A galaxy is a cluster of stars, dust, and gas which is held together by gravity. Galaxies are scattered throughout the Universe and they vary greatly in size. Our Sun is a star in the Milky Way Galaxy. Our Galaxy is a spiral galaxy that formed approximately 12-13 billion years ago. Contained in the Milky Way are stars, clouds of dust and gas called nebulae, planets, and asteroids. All objects in our Galaxy revolve around the Galaxy's center. It takes 250 million years for our Sun to pull us through one revolution around the center of the Milky Way. The stars we see over our head every night are also members of the Milky Way family.