Galaxies
Middle School
Definition
Vast systems of stars, gas, and dust held together by gravity. Galaxies contain millions to billions of stars and come in shapes such as spiral, elliptical, and irregular; our Solar System lies within the Milky Way galaxy.
Worked examples
The Milky Way is a spiral galaxy containing over 200 billion stars, including our Sun.
Our Solar System is just one tiny part of this vast collection of stars, gas, and dust.
The Andromeda Galaxy (M31) is the nearest major galaxy to the Milky Way, about 2.5 million light-years away.
Even the closest large galaxy is incredibly distant, showing the immense scale of the universe.
Elliptical galaxies are smooth and rounded with older stars; irregular galaxies have no defined shape.
Galaxies come in different forms based on their structure and the ages of their stars.
Common mistakes
- A galaxy is the same as a solar system → A galaxy contains millions to billions of stars; a solar system is just one star with its planets Solar systems are tiny parts of galaxies — galaxies are vastly larger structures.
- All galaxies are spiral-shaped like the Milky Way → Galaxies come in spiral, elliptical, and irregular shapes Galaxy shape depends on star formation history, collisions, and gravitational interactions.
- Galaxies are empty space with a few stars scattered around → Galaxies are dense systems with billions of stars plus gas and dust held by gravity Though stars appear far apart, galaxies are gravitationally bound collections of enormous amounts of matter.
Where you'll use it next
You'll study galaxy formation and evolution when learning cosmology and the Big Bang, explore how telescopes detect distant galaxies in astronomy, and understand galaxy collisions and dark matter in advanced physics.
See also
Reviewed by Pat Cheng, M.Ed. — StudyPug Curriculum Lead · Last updated June 6, 2026