The Milky Way is our galaxy, home to our Solar System. It formed more than 13 billion years ago, less than a billion years after the Big Bang. The galaxy is estimated to be about 100,000 light years in diameter and 1,000 light years thick. It is part of a system of about 50 galaxies known as the Local Group, which is part of the Virgo Supercluster. Containing as many as 100 billion planets and 400 billion stars, the Milky Way is a spiral galaxy. It has a centre, known as a ‘bulge’, which is surrounded by a flat disc comprising several loose arms that contain stars and their orbiting bodies as well as gases and dust. The centre is thought to contain a massive black hole and a complex radio source known as ‘Sagittarius A*’. Around the outside of the Milky Way there is a halo containing dark matter and a very small percentage of the galaxy’s total number of stars. Astronomers have observed that the Milky Way is actually a special type of spiral galaxy called a barred spiral, meaning that it has a bar-shaped distribution of stars running across its centre.
Timeline of the universe