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

Read Next

Colorful winter sunset

What's the Difference Between a Solstice and an Equinox?

Galaxy clusters like Abell 2744 can act as a natural cosmic lens, magnifying light from more distant, background objects through gravity. NASA's James Webb Space Telescope may be able to detect light from the first stars in the universe if they are gravitationally lensed by such clusters. (astronomy, space exploration, galaxies)

How Fast Is the Universe Expanding?

1 July 2002: The Solar and Heliospheric Observatory (SOHO) satellite reveals a massive solar eruption more than 30 times the Earth's diameter. The eruption formed when a loop of a magnetic field over the surface of the Sun trapped hot gas.

6 Questions About the Sun Answered

Infinite space background with silhouette of telescope. This image elements furnished by NASA.

Telescopes: Seeing Stars

M17, the Swan Nebula, is a star-making cloud in the constellation Sagittarius.

How Do We Know How Far Away the Stars Are?

Discover

New Zealand. Political/Physical map. Includes locator.

Where Is “Old Zealand”?

Rural Irish landscape, Sligo, Ireland.

Why Is Ireland Two Countries?

Two chicks near an egg with a white background (poultry, chick, chickens, birds).

What Do Eggs Have to Do with Easter?

Ahura Mazda - relief of the Zoroastrian god Ahura Mazda at the ancient ruins of Persepolis in Iran. Also known as Ormazd Zoroastrianism,

Which Religion Is the Oldest?

Easter Procession of Maria Santisima del Sacromonte in Granada, Spain. This statue is known as Virgen de los Gitanos or The Virgin of the Gypsies.

What is Holy Week?

Hemp (Cannabis sativa).

9 Mind-Altering Plants

illustration of the walking titanosaurus, Patagotitan mayorum

Titanosaurs: 8 of the World's Biggest Dinosaurs

galaxy: Related Content

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

News •

Scientists detect record-setting explosion outside our galaxy • Apr. 18, 2025, 11:36 PM ET (Live Science)

Astronomers Discover Most Distant Milky Way-like Galaxy • Apr. 16, 2025, 11:56 PM ET (Newsweek)

Astronomer Proposes Entirely New Category of Galaxy • Apr. 11, 2025, 6:55 AM ET (Newsweek)

Webb Detects Lyman-Apha Emission from Galaxy in Early Universe • Mar. 30, 2025, 6:19 AM ET (Sci.News)

Webb Telescope Reveals Hidden Galaxy Behind “Cosmic Tornado” • Mar. 24, 2025, 5:56 PM ET (SciTechDaily)

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 ultraluminous infrared galaxy nebula galactic coordinate cluster of galaxies spiral galaxy radio galaxy X-ray galaxy de Vaucouleurs classification nucleus cosmic microwave background planet X-ray source black hole radio source intergalactic medium stellar association infrared source constellation cosmic X-ray background

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

Quiz

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

Astronomy and Space Quiz

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

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

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

The Night Sky: Galaxies and Constellations

Demystified

Colorful winter sunset

What's the Difference Between a Solstice and an Equinox?