Earth Science

Beyond the Solar System

Galaxies

Study Tip
The Milky Way Galaxy is one of the many spiral galaxies in our universe.

Galaxies

  • Galaxies are the biggest groups of stars, each containing millions or even billions of stars.
  • With the naked eye, we are able to see stars within the Milky Way galaxy and parts of the Andromeda galaxy.
Study Tip
The Milky Way Galaxy is one of the many spiral galaxies in our universe.

Types of Galaxies

  • Galaxies are divided into spiral, elliptical, or irregular galaxies, based on their shapes.
  • A spiral galaxy is a rotating disk of stars and dust, often with a bulge in its center. The stars in a spiral galaxy tend to be rather young. Spiral galaxies also have spiral arms that branch outwards from the center of the galaxy.
  • An elliptical galaxy is egg shaped, and can vary greatly in size from small clusters to giant galaxies housing trillions of stars. Elliptical galaxies tend to be reddish or yellowish due to the relatively old age of the stars in these galaxies.
  • An irregular galaxy is neither spiral nor elliptical. Most irregular galaxies are spiral or elliptical galaxies that have been deformed by gravity or collisions.
  • A dwarf galaxy is a tiny galaxy that only houses a few million or billion stars. They are the most common type of galaxy, but most of them are not visible from the Earth. Most dwarf galaxies are irregular in shape, but sometimes they can collide and merge with other galaxies.
Spiral Galaxy
Spiral Galaxy
Elliptical Galaxy
Elliptical Galaxy
Irregular Galaxy
Irregular Galaxy
Concept Check
  • What are galaxies and which one do we live in?
  • Describe the differences between spiral, elliptical, and irregular galaxies?
  • Where are most dwarf galaxies found?