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Milky Way Galaxy | Size, Definition, & Facts | Britannica
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[Milky Way Galaxy](https://www.britannica.com/place/Milky-Way-Galaxy)

Table of Contents


- [Introduction & Top Questions](/place/Milky-Way-Galaxy)

- [Major components of the Galaxy](/place/Milky-Way-Galaxy#ref68064)



- [Star clusters and stellar associations](/place/Milky-Way-Galaxy#ref68068)

  - [Globular clusters](/place/Milky-Way-Galaxy#ref68069)

  - [Open clusters](/place/Milky-Way-Galaxy#ref68070)

  - [Stellar associations](/place/Milky-Way-Galaxy#ref68071)

  - [Moving groups](/place/Milky-Way-Galaxy#ref68072)

- [Emission nebulae](/place/Milky-Way-Galaxy#ref68073)

- [Planetary nebulae](/place/Milky-Way-Galaxy#ref68074)

- [Supernova remnants](/place/Milky-Way-Galaxy#ref68075)

- [Dust clouds](/place/Milky-Way-Galaxy#ref68076)

- [The general interstellar medium](/place/Milky-Way-Galaxy#ref68077)

- [Companion galaxies](/place/Milky-Way-Galaxy#ref272491)

- [Star populations and movement](/place/Milky-Way-Galaxy/Star-populations-and-movement)



- [Stars and stellar populations](/place/Milky-Way-Galaxy/Star-populations-and-movement#ref68065)

  - [Principal population types](/place/Milky-Way-Galaxy/Star-populations-and-movement#ref68066)

  - [The stellar luminosity function](/place/Milky-Way-Galaxy/Star-populations-and-movement#ref68067)

- [Density distribution](/place/Milky-Way-Galaxy/Star-populations-and-movement#ref68086)

  - [The stellar density near the Sun](/place/Milky-Way-Galaxy/Star-populations-and-movement#ref68087)

  - [Density distribution of various types of stars](/place/Milky-Way-Galaxy/Star-populations-and-movement#ref68088)

  - [Variations in the stellar density](/place/Milky-Way-Galaxy/Star-populations-and-movement#ref68089)

  - [Variation of star density with _z_ distances](/place/Milky-Way-Galaxy/Star-populations-and-movement#ref68090)

- [Stellar motions](/place/Milky-Way-Galaxy/Star-populations-and-movement#ref68091)

  - [Proper motions](/place/Milky-Way-Galaxy/Star-populations-and-movement#ref68092)

  - [Radial velocities](/place/Milky-Way-Galaxy/Star-populations-and-movement#ref68093)

  - [Space motions](/place/Milky-Way-Galaxy/Star-populations-and-movement#ref68094)

  - [High-velocity stars](/place/Milky-Way-Galaxy/Star-populations-and-movement#ref68095)

- [Solar motion](/place/Milky-Way-Galaxy/Star-populations-and-movement#ref68096)

  - [Solar motion calculations from radial velocities](/place/Milky-Way-Galaxy/Star-populations-and-movement#ref68097)

  - [Solar motion calculations from proper motions](/place/Milky-Way-Galaxy/Star-populations-and-movement#ref68098)

  - [Solar motion calculations from space motions](/place/Milky-Way-Galaxy/Star-populations-and-movement#ref68099)

  - [Solar motion solutions](/place/Milky-Way-Galaxy/Star-populations-and-movement#ref68100)

- [The structure and dynamics of the Milky Way Galaxy](/place/Milky-Way-Galaxy/The-structure-and-dynamics-of-the-Milky-Way-Galaxy)



- [Size](/place/Milky-Way-Galaxy/The-structure-and-dynamics-of-the-Milky-Way-Galaxy#ref272493)

- [Structure of the spiral system](/place/Milky-Way-Galaxy/The-structure-and-dynamics-of-the-Milky-Way-Galaxy#ref68079)

  - [The nucleus](/place/Milky-Way-Galaxy/The-structure-and-dynamics-of-the-Milky-Way-Galaxy#ref68080)

  - [The central bulge](/place/Milky-Way-Galaxy/The-structure-and-dynamics-of-the-Milky-Way-Galaxy#ref68081)

  - [The disk](/place/Milky-Way-Galaxy/The-structure-and-dynamics-of-the-Milky-Way-Galaxy#ref68082)

  - [The spiral arms](/place/Milky-Way-Galaxy/The-structure-and-dynamics-of-the-Milky-Way-Galaxy#ref68083)

  - [The spherical component](/place/Milky-Way-Galaxy/The-structure-and-dynamics-of-the-Milky-Way-Galaxy#ref68084)

  - [The massive halo](/place/Milky-Way-Galaxy/The-structure-and-dynamics-of-the-Milky-Way-Galaxy#ref68085)

- [Magnetic field](/place/Milky-Way-Galaxy/The-structure-and-dynamics-of-the-Milky-Way-Galaxy#ref68101)

- [Rotation](/place/Milky-Way-Galaxy/The-structure-and-dynamics-of-the-Milky-Way-Galaxy#ref68102)

- [Mass](/place/Milky-Way-Galaxy/The-structure-and-dynamics-of-the-Milky-Way-Galaxy#ref272494)

[References & Edit History](https://www.britannica.com/place/Milky-Way-Galaxy/additional-info) [Related Topics](/facts/Milky-Way-Galaxy)

[Images & Videos](https://www.britannica.com/place/Milky-Way-Galaxy/images-videos)

