The observable universe is believed to be formed of 100 billion galaxies. A galaxy is a gravitationally bound group of stars, planets, stellar remnants, gas dust and dark matter. Earth and our Solar System are in the Milky Way galaxy, which is thought to contain 100 billion stars. The Milky way is part of the Local group, which also includes the Andromeda galaxy, our nearest neighbouring galaxy. Galaxies come in varying sizes and shapes, and can be categorised as spiral, elliptical or irregular galaxies.