Galaxy

space · stars · gravity · cosmos

Billions of stars together. A galaxy is a massive group of stars, gas, and dust all stuck together by gravity. Our own Milky Way galaxy has about 100 million stars, but some galaxies have way more, up to 100 trillion stars.

Mostly invisible dark matter. Most of the stuff in a galaxy is something called dark matter, which we can't see. Even though stars look bright and important, they only make up a tiny bit of a galaxy's total mass.

Black holes at the center. Almost every galaxy has a supermassive black hole spinning at its very center. These cosmic vacuums are so powerful that they help hold the whole galaxy together.

The name tells a story. The word 'galaxy' comes from the Greek word meaning 'milky,' because our Milky Way looks like a milky band across the night sky. Ancient people named all galaxies after the one they could see.

Galaxies come in all sizes. Some dwarf galaxies have fewer than 1,000 stars, while supergiant galaxies have 100 trillion. That's like comparing a small town to an entire country.