[![Milky Way Galaxy](https://cdn.britannica.com/90/2590-004-016E7594/Milky-Way-Galaxy-Earth.jpg)](https://cdn.britannica.com/90/2590-050-6C33CB5C/Milky-Way-Galaxy-Earth.jpg)[![Milky Way Galaxy](https://cdn.britannica.com/79/150779-004-C687FDC1/Milky-Way-Galaxy-Tuolumne-Meadows-California-Yosemite.jpg)](https://cdn.britannica.com/79/150779-050-601CB582/Milky-Way-Galaxy-Tuolumne-Meadows-California-Yosemite.jpg)[![Milky Way Galaxy](https://cdn.britannica.com/29/148329-004-434FCBE6/night-sky-Milky-Way-Galaxy.jpg)](https://cdn.britannica.com/29/148329-050-269A9EFE/night-sky-Milky-Way-Galaxy.jpg)[![globular cluster M80](https://cdn.britannica.com/47/111347-004-8A438D21/cluster-M80-image-Hubble-Space-Telescope-hundreds.jpg)](https://cdn.britannica.com/47/111347-050-F03ABE95/cluster-M80-image-Hubble-Space-Telescope-hundreds.jpg)[![open cluster NGC 290](https://cdn.britannica.com/38/111338-004-D4617D83/Stars-NGC-290-Hubble-Space-Telescope.jpg)](https://cdn.britannica.com/38/111338-050-D23BE7C8/Stars-NGC-290-Hubble-Space-Telescope.jpg)[![Pleiades](https://cdn.britannica.com/58/2958-004-D3B94115/nebulosity-Pleiades-distance-clouds-Cluster-stars-light.jpg)](https://cdn.britannica.com/58/2958-050-C1B86BCF/nebulosity-Pleiades-distance-clouds-Cluster-stars-light.jpg)[![Orion Nebula (M42)](https://cdn.britannica.com/54/21254-004-A2E0EEF5/Centre-Astronomers-Orion-Nebula-picture-systems-disks.jpg)](https://cdn.britannica.com/54/21254-050-9D8F727F/Centre-Astronomers-Orion-Nebula-picture-systems-disks.jpg)[![Cat's Eye Nebula](https://cdn.britannica.com/36/21236-004-DDD48DFB/picture-Cats-Eye-Nebula-details-images-planetary.jpg)](https://cdn.britannica.com/36/21236-050-DD862016/picture-Cats-Eye-Nebula-details-images-planetary.jpg)[![Crab Nebula](https://cdn.britannica.com/88/91388-004-F84F5F3A/Crab-Nebula-exposures-image-astronomers-explosion-NASA.jpg)](https://cdn.britannica.com/88/91388-050-57AD9D69/Crab-Nebula-exposures-image-astronomers-explosion-NASA.jpg)[![Eagle Nebula](https://cdn.britannica.com/25/92825-004-C6A93067/Eagle-Nebula-column-Stars-cold-dust-gas.jpg)](https://cdn.britannica.com/25/92825-050-AA387705/Eagle-Nebula-column-Stars-cold-dust-gas.jpg)

For Students


[![centre of the Milky Way Galaxy](https://cdn.britannica.com/42/442-050-2F14CF45/centre-Image-Milky-Way-Galaxy-observations-clouds-Jan-25-1983.jpg?w=200&h=200&c=crop)](/summary/Milky-Way-Galaxy)

[Milky Way Galaxy summary](/summary/Milky-Way-Galaxy)

Quizzes


[![View of the Andromeda Galaxy (Messier 31, M31).](https://cdn.britannica.com/41/155241-131-FB2C633F/View-Andromeda-Galaxy.jpg?w=200&h=200&c=crop)](/quiz/astronomy-and-space-quiz)

[Astronomy and Space Quiz](/quiz/astronomy-and-space-quiz)

[![The orbits of the planets and other elements of the solar system, including asteroids, Kuiper belt, Oort cloud, comet](https://cdn.britannica.com/92/95392-131-F6284677/orbits-planets-bodies-solar-system.jpg?w=200&h=200&c=crop)](/quiz/space-odyssey)

[Space Odyssey](/quiz/space-odyssey)

[![Encyclopaedia Britannica thistle graphic to be used with a Mendel/Consumer quiz in place of a photograph.](https://cdn.britannica.com/05/215905-131-626D860A/Encyclopaedia-Britannica-thistle-graphic-logo.jpg?w=200&h=200&c=crop)](/quiz/27-true-or-false-questions-from-britannicas-most-difficult-science-quizzes)

[27 True-or-False Questions from Britannica’s Most Difficult Science Quizzes](/quiz/27-true-or-false-questions-from-britannicas-most-difficult-science-quizzes)

Related Questions


- [How big is the Milky Way Galaxy?](/question/How-big-is-the-Milky-Way-Galaxy)
- [How has Galileo influenced science?](/question/How-has-Galileo-influenced-science)

Read Next


[![Solar system illustration. (Sun; planets)](https://cdn.britannica.com/51/157551-131-44092D74/Solar-system-illustration.jpg?w=200&h=200&c=crop)](/story/a-model-of-the-cosmos)

[A Model of the Cosmos](/story/a-model-of-the-cosmos)

[![Infinite space background with silhouette of telescope. This image elements furnished by NASA.](https://cdn.britannica.com/60/190760-131-F1576E0B/space-background-telescope-silhouette-NASA-image-elements.jpg?w=200&h=200&c=crop)](/story/telescopes-seeing-stars)

[Telescopes: Seeing Stars](/story/telescopes-seeing-stars)

[![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)](https://cdn.britannica.com/27/238127-131-9A610712/galaxy-cluster-Abell-2744.jpg?w=200&h=200&c=crop)](/story/how-fast-is-the-universe-expanding)

[How Fast Is the Universe Expanding?](/story/how-fast-is-the-universe-expanding)

[![Planet Jupiter with its moon Io at left, photographed by the Cassini orbiter during the Cassini-Huygens mission, 2000. spacecraft](https://cdn.britannica.com/90/186790-131-AEA5CAE9/Planet-Jupiter-moon-left-orbiter-Io-mission-2000.jpg?w=200&h=200&c=crop)](/story/7-important-dates-in-jupiter-history)

[7 Important Dates in Jupiter History](/story/7-important-dates-in-jupiter-history)

[![Colorful winter sunset](https://cdn.britannica.com/90/202690-131-1D29B008/colorful-winter-sunset.jpg?w=200&h=200&c=crop)](/story/whats-the-difference-between-a-solstice-and-an-equinox)

[What's the Difference Between a Solstice and an Equinox?](/story/whats-the-difference-between-a-solstice-and-an-equinox)

Discover


[![Presidential Debate Bingo. As candidates for the Republican Party's nomination for the 2024 presidential election take to the stage to debate the issues and each other, keep track of the facts you need to know with updated Britannica entries. As with every Bingo game, if you connect five tiles in a row, celebrate!](https://cdn.britannica.com/66/249866-004-B67557FB/presidential-debate-bingo.jpg?w=200&h=200&c=crop)](/story/presidential-debate-bingo)

[Presidential Debate Bingo](/story/presidential-debate-bingo)

[![Close-up of ostriches (Struthio camelus) necks and heads; location unknown.](https://cdn.britannica.com/58/152058-131-0EB52C8F/Ostriches.jpg?w=200&h=200&c=crop)](/list/6-of-the-worlds-most-dangerous-birds)

[6 of the World's Most Dangerous Birds](/list/6-of-the-worlds-most-dangerous-birds)

[![United States Electoral College votes by state](https://cdn.britannica.com/68/220368-131-C835E48E/United-States-electoral-college-votes-by-state.jpg?w=200&h=200&c=crop)](/story/how-many-electoral-college-votes-does-each-us-state-have)

[How Many Electoral College Votes Does Each U.S. State Have?](/story/how-many-electoral-college-votes-does-each-us-state-have)

