Galaxies are immense systems containing billions of stars.
Astronomers have estimated that the universe could contain 40 to 50 billion galaxies. The Milky Way is our own galaxy. The Sun is the closest star to Earth and the center of our solar system. The Sun weighs more than 300,000 times as much as Earth and is 109 times larger. Sunspots appear as dark spots on the Sun, and are believed to be cooler than the rest of the Sun.