star. Planets and moons give out no light of their own: we see them by the sunlight they reflect.
- The Sun is one of hundreds of billions of stars in our galaxy, the Milky Way, which is one of billions of galaxies.
- A light year is the distance light travels in a year, about 9.5 million million km. The next nearest star is 4.2 light years away.
Going further at GCSE