The sun with its associated planets is just one star in this galaxy. If you looks up at the sky on a clear night, you see that the sky is full of stars. During the summer months in the Northern Hemisphere, a faint band of light stretches from horizon to horizon, a swath of pale white cutting across a background of deepest black. This band is composed of countless stars in a flattened disk seen edge on. The stars are so close to one another that the unaided eye has difficulty discerning the individual members.
Astronomers find myriads of like systems sprinkled throughout the depths of space .
They call such vast collections of stars galaxies , after the Greek word for milk, and call the local galaxy to which the Sun belongs the Milky Way Galaxy or simply the Galaxy . The Milky Way Galaxy is a flattened system of stars and nebulae. During the early 1920s Edwin Powell Hubble determined conclusively that galaxies exist beyond the Milky Way system. A galaxy contains gas, dust, and billions of stars . It is held together by the gravitational attraction between its parts, and its rotational motion prevents it from collapsing on itself.