[![Aphrodite. Greek mythology. Sculpture. Aphrodite is the Greek goddess of love and beauty.](https://cdn.britannica.com/23/176123-131-94DCF6F8/Aphrodite.jpg?w=200&h=200&c=crop)](/list/12-greek-gods-and-goddesses)

[12 Greek Gods and Goddesses](/list/12-greek-gods-and-goddesses)

[![Encyclopaedia Britannica First Edition: Volume 1, Plate XLIII, Figure 1, Astronomy, Solar System, Equation of Time, Precession of Equinoxes, Earth, orbit, ecliptic, apogee, perigee, line of apsides, mean anomaly, tropical year, Sydereal, Julian](https://cdn.britannica.com/30/119830-131-D828BB6C/Encyclopaedia-Britannica-First-Edition-Volume-1-Figure.jpg?w=200&h=200&c=crop)](/story/first-day-of-fall)

[First Day of Fall](/story/first-day-of-fall)

[![Russian grand duchess Anastasia; undated photograph. (Anastasiya Nikolayevna, Tsar Nicholas II)](https://cdn.britannica.com/68/194168-131-CFE3851A/Anastasia.jpg?w=200&h=200&c=crop)](/story/did-duchess-anastasia-survive-her-familys-execution)

[Did Duchess Anastasia Survive Her Family’s Execution?](/story/did-duchess-anastasia-survive-her-familys-execution)

[![The Colosseum, Rome, Italy.  Giant amphitheatre built in Rome under the Flavian emperors. (ancient architecture; architectural ruins)](https://cdn.britannica.com/36/162636-131-E4AA93A0/Colosseum-Rome-Italy.jpg?w=200&h=200&c=crop)](/list/new-seven-wonders-of-the-world)

[New Seven Wonders of the World](/list/new-seven-wonders-of-the-world)


Contents


[Science](/browse/Science)[Astronomy](/browse/Astronomy)

# Milky Way Galaxy

astronomy



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External Websites


- [Space.com - Milky Way galaxy: Facts about our cosmic neighborhood](https://www.space.com/19915-milky-way-galaxy.html)
- [Chandra X-Ray Observatory - Learn About the Milky Way](https://chandra.harvard.edu/learn_mw.html)
- [The Planets - Milky Way](https://theplanets.org/milky-way/)
- [NASA - The Milky Way Galaxy](https://imagine.gsfc.nasa.gov/science/objects/milkyway1.html)
- [Library of Congress - The Milky Way: One of the Many Galaxies](https://www.loc.gov/collections/finding-our-place-in-the-cosmos-with-carl-sagan/articles-and-essays/modeling-the-cosmos/the-milky-way-one-of-the-many-galaxies)
- [Las Cumbres Observatory - The Milky Way Galaxy](https://lco.global/spacebook/galaxies/the-milky-way-galaxy/)
- [Physics LibreTexts - The Milky Way Galaxy](https://phys.libretexts.org/Bookshelves/Astronomy__Cosmology/Astronomy_1e_(OpenStax)/25%3A_The_Milky_Way_Galaxy)

Britannica Websites

Articles from Britannica Encyclopedias for elementary and high school students.

- [Milky Way - Children's Encyclopedia (Ages 8-11)](https://kids.britannica.com/kids/article/Milky-Way/353466)
- [Milky Way Galaxy - Student Encyclopedia (Ages 11 and up)](https://kids.britannica.com/students/article/Milky-Way-Galaxy/601151)



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External Websites


- [Space.com - Milky Way galaxy: Facts about our cosmic neighborhood](https://www.space.com/19915-milky-way-galaxy.html)
- [Chandra X-Ray Observatory - Learn About the Milky Way](https://chandra.harvard.edu/learn_mw.html)
- [The Planets - Milky Way](https://theplanets.org/milky-way/)
- [NASA - The Milky Way Galaxy](https://imagine.gsfc.nasa.gov/science/objects/milkyway1.html)
- [Library of Congress - The Milky Way: One of the Many Galaxies](https://www.loc.gov/collections/finding-our-place-in-the-cosmos-with-carl-sagan/articles-and-essays/modeling-the-cosmos/the-milky-way-one-of-the-many-galaxies)
- [Las Cumbres Observatory - The Milky Way Galaxy](https://lco.global/spacebook/galaxies/the-milky-way-galaxy/)
- [Physics LibreTexts - The Milky Way Galaxy](https://phys.libretexts.org/Bookshelves/Astronomy__Cosmology/Astronomy_1e_(OpenStax)/25%3A_The_Milky_Way_Galaxy)

Britannica Websites

Articles from Britannica Encyclopedias for elementary and high school students.

- [Milky Way - Children's Encyclopedia (Ages 8-11)](https://kids.britannica.com/kids/article/Milky-Way/353466)
- [Milky Way Galaxy - Student Encyclopedia (Ages 11 and up)](https://kids.britannica.com/students/article/Milky-Way-Galaxy/601151)

Also known as: The Galaxy


Written by


[Paul W. Hodge\
\
Professor Emeritus, Department of Astronomy, University of Washington, Seattle. Author of _The Andromeda Galaxy_; _Higher then Everest: An Adventurer's Guide to the Solar System_; and others.](/contributor/Paul-W-Hodge/1346)

Paul W. Hodge

Fact-checked by

[The Editors of Encyclopaedia Britannica\
\
Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.](/editor/The-Editors-of-Encyclopaedia-Britannica/4419)

The Editors of Encyclopaedia Britannica

Last Updated:

Sep 12, 2024
• [Article History](https://www.britannica.com/place/Milky-Way-Galaxy/additional-info#history)


Table of Contents

Ask the Chatbot a Question

Ask the Chatbot a Question

Top Questions


### Why is it called the Milky Way Galaxy?

The Milky Way Galaxy takes its name from the Milky Way, the irregular luminous band of stars and gas clouds that stretches across the sky as seen from Earth.

### How big is the Milky Way Galaxy?

The first reliable measurement of the size of the Milky Way Galaxy was made in 1917 by American astronomer [Harlow Shapley](https://www.britannica.com/biography/Harlow-Shapley). Assuming that the globular clusters outlined the Galaxy, he determined that it has a diameter of about 100,000 light-years. His values have held up remarkably well over the years.

### Is Earth in the centre of the Milky Way Galaxy?

The solar system is about 30,000 light-years from the centre of the Milky Way Galaxy. The Galaxy itself is thought to be about 100,000 light-years in diameter.

