There are thousands of millions of galaxies in space. We can not say how many galaxies there are because we do not know the extension of space. The Milky Way Galaxy in which we live is probably an average galaxy. It contains around 400 billion stars. Some of these have planets orbiting around them. The Milky Way Galaxy is a spiral galaxy with two arms. It is estimated to have a diameter of about 100000 light years and to have a thickness, or depth of about 20000 light years. It has a stellar population of about 100 billion stars and contains additional dust and gas — about 10% of the total mass of galaxy. Most of the dust and gas is concentrated in the arm structure, where it may amount about 50% of the mass of the arms.

It is rotating about a point at its centre. The rotation of the Galaxy carried our Sun and us about the centre in about 225 million years. The Sun will travel completely around the centre of the galaxy in about 225 million years.

We are in the plane of the disc of our Galaxy and about 1/3 the distance between one edge and the centre of the Galaxy — about 15000 light years from the edge and about 30000 light" years from the centre.

Through the telescopes we can see that the Milky Way is made up of countless stars. This is the edge of our Galaxy, seen from our position inside the Galaxy. The edge of our Galaxy lies in Taurus and Sagittarius.