What is a star?
A star is a huge ball of hot, glowing gas that produces its own light and heat. Stars shine because of nuclear reactions deep in their cores. Our nearest star is the Sun, which is why it looks so large and bright — other stars are much farther away and appear only as tiny points of light at night.
Key idea
The Sun is a star. It only looks bigger and brighter than other stars because it is by far the closest one to Earth.
Galaxies
Stars are not spread evenly through space. Gravity pulls billions of stars together into enormous groups called galaxies. Our Sun, together with more than a hundred billion other stars, belongs to a galaxy called the Milky Way. The Milky Way is shaped like a giant spinning spiral.
| Object | Description |
| Star | Ball of hot gas that gives out light |
| Galaxy | Huge group of billions of stars |
| Universe | Everything that exists — all galaxies, space and matter |
The universe
The universe is everything that exists: all the stars, galaxies, planets, gas, dust and the space between them. It is unimaginably large and contains billions of galaxies, each holding billions of stars. Distances in the universe are so great that astronomers measure them in light years — the distance light travels in one year.
Example
Light from the Sun takes about 8 minutes to reach Earth, but light from the next nearest star takes more than 4 years — so that star is more than 4 light years away.
Remember
- A star makes its own light; the Sun is a star.
- A galaxy is a huge group of stars; ours is the Milky Way.
- The universe contains all galaxies, matter and space.