Our own galaxy is called the Milky Way. It contains about 200 billion stars and can be seen on a very dark night as a bright band stretching across the sky. It is a spiral galaxy, with a diameter of 100 000 light years and a thickness of about 5000 light years. Our Sun is located in one of the spiral arms, called the Orion arm, at a distance of about 30 000 light years from the galactic centre and orbits every 200 million years.
The centre of the Milky Way lies in the constellation of Sagittarius. When observations were made of the stellar velocities in the area, they were found to be particularly fast, which suggests that there is a high gravitational force acting on the stars, and therefore a high mass object in the centre. This is now believed to be a black hole.
Our position in the galaxy means that we are unable to see the spiral structure of the Milky Way. Instead we look at other spiral galaxies in order to understand our own.
The images below show two spiral galaxies similar to our own. If they were the Milky Way, the Sun would be about two-thirds of the way to the edge. This has been marked on the diagrams to give an idea of the Sun's position in our own galaxy.
Click on the links below to find out more about galaxies.
Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001