How is a galaxy different from a solar system?

The Sun and its planets form one solar system. A galaxy is a far larger collection containing many stars, gas, dust and other matter held together by gravity. Our solar system lies inside the Milky Way galaxy.

Planets travel around the Sun because its gravity strongly shapes their nearby motion. The Sun itself moves through the Milky Way, affected by the gravity of the galaxy's combined matter. Other stars have their own paths, and many have planets too. A galaxy is not a single giant star with tiny stars fixed to its surface.

The Milky Way contains a disk with spiral structure, but galaxies do not all share that shape. Some look rounded or stretched, and some are irregular. Their histories of star formation and encounters with other galaxies help explain their different appearances. Clouds of gas within galaxies can supply material for new stars.

When we see the pale Milky Way across a dark sky, we are looking through part of our own galaxy from inside it. A photograph of another galaxy gives an outside view. Keeping those viewpoints separate helps make sense of cosmic scale: a planet can belong to a system around one star, while that whole system belongs to a much larger galactic neighborhood.