The Milky Way Galaxy
Our own galaxy, the Galaxy, commonly referred to as the Milky Way (a term also used to describe the faint band of stars that can be seen running across the sky on a clear night), is thought by astronomers to be a spiral or barred spiral with a disk, consisting of four major arms, about 800,000 to 100,000 light years across and 2000 light years in thickness. The main disk is surrounded in space by a halo of globular clusters, groupings of old stars arranged in ball-like masses. The Milky Way formed about 5 billion years after the Big Bang, and like all other galaxies, from clouds of hydrogen and helium which collapsed under the influence of gravity. Studying the structure of the Milky Way is not easy. Interstellar material hides much of it from view. Astronomers use infrared and radio waves to penetrate this material to map it out in as much detail as they can.
The Sun, our nearest star and one of billions of other stars in the Milky Way, is located about 25,000 light years from the galactic nucleus in a less well developed arm known as the Orion arm. As the Milky Way galaxy rotates, astronomers have estimated that the Sun takes about 200 million years to complete one orbit of the galactic nucleus, travelling at about 800,000 kilometres per hour. The Milky Way galaxy belongs to a poor cluster of about 30 other galaxies known as the Local Group. Our nearest galactic neighbours are the Large and Small Magellanic clouds, irregulars which lie about 150,000 to 200,000 light years away. The largest galaxy in the Local Group is the Andromeda galaxy, a spiral over 2 million light years away.