## Recent News

Sep. 10, 2024, 9:08 AM ET (Deutsche Welle)


[Milky Way is bigger than we thought, even touching Andromeda](https://www.dw.com/en/milky-way-is-bigger-than-we-thought-even-touching-andromeda/a-70154211)

Sep. 6, 2024, 6:18 AM ET (Deutsche Welle)


[Galaxies are way bigger than we thought, new research shows](https://www.dw.com/en/galaxies-are-much-bigger-than-we-thought-study-reveals/a-70154211)

Sep. 4, 2024, 11:40 PM ET (NASA Science (.gov))


[NASA’s Webb Reveals Distorted Galaxy Forming Cosmic Question Mark](https://science.nasa.gov/missions/webb/nasas-webb-reveals-distorted-galaxy-forming-cosmic-question-mark/)

Sep. 1, 2024, 3:26 AM ET (ScienceAlert)


[NASA Reveals Incredible Image of Star Cluster Shining in Ultraviolet](https://www.sciencealert.com/nasa-reveals-incredible-image-of-star-cluster-shining-in-ultraviolet)

Aug. 30, 2024, 12:48 PM ET (TechSpot)


[Researchers are now looking for advanced alien civilizations beyond the Milky Way](https://www.techspot.com/news/104508-researchers-now-looking-advanced-alien-civilizations-beyond-milky.html)

[![](https://cdn.britannica.com/90/2590-050-6C33CB5C/Milky-Way-Galaxy-Earth.jpg?w=300)](https://cdn.britannica.com/90/2590-050-6C33CB5C/Milky-Way-Galaxy-Earth.jpg)

[Milky Way Galaxy](https://cdn.britannica.com/90/2590-050-6C33CB5C/Milky-Way-Galaxy-Earth.jpg) The Milky Way Galaxy as seen from Earth.(more)

[![](https://cdn.britannica.com/79/150779-050-601CB582/Milky-Way-Galaxy-Tuolumne-Meadows-California-Yosemite.jpg?w=300)](https://cdn.britannica.com/79/150779-050-601CB582/Milky-Way-Galaxy-Tuolumne-Meadows-California-Yosemite.jpg)

[Milky Way Galaxy](https://cdn.britannica.com/79/150779-050-601CB582/Milky-Way-Galaxy-Tuolumne-Meadows-California-Yosemite.jpg) The Milky Way Galaxy viewed at night from Tuolumne Meadows, Yosemite National Park, California.(more)

**Milky Way Galaxy**, large spiral system consisting of several hundred billion [stars](https://www.britannica.com/science/star-astronomy), one of which is the [Sun](https://www.britannica.com/place/Sun). It takes its name from the Milky Way, the irregular [luminous](https://www.britannica.com/dictionary/luminous) band of stars and gas clouds that stretches across the sky as seen from [Earth](https://www.britannica.com/place/Earth). Although Earth lies well within the Milky Way Galaxy (sometimes simply called the Galaxy), astronomers do not have as complete an understanding of its nature as they do of some external star systems. A thick layer of [interstellar dust](https://www.britannica.com/science/micrometeoroid) obscures much of the Galaxy from scrutiny by optical [telescopes](https://www.britannica.com/science/optical-telescope), and astronomers can determine its large-scale structure only with the aid of [radio](https://www.britannica.com/science/radio-telescope) and [infrared telescopes](https://www.britannica.com/science/infrared-telescope), which can detect the forms of radiation that penetrate the obscuring matter.

[![](https://cdn.britannica.com/29/148329-050-269A9EFE/night-sky-Milky-Way-Galaxy.jpg?w=300)](https://cdn.britannica.com/29/148329-050-269A9EFE/night-sky-Milky-Way-Galaxy.jpg)

[Milky Way Galaxy](https://cdn.britannica.com/29/148329-050-269A9EFE/night-sky-Milky-Way-Galaxy.jpg) The Milky Way Galaxy in the night sky.(more)

This article discusses the structure, properties, and component parts of the Milky Way Galaxy. For a full-length discussion of the cosmic [universe](https://www.britannica.com/science/universe) of which the Galaxy is only a small part, _see_ [cosmology](https://www.britannica.com/science/cosmology-astronomy). For the star system within the Galaxy that is the home of Earth, _see_ [solar system](https://www.britannica.com/science/solar-system).

## Major components of the Galaxy

## [Star clusters](https://www.britannica.com/science/star-cluster) and stellar associations

Although most [stars](https://www.britannica.com/science/star-astronomy) in the Galaxy exist either as single stars like the [Sun](https://www.britannica.com/place/Sun) or as [double stars](https://www.britannica.com/science/binary-star), there are many [conspicuous](https://www.merriam-webster.com/dictionary/conspicuous) groups and clusters of stars that contain tens to thousands of members. These objects can be subdivided into three types: [globular clusters](https://www.britannica.com/science/star-cluster#ref52777), [open clusters](https://www.britannica.com/science/star-cluster/Open-clusters#ref52778), and [stellar associations](https://www.britannica.com/science/stellar-association). They differ primarily in age and in the number of member stars.

## [Globular clusters](https://www.britannica.com/science/globular-cluster)

[![](https://cdn.britannica.com/47/111347-050-F03ABE95/cluster-M80-image-Hubble-Space-Telescope-hundreds.jpg?w=300)](https://cdn.britannica.com/47/111347-050-F03ABE95/cluster-M80-image-Hubble-Space-Telescope-hundreds.jpg)

[globular cluster M80](https://cdn.britannica.com/47/111347-050-F03ABE95/cluster-M80-image-Hubble-Space-Telescope-hundreds.jpg) Globular cluster M80 (also known as NGC 6093) in an optical image taken by the Hubble Space Telescope. M80 is located 28,000 light-years from Earth and contains hundreds of thousands of stars.(more)

The largest and most massive star clusters are the globular clusters, so called because of their roughly spherical appearance. The Galaxy contains more than 150 globular clusters (the exact number is uncertain because of obscuration by dust in the Milky Way band, which probably prevents some globular clusters from being seen). They are arranged in a nearly spherical halo around the Milky Way, with relatively few toward the galactic plane but a heavy concentration toward the centre. The [radial](https://www.britannica.com/dictionary/radial) distribution, when plotted as a function of distance from the galactic centre, fits a mathematical expression of a form identical to the one describing the star distribution in elliptical [galaxies](https://www.britannica.com/science/galaxy).

