galaxy

Table of Contents

Introduction & Top Questions
Historical survey of the study of galaxies
- Early observations and conceptions
  - The problem of the Magellanic Clouds
  - Novae in the Andromeda Nebula
  - The scale of the Milky Way Galaxy
  - The van Maanen rotation
  - The Shapley-Curtis debate
  - Hubble’s discovery of extragalactic objects
  - The distance to the Andromeda Nebula
- The golden age of extragalactic astronomy
Types of galaxies
- Principal schemes of classification
  - Elliptical galaxies
  - Spiral galaxies
    - S0 galaxies
    - Sa galaxies
    - Sb galaxies
    - Sc galaxies
    - SB galaxies
  - Irregular galaxies
- Other classification schemes and galaxy types
The external galaxies
- The extragalactic distance scale
- Physical properties of external galaxies
  - Size and mass
  - Luminosity
  - Age
  - Composition
- Structure
  - The spheroidal component
  - The disk component
  - Spiral arms
  - Gas distribution
- Clusters of galaxies
  - Types of clusters
    - Groups
    - Irregular clusters
    - Spherical clusters
  - Distribution
  - Interactions between cluster members
- Extragalactic radio and X-ray sources
  - Radio galaxies
  - X-ray galaxies
  - Clusters of galaxies as radio and X-ray sources
  - Quasars
  - Gamma-ray bursts
Evolution of galaxies and quasars

References & Edit History Related Topics

Images & Videos

Whirlpool Galaxy (M51); NGC 5195

Large Magellanic Cloud

Small Magellanic Cloud

globular cluster M80

centre of the Milky Way Galaxy

Andromeda Galaxy

Hubble's system of classification for galaxies

For Students

galactic types

galaxy: Related Content

Written and fact-checked by The Information Architects of Encyclopaedia Britannica

Last Updated: Jul 07, 2025

News •

Scientists spot mystery object believed to come from beyond solar system • July 4, 2025, 3:18 AM ET (The Guardian)

Our Galaxy's Monster Black Hole Is Spinning Almost as Fast as Physics Allows • June 19, 2025, 7:26 AM ET (ScienceAlert)

Photos and Videos

Whirlpool Galaxy (M51); NGC 5195

Large Magellanic Cloud

Small Magellanic Cloud

globular cluster M80

centre of the Milky Way Galaxy

Andromeda Galaxy

Hubble's system of classification for galaxies

What are the three types of galaxies?

How the Hubble Space Telescope works—and why it's important

Understand gravitational lenses and the importance of tracking the dark matter

Learn about galaxy clusters, superclusters, and the Local Group

Understand the theoretical model of dark energy and the acceleration of the universe

See All Videos and Images →

Topics

star spiral galaxy de Vaucouleurs classification X-ray galaxy radio galaxy irregular galaxy galactic centre stellar population external galaxy nucleus cosmic microwave background planet X-ray source black hole radio source intergalactic medium stellar association infrared source cosmic X-ray background constellation

Dig Deeper: More Articles That Discuss This Topic

Key People

Neil deGrasse Tyson

Neil deGrasse Tyson

American astronomer

Harlow Shapley

American astronomer

Telescope at Birr Castle

William Parsons, 3rd earl of Rosse

Irish astronomer

Sir Martin Ryle.

Sir Martin Ryle

British astronomer

Bertil Lindblad

Swedish astronomer

Nicolaus Copernicus. Nicolas Copernicus (1473-1543) Polish astronomer. In 1543 he published, forward proof of a Heliocentric (sun centered) universe. Coloured stipple engraving published London 1802. De revolutionibus orbium coelestium libri vi.

All About Astronomy

27 True-or-False Questions from Britannica’s Most Difficult Science Quizzes

Nicolaus Copernicus. Nicolas Copernicus (1473-1543) Polish astronomer. In 1543 he published, forward proof of a Heliocentric (sun centered) universe. Coloured stipple engraving published London 1802. De revolutionibus orbium coelestium libri vi.

All About Astronomy

View of the Andromeda Galaxy (Messier 31, M31).

The Night Sky: Galaxies and Constellations

View of the Andromeda Galaxy (Messier 31, M31).

Astronomy and Space Quiz