The Milky Way is a barred spiral galaxy that is part of the Local Group of galaxies. It is estimated to contain between 200 and 400 billion stars and has a diameter of about 100,000 light-years. The Milky Way is thought to have formed around 13.6 billion years ago from the gravitational collapse of a giant molecular cloud. Its structure consists of a central bulge, four major spiral arms, and several minor arms, all of which are surrounded by an extended halo of gas and dust.