[![Encyclopaedia Britannica thistle graphic to be used with a Mendel/Consumer quiz in place of a photograph.](https://cdn.britannica.com/05/215905-131-626D860A/Encyclopaedia-Britannica-thistle-graphic-logo.jpg)\
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Globular clusters are extremely luminous objects. Their mean luminosity is the equivalent of approximately 25,000 [Suns](https://www.britannica.com/place/Sun). The most luminous are 50 times brighter. The masses of globular clusters, measured by determining the dispersion in the velocities of individual stars, range from a few thousand to more than 1,000,000 solar masses. The clusters are very large, with diameters measuring from 10 to as much as 300 light-years. Most globular clusters are highly concentrated at their centres, having stellar distributions that resemble isothermal gas spheres with a cutoff that corresponds to the tidal effects of the Galaxy. A precise model of star distribution within a cluster can be derived from stellar [dynamics](https://www.merriam-webster.com/dictionary/dynamics), which takes into account the kinds of orbits that stars have in the cluster, encounters between these member stars, and the effects of exterior influences. The American astronomer Ivan R. King, for instance, derived [dynamical](https://www.merriam-webster.com/dictionary/dynamical) models that fit observed stellar distributions very closely. He finds that a cluster’s structure can be described in terms of two numbers: (1) the core radius, which measures the degree of concentration at the centre, and (2) the tidal radius, which measures the cutoff of star densities at the edge of the cluster.

A key distinguishing feature of globular clusters in the Galaxy is their uniformly [old age](https://www.britannica.com/science/old-age). Determined by comparing the stellar population of globular clusters with stellar evolutionary models, the ages of all those so far measured range from 11 billion to 13 billion years. They are the oldest objects in the Galaxy and so must have been among the first formed. That this was the case is also indicated by the fact that the globular clusters tend to have much smaller amounts of heavy elements than do the stars in the plane of the Galaxy, e.g., the Sun. Composed of stars belonging to the extreme Population II ( _see below_ [Stars and stellar populations](https://www.britannica.com/place/Milky-Way-Galaxy/Star-populations-and-movement#ref68065)), as well as the high-latitude halo stars, these nearly spherical assemblages apparently formed before the material of the Galaxy flattened into the present thin disk. As their component stars evolved, they gave up some of their gas to interstellar space. This gas was enriched in the heavy [elements](https://www.britannica.com/science/chemical-element) (i.e., elements heavier than helium) produced in stars during the later stages of their evolution, so that the interstellar gas in the Galaxy is continually being changed. [Hydrogen](https://www.britannica.com/science/hydrogen) and [helium](https://www.britannica.com/science/helium-chemical-element) have always been the major [constituents](https://www.merriam-webster.com/dictionary/constituents), but heavy elements have gradually grown in importance. The present interstellar gas contains elements heavier than [helium](https://www.britannica.com/science/helium-chemical-element) at a level of about 2 percent by mass, while the globular clusters contain as little as 0.02 percent of the same elements.

## [Open clusters](https://www.britannica.com/science/open-cluster)

[![](https://cdn.britannica.com/38/111338-050-D23BE7C8/Stars-NGC-290-Hubble-Space-Telescope.jpg?w=300)](https://cdn.britannica.com/38/111338-050-D23BE7C8/Stars-NGC-290-Hubble-Space-Telescope.jpg)

[open cluster NGC 290](https://cdn.britannica.com/38/111338-050-D23BE7C8/Stars-NGC-290-Hubble-Space-Telescope.jpg) Open cluster NGC 290, as seen by the Hubble Space Telescope.(more)

![](https://cdn.britannica.com/marketing/BlueThistle.webp)

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Clusters smaller and less massive than the globular clusters are found in the plane of the Galaxy intermixed with the majority of the system’s [stars](https://www.britannica.com/science/star-astronomy), including the [Sun](https://www.britannica.com/place/Sun). These objects are the open clusters, so called because they generally have a more open, loose appearance than typical globular clusters.

Open clusters are distributed in the Galaxy very similarly to young stars. They are highly concentrated along the plane of the Galaxy and slowly decrease in number outward from its centre. The large-scale distribution of these clusters cannot be learned directly because their existence in the Milky Way plane means that dust obscures those that are more than a few thousand [light-years](https://www.britannica.com/science/light-year) from the Sun. By [analogy](https://www.merriam-webster.com/dictionary/analogy) with open clusters in external galaxies similar to the Galaxy, it is surmised that they follow the general distribution of [integrated](https://www.merriam-webster.com/dictionary/integrated) light in the Galaxy, except that there are probably fewer of them in the central areas. There is some evidence that the younger open clusters are more densely concentrated in the [Galaxy’s](https://www.britannica.com/science/galaxy) spiral arms, at least in the neighbourhood of the Sun where these arms can be discerned.

The brightest open clusters are considerably fainter than the brightest globular clusters. The peak absolute luminosity appears to be about 50,000 times the luminosity of the Sun, but the largest percentage of known open clusters has a brightness equivalent to 500 solar luminosities. Masses can be determined from the dispersion in the measured velocities of individual stellar members of clusters. Most open clusters have small masses on the order of 50 solar masses. Their total populations of stars are small, ranging from tens to a few thousand.

Open [clusters](https://www.britannica.com/dictionary/clusters) have diameters of only 2 or 3 to about 20 light-years, with the majority being less than 5 light-years across. In structure they look very different from globular clusters, though they can be understood in terms of similar dynamical models. The most important structural difference is their small total mass and relative looseness, which result from their comparatively large core radii. These two features have disastrous consequences as far as their ultimate fate is concerned, because open clusters are not sufficiently gravitationally bound to be able to withstand the disruptive tidal effects in the Galaxy ( _see_ [star cluster: Open clusters](https://www.britannica.com/science/star-cluster/Open-clusters#ref52778)). Judging from the sample of open clusters within 3,000 light-years of the Sun, only half of them can withstand such tidal forces for more than 200 million years, and a mere 2 percent have life expectancies as high as 1 billion years.

Measured ages of open clusters agree with the conclusions that have been reached about their life expectancies. They tend to be young objects; only a few are known to exceed 1 billion years in age. Most are younger than 200 million years, and some are 1 or 2 million years old. Ages of open clusters are determined by comparing their stellar membership with theoretical models of stellar evolution. Because all the stars in a cluster have very nearly the same age and chemical [composition](https://www.merriam-webster.com/dictionary/composition), the differences between the member stars are entirely the result of their different masses. As time progresses after the formation of a cluster, the massive stars, which evolve the fastest, gradually disappear from the cluster, becoming [white dwarf stars](https://www.britannica.com/science/white-dwarf-star) or other underluminous stellar remnants. Theoretical models of clusters show how this effect changes the stellar content with time, and direct comparisons with real clusters give reliable ages for them. To make this comparison, astronomers use a diagram (the colour-magnitude diagram) that plots the temperatures of the stars against their luminosities. Colour-magnitude diagrams have been obtained for more than 1,000 open clusters, and ages are thus known for this large sample.

