A galaxy is the large group of billions of stars orbiting around a center. It is estimated that here are about 50 billion galaxies in the universe. The largest known galaxy has about 13 times as many stars as the Milky Way, the galaxy that the earth is in. Besides stars and planets, galaxies contain hydrogen gas, complex molecules, and cosmic rays. They radiate visible light, radio waves, infrared radiation, ultraviolet radiation, and X rays.

Milky Way is the large, disk-shaped galaxy that includes the sun and its solar system, its name is derived from its appearance as a luminous band that stretches across earth’s sky at night.