Stars and galaxies

Our Sun is a star. It seems much bigger than other stars in the sky because it is much closer to Earth. Stars form immense groups called galaxies. A galaxy can contain many millions of stars, held together by the force of gravity.

Our Sun is in a spiral galaxy called the Milky Way. The Sun is about half-way from the centre of the galaxy, on one of the arms.

Galaxy
Our galaxy contains millions of stars, including our Sun. It is just one of more than 100 billion galaxies in the universe

The light year

The distances between objects in space are huge:

  • the distance from one star to another in a galaxy is millions of times more than the distance between the planets in the solar system
  • the distance from one galaxy to another is millions of times more than the distance between the stars in a galaxy

This means that the numbers used to describe distances in space become very difficult to understand and to write down. For example, the distance between the Earth and the Sun is about 150,000,000,000 m but the distance to the next nearest star (Proxima Centauri) is 39,900,000,000,000,000 m.

To get around this problem, scientists use the light year as the unit of astronomical distance. It is the distance travelled by light in one year. So, for example:

  • Sun to Proxima Centauri distance is about 4.24 light years
  • Milky Way to Andromeda (the next nearest spiral galaxy) distance is about 2.5 million light years

It takes light from our Sun about 8 minutes to reach the Earth.

The Sun and its role in our galaxy