Because open clusters are mostly young objects, they have chemical [compositions](https://www.merriam-webster.com/dictionary/compositions) that correspond to the enriched [environment](https://www.merriam-webster.com/dictionary/environment) from which they formed. Most of them are like the Sun in their abundance of the heavy elements, and some are even richer. For instance, the [Hyades](https://www.britannica.com/topic/Hyades-astronomy), which compose one of the nearest clusters, have almost twice the abundance of heavy elements as the Sun. It became possible in the 1990s to discover very young open clusters that previously had been entirely hidden in deep, dusty regions. Using [infrared](https://www.britannica.com/science/infrared-radiation) array detectors, astronomers found that many [molecular clouds](https://www.britannica.com/science/molecular-cloud) contained very young groups of stars that had just formed and, in some cases, were still forming.

## [Stellar associations](https://www.britannica.com/science/stellar-association)

Even younger than open clusters, stellar associations are very loose groupings of young [stars](https://www.britannica.com/science/star-astronomy) that share a common place and time of origin but that are not generally tied closely enough together gravitationally to form a stable cluster. Stellar associations are limited strictly to the plane of the Galaxy and appear only in regions of the system where star formation is occurring, notably in the spiral arms. They are very luminous objects. The brightest are even brighter than the brightest globular clusters, but this is not because they contain more stars; instead it is the result of the fact that their [constituent](https://www.merriam-webster.com/dictionary/constituent) stars are very much brighter than the stars [constituting](https://www.merriam-webster.com/dictionary/constituting) globular clusters. The most luminous stars in stellar associations are very young stars of spectral types [O](https://www.britannica.com/science/O-type-star) and [B](https://www.britannica.com/science/B-type-star). They have absolute luminosities as bright as any star in the Galaxy—on the order of one million times the luminosity of the Sun. Such stars have very short lifetimes, only lasting a few million years. With luminous stars of this type there need not be very many to make up a highly luminous and conspicuous grouping. The total masses of stellar associations amount to only a few hundred solar masses, with the population of stars being in the hundreds or, in a few cases, thousands.

The sizes of stellar associations are large; the average [diameter](https://www.britannica.com/dictionary/diameter) of those in the Galaxy is about 250 [light-years](https://www.britannica.com/science/light-year). They are so large and loosely structured that their self-gravitation is insufficient to hold them together, and in a matter of a few million years the members disperse into surrounding space, becoming separate and unconnected stars in the galactic field.

## [Moving groups](https://www.britannica.com/science/moving-cluster)

[![](https://cdn.britannica.com/58/2958-050-C1B86BCF/nebulosity-Pleiades-distance-clouds-Cluster-stars-light.jpg?w=300)](https://cdn.britannica.com/58/2958-050-C1B86BCF/nebulosity-Pleiades-distance-clouds-Cluster-stars-light.jpg)

[Pleiades](https://cdn.britannica.com/58/2958-050-C1B86BCF/nebulosity-Pleiades-distance-clouds-Cluster-stars-light.jpg) Bright nebulosity in the Pleiades (M45, NGC 1432), distance 490 light-years. Cluster stars provide the light, and surrounding clouds of dust reflect and scatter the rays from the stars.(more)

These objects are organizations of stars that share common measurable motions. Sometimes these do not form a noticeable cluster. This definition allows the term to be applied to a range of objects from the nearest gravitationally bound clusters to groups of widely spread stars with no apparent gravitational identity, which are discovered only by searching the catalogs for stars of common motion. Among the best known of the moving groups is the [Hyades](https://www.britannica.com/topic/Hyades-astronomy) in the [constellation](https://www.britannica.com/science/constellation) [Taurus](https://www.britannica.com/place/Taurus). Also known as the Taurus moving cluster or the Taurus stream, this system [comprises](https://www.merriam-webster.com/dictionary/comprises) the relatively dense Hyades cluster along with a few very distant members. It contains a total of about 350 stars, including several white dwarfs. Its centre lies about 150 light-years away. Other notable moving stellar groups include the [Ursa Major](https://www.britannica.com/place/Ursa-Major), [Scorpius](https://www.britannica.com/place/Scorpius)-Centaurus, and [Pleiades](https://www.britannica.com/place/Pleiades-astronomy) groups. Besides these remote organizations, investigators have observed what appear to be groups of high-velocity stars near the Sun. One of these, called the Groombridge 1830 group, consists of a number of subdwarfs and the star [RR Lyrae](https://www.britannica.com/science/RR-Lyrae-star), after which the RR Lyrae variables were named.

Recent advances in the study of moving groups have had an impact on the investigation of the kinematic history of stars and on the absolute calibration of the distance scale of the Galaxy. Moving groups have proved particularly useful with respect to the latter because their [commonality](https://www.britannica.com/dictionary/commonality) of motion enables astronomers to determine accurately (for the nearer examples) the distance of each individual member. Together with nearby [parallax](https://www.britannica.com/science/parallax) stars, moving-group parallaxes provide the basis for the galactic distance scale. Astronomers have found the Hyades moving cluster well suited for their purpose: it is close enough to permit the reliable application of the method, and it has enough members for deducing an accurate age.

One of the basic problems of using moving groups for distance determination is the selection of members. In the case of the Hyades, this has been done very carefully but not without considerable dispute. The members of a moving group (and its actual existence) are established by the degree to which their motions define a common convergent point in the sky. One technique is to determine the coordinates of the poles of the great circles defined by the [proper motions](https://www.britannica.com/science/proper-motion) and positions of individual stars. The positions of the poles will define a great circle, and one of its poles will be the convergent point for the moving group. Membership of stars can be established by [criteria](https://www.merriam-webster.com/dictionary/criteria) applied to the distances of proper-motion poles of individual stars from the mean great circle. The reliability of the existence of the group itself can be measured by the dispersion of the great circle points about their mean.

As radial velocities will not have been used for the preliminary selection of members, they can be subsequently examined to eliminate further nonmembers. The final list of members should contain only a very few nonmembers—either those that appear to agree with the group motion because of observational errors or those that happen to share the group’s motion at the present time but are not related to the group historically.

The distances of individual stars in a moving group may be determined if their radial velocities and proper motions are known ( _see below_ [Stellar motions](https://www.britannica.com/place/Milky-Way-Galaxy/Star-populations-and-movement#ref68091)) and if the exact position of the radiant is determined. If the angular distance of a star from the radiant is λ and if the velocity of the cluster as a whole with respect to the Sun is _V_, then the radial velocity of the star, _V_ _r_, is _V_ _r_= _V_ cos λ.  The transverse (or tangential) velocity, _T_, is given by _T_ = _V_ sin λ = 4.74 μ/ _p_ where _p_ is the star’s parallax in arc seconds. Thus, the parallax of a star is given by _p_ = 4.74 μ cot λ/ _V_ _r_.

