A galaxy is an enormous collection of stars, planets, gas, and dust all held together by gravity, spinning through space like a vast cosmic island. Our own galaxy, the Milky Way, contains hundreds of billions of stars, including our Sun. When you look up at the night sky and see that hazy band of light stretching overhead (in places dark enough to see it), you're actually looking at millions of stars in our own galaxy, all so far away they blur together.

Galaxies come in different shapes. Some are spiral galaxies with graceful, pinwheel-like arms of stars. Others are elliptical galaxies, shaped like stretched-out footballs or spheres. Still others are irregular galaxies with no clear shape at all. The universe contains billions of galaxies, each one separated from the others by mind-boggling distances of empty space.

Scientists estimate the Milky Way is about 100,000 light-years across, meaning light traveling at 186,000 miles per second would take 100,000 years to cross it. The closest major galaxy to ours, Andromeda, is about 2.5 million light-years away. That distance helps you appreciate just how vast the universe really is: even with countless galaxies scattered throughout space, there's far more emptiness between them than anything else.