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What is a galaxy?

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Quick answer

A galaxy is a vast collection of stars, gas, dust, and dark matter bound together by gravity. Our Sun is one of hundreds of billions of stars in the Milky Way galaxy, and the universe contains hundreds of billions of galaxies.

Key things to understand

  1. A galaxy is an enormous, gravity-bound system of stars, planets, gas, dust, and dark matter.
  2. Our galaxy is the Milky Way, home to the Sun and hundreds of billions of other stars.
  3. Galaxies come in shapes — spiral (like the Milky Way), elliptical, and irregular.
  4. Most galaxies have a supermassive black hole at their centre.
  5. The universe holds hundreds of billions of galaxies, separated by vast stretches of empty space.

People also ask

What galaxy do we live in?
The Milky Way — a spiral galaxy containing the Sun and hundreds of billions of other stars. On a dark night you can see part of it as a faint band of light.
How many galaxies are there?
Estimates run into the hundreds of billions — possibly two trillion — each containing millions to trillions of stars.
What's at the centre of a galaxy?
Most large galaxies have a supermassive black hole at their core, around which the galaxy's stars orbit.
What holds a galaxy together?
Gravity — including the pull of dark matter, which makes up most of a galaxy's mass.

Part of the guide: Space & Astronomy →