The key to achieving reliable distances by this method is to locate the convergent point of the group as accurately as possible. The various techniques used (e.g., Charlier’s method) are capable of high accuracy, provided that the measurements themselves are free of [systematic](https://www.britannica.com/dictionary/systematic) errors. For the Taurus moving group, for example, it has been estimated that the accuracy for the best-observed stars is on the order of 3 percent in the parallax, discounting any errors due to systematic problems in the proper motions. Accuracies of this order were not possible by other means until the space-based telescope [Hipparcos](https://www.britannica.com/topic/Hipparcos) was able to measure highly precise stellar parallaxes for thousands of individual stars.

## [Emission nebulae](https://www.britannica.com/science/emission-nebula)

[![](https://cdn.britannica.com/54/21254-050-9D8F727F/Centre-Astronomers-Orion-Nebula-picture-systems-disks.jpg?w=300)](https://cdn.britannica.com/54/21254-050-9D8F727F/Centre-Astronomers-Orion-Nebula-picture-systems-disks.jpg)

[Orion Nebula (M42)](https://cdn.britannica.com/54/21254-050-9D8F727F/Centre-Astronomers-Orion-Nebula-picture-systems-disks.jpg) Centre of the Orion Nebula (M42). Astronomers have identified some 700 young stars in this 2.5-light-year-wide area. They have also detected over 150 protoplanetary disks, or proplyds, which are believed to be embryonic solar systems that will eventually form planets. These stars and proplyds generate most of the nebula's light. This picture is a mosaic combining 45 images taken by the Hubble Space Telescope.(more)

A conspicuous component of the Galaxy is the collection of large, bright, diffuse gaseous objects generally called [nebulae](https://www.britannica.com/science/nebula). The brightest of these cloudlike objects are the emission nebulae, large complexes of [interstellar](https://www.britannica.com/dictionary/interstellar) gas and stars in which the gas exists in an ionized and excited state (with the [electrons](https://www.britannica.com/science/electron) of the [atoms](https://www.britannica.com/science/atom) excited to a higher than normal energy level). This condition is produced by the strong [ultraviolet light](https://www.britannica.com/science/ultraviolet-radiation) emitted from the very luminous, hot [stars](https://www.britannica.com/science/star-astronomy) embedded in the gas. Because emission nebulae consist almost entirely of ionized [hydrogen](https://www.britannica.com/science/hydrogen), they are usually referred to as [H II regions.](https://www.britannica.com/science/H-II-region)

H II regions are found in the plane of the Galaxy intermixed with young stars, stellar associations, and the youngest of the open clusters. They are areas where very massive stars have recently formed, and many contain the uncondensed gas, dust, and molecular complexes commonly associated with ongoing star formation. The H II regions are concentrated in the spiral arms of the Galaxy, though some exist between the arms. Many of them are found at intermediate distances from the centre of the Milky Way Galaxy, with the largest number occurring at a distance of 10,000 light-years. This latter fact can be [ascertained](https://www.merriam-webster.com/dictionary/ascertained) even though the H II regions cannot be seen clearly beyond a few thousand light-years from the [Sun](https://www.britannica.com/place/Sun). They emit radio radiation of a characteristic type, with a thermal spectrum that indicates that their temperatures are about 10,000 kelvins. This thermal radio radiation enables astronomers to map the distribution of H II regions in distant parts of the Galaxy.

The largest and brightest H II regions in the Galaxy rival the brightest star clusters in total [luminosity](https://www.britannica.com/science/luminosity). Even though most of the [visible radiation](https://www.britannica.com/science/light) is concentrated in a few discrete emission lines, the total apparent brightness of the brightest is the equivalent of tens of thousands of solar luminosities. These H II regions are also remarkable in size, having [diameters](https://www.britannica.com/dictionary/diameters) of about 1,000 light-years. More typically, common H II regions such as the [Orion Nebula](https://www.britannica.com/place/Orion-Nebula) are about 50 light-years across. They contain gas that has a total mass ranging from one or two solar masses up to several thousand. H II regions consist primarily of hydrogen, but they also contain measurable amounts of other gases. [Helium](https://www.britannica.com/science/helium-chemical-element) is second in abundance, and large amounts of [carbon](https://www.britannica.com/science/carbon-chemical-element), [nitrogen](https://www.britannica.com/science/nitrogen), and [oxygen](https://www.britannica.com/science/oxygen) occur as well. Preliminary evidence indicates that the ratio of the [abundance](https://www.britannica.com/science/abundance-of-the-elements) of the heavier elements among the detected gases to hydrogen decreases outward from the centre of the Galaxy, a tendency that has been observed in other spiral galaxies.

## [Planetary nebulae](https://www.britannica.com/science/planetary-nebula)

[![](https://cdn.britannica.com/36/21236-050-DD862016/picture-Cats-Eye-Nebula-details-images-planetary.jpg?w=300)](https://cdn.britannica.com/36/21236-050-DD862016/picture-Cats-Eye-Nebula-details-images-planetary.jpg)

[Cat's Eye Nebula](https://cdn.britannica.com/36/21236-050-DD862016/picture-Cats-Eye-Nebula-details-images-planetary.jpg) Composite picture of the Cat's Eye Nebula (NGC 6543), combining three images taken by the Hubble Space Telescope. This planetary nebula has an unusually complicated structure, with concentric shells (seen as bright rings), jets (the projections at upper left and lower right), and a number of details that suggest complex interactions of shock waves. (more)

The gaseous clouds known as planetary nebulae are only superficially similar to other types of nebulae. So called because the smaller varieties almost resemble planetary disks when viewed through a telescope, planetary nebulae represent a stage at the end of the stellar life cycle rather than one at the beginning. The distribution of such nebulae in the Galaxy is different from that of H II regions. Planetary nebulae belong to an intermediate population and are found throughout the disk and the inner halo. There are more than 1,000 known planetary nebulae in the Galaxy, but more might be overlooked because of obscuration in the Milky Way region.

## [Supernova remnants](https://www.britannica.com/science/supernova-remnant)

[![](https://cdn.britannica.com/88/91388-050-57AD9D69/Crab-Nebula-exposures-image-astronomers-explosion-NASA.jpg?w=300)](https://cdn.britannica.com/88/91388-050-57AD9D69/Crab-Nebula-exposures-image-astronomers-explosion-NASA.jpg)

[Crab Nebula](https://cdn.britannica.com/88/91388-050-57AD9D69/Crab-Nebula-exposures-image-astronomers-explosion-NASA.jpg) The Crab Nebula, which was formed by a supernova explosion recorded in 1054. This image was made by combining two dozen exposures from the Hubble Space Telescope.(more)

Another type of [nebulous](https://www.merriam-webster.com/dictionary/nebulous) object found in the Galaxy is the remnant of the gas blown out from an exploding star that forms a [supernova](https://www.britannica.com/science/supernova). Occasionally these objects look something like planetary nebulae, as in the case of the [Crab Nebula](https://www.britannica.com/place/Crab-Nebula), but they differ from the latter in three ways: (1) the total mass of their gas (they involve a larger mass, essentially all the mass of the exploding star), (2) their kinematics (they are expanding with higher velocities), and (3) their lifetimes (they last for a shorter time as visible nebulae). The best-known supernova remnants are those resulting from three historically observed supernovae: that of 1054, which made the Crab Nebula its remnant; that of 1572, called [Tycho’s Nova](https://www.britannica.com/place/Tychos-Nova); and that of 1604, called [Kepler’s Nova](https://www.britannica.com/place/Keplers-Nova). These objects and the many others like them in the Galaxy are detected at radio wavelengths. They release radio energy in a nearly flat spectrum because of the emission of radiation by charged particles moving spirally at nearly the [speed of light](https://www.britannica.com/science/speed-of-light) in a magnetic field enmeshed in the gaseous remnant. Radiation generated in this way is called [synchrotron radiation](https://www.britannica.com/science/synchrotron-radiation) and is associated with various types of violent [cosmic](https://www.britannica.com/science/micrometeoroid) phenomena besides supernova remnants, as, for example, radio galaxies.

## Dust clouds

[![](https://cdn.britannica.com/25/92825-050-AA387705/Eagle-Nebula-column-Stars-cold-dust-gas.jpg?w=300)](https://cdn.britannica.com/25/92825-050-AA387705/Eagle-Nebula-column-Stars-cold-dust-gas.jpg)

[Eagle Nebula](https://cdn.britannica.com/25/92825-050-AA387705/Eagle-Nebula-column-Stars-cold-dust-gas.jpg) The Eagle Nebula. Stars are forming in this column of cold dust and gas, which is 9.5 light-years in length.(more)

[![](https://cdn.britannica.com/35/111335-050-47DB3358/NGC-4013-dust-lane-Milky-Way-Galaxy.jpg?w=300)](https://cdn.britannica.com/35/111335-050-47DB3358/NGC-4013-dust-lane-Milky-Way-Galaxy.jpg)

[NGC 4013](https://cdn.britannica.com/35/111335-050-47DB3358/NGC-4013-dust-lane-Milky-Way-Galaxy.jpg) NGC 4013, a spiral galaxy, which has a prominent dust lane like the Milky Way Galaxy, in an image taken by the Hubble Space Telescope.(more)

The dust clouds of the Galaxy are narrowly limited to the plane of the Milky Way, though very low-density dust can be detected even near the galactic poles. Dust clouds beyond 2,000 to 3,000 light-years from the [Sun](https://www.britannica.com/place/Sun) cannot be detected optically, because intervening clouds of dust and the general dust layer obscure more distant views. Based on the distribution of dust clouds in other galaxies, it can be concluded that they are often most conspicuous within the spiral arms, especially along the inner edge of well-defined ones. The best-observed dust clouds near the Sun have masses of several hundred solar masses and sizes ranging from a maximum of about 200 light-years to a fraction of a light-year. The smallest tend to be the densest, possibly partly because of evolution: as a dust complex contracts, it also becomes denser and more [opaque](https://www.merriam-webster.com/dictionary/opaque). The very smallest dust clouds are the so-called [Bok globules](https://www.britannica.com/science/Bok-globule), named after the Dutch American astronomer [Bart J. Bok](https://www.britannica.com/biography/Bart-J-Bok); these objects are about one light-year across and have masses of 1–20 solar masses.

More complete information on the dust in the Galaxy comes from [infrared](https://www.britannica.com/science/infrared-radiation) observations. While optical instruments can detect the dust when it obscures more distant objects or when it is [illuminated](https://www.merriam-webster.com/dictionary/illuminated) by very nearby stars, [infrared telescopes](https://www.britannica.com/science/infrared-telescope) are able to register the long-wavelength radiation that the cool dust clouds themselves emit. A complete survey of the sky at infrared wavelengths made during the early 1980s by an unmanned [orbiting observatory](https://www.britannica.com/science/satellite-observatory), the [Infrared Astronomical Satellite](https://www.britannica.com/topic/Infrared-Astronomical-Satellite) (IRAS), revealed a large number of dense dust clouds in the Milky Way. Twenty years later the [Spitzer Space Telescope](https://www.britannica.com/topic/Spitzer-Space-Telescope), with greater sensitivity, greater wavelength coverage, and better resolution, mapped many dust complexes in the Milky Way. In some it was possible to view massive star clusters still in the process of formation.

Thick clouds of dust in the Milky Way can be studied by still another means. Many such objects contain detectable amounts of [molecules](https://www.britannica.com/science/molecule) that [emit](https://www.britannica.com/dictionary/emit) [radio](https://www.britannica.com/science/radio-wave) radiation at wavelengths that allow them to be identified and analyzed. More than 50 different molecules, including [carbon monoxide](https://www.britannica.com/science/carbon-monoxide) and [formaldehyde](https://www.britannica.com/science/formaldehyde), and radicals have been detected in dust clouds.

## The general [interstellar medium](https://www.britannica.com/science/interstellar-medium)

[![](https://cdn.britannica.com/80/1280-004-76E2D8EB/Horsehead-Nebula.jpg?w=300)](https://cdn.britannica.com/80/1280-004-76E2D8EB/Horsehead-Nebula.jpg)

[Horsehead Nebula](https://cdn.britannica.com/80/1280-004-76E2D8EB/Horsehead-Nebula.jpg)(more)

[![](https://cdn.britannica.com/36/111336-004-05E193D2/regions-Milky-Way-Galaxy-image-light-left.jpg?w=300)](https://cdn.britannica.com/36/111336-004-05E193D2/regions-Milky-Way-Galaxy-image-light-left.jpg)

[centre of the Milky Way Galaxy](https://cdn.britannica.com/36/111336-004-05E193D2/regions-Milky-Way-Galaxy-image-light-left.jpg) Central regions of the Milky Way Galaxy. The image on the left is in visible light, and the image on the right is in infrared; the marked difference between the two images shows how infrared radiation can penetrate galactic dust. The infrared image is part of the Two Micron All Sky Survey (2MASS), a survey of the entire sky in infrared light.(more)

The stars in the Galaxy, especially along the Milky Way, reveal the presence of a general, all-pervasive interstellar medium by the way in which they gradually fade with distance. This occurs primarily because of interstellar [dust](https://www.britannica.com/science/micrometeoroid), which obscures and reddens starlight. On the average, stars near