The word galaxy comes from the Greek word meaning "milky circle" or, more familiarly, "milky way." The white band of light across the night sky that we call the Milky Way was poetically described long before Galileo. But with his small telescope, what he discovered was a multitude of individual stars, "so numerous as almost to surpass belief."

Today we know that the Milky Way is our home galaxy-a vast rotating spiral of gas, dust, and hundreds of billions of stars. The Sun and its planetary system formed in the outer reaches of the Milky Way about 4.5 billion years ago.  More >