Diagram showing the structure of the Milky Way galaxy, of which the Sun is a part, from the top and the side. The Sun is located in one of the spiral arms. At the centre of the Milky Way is the nucleus, a bulge of stars that is made up of older, redder stars than the arms. The very centre is marked by an intense radio source, called Sagittarius A, thought to be a supermassive black hole. The Milky Way is at least 100,000 light years in diameter and is estimated to contain 100?00 billion stars.