The milky way is the name given to the faint band of light that stretches z cross the night sky. The light comes from the stars and nebulae in our galaxy, known as the Milky Way Galaxy or simply as “the Galaxy” . The Galaxy is shaped like a spiral, with a dense central bulge that is encircled by four arms spiraling outwards and surrounded by a less dense halo. We cannot see the spiral shape because the Solar System is in one of the spiral arms, the orion Arm (also called the Local Arm) . From our position, the center of the Galaxy is completely obscured by dust clouds ; as a result, optical maps give only a limited view of the Galaxy . However, a more complete picture can be obtained by studying radio, infra-red, and other radiation. The central bulge of the Galaxy is a relatively small, dense sphere that contains mainly older red and yellow stars. The halo is a less dense region in which the oldest stars are situated, some of these stars maybe as old as the Galaxy itself (possibly 15 billion years) . The spiral arms mainly contain hot, young, blue stars , as well as nebulae (clouds of dust and gas inside which stars are born). The galaxy is vast, about 100,000 light years across ( a light year is about 9,460 billion kilometers) , in comparison, the solar system seems small, all about 12 light hours across( about 13 billion kilometers ). The entire galaxy is rotating in space, although the inner stars travel faster than those further out. The Sun, which is about two-third out from the center, completes one lap of the Galaxy about every 20 million years.
