The Milky Way

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, so wide that light would take 100 000 years to travel across it, and so thick that it would take 5000 years for light to pass through it. Our Sun is in one of the spiral arms, called the Orion arm, about two-thirds of the way from the centre. It moves around the centre of the galaxy just like the planets move around the Sun, and each of these orbits takes 200 million years.

The centre of the Milky Way appears to be in the constellation of Sagittarius. Astronomers now think that there is a black hole there.

We cannot see the spiral structure of the Milky Way because we are inside it. Instead we look at other spiral galaxies to try to understand our own.

The pictures below show two spiral galaxies like 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 pictures to show where the Sun would be in our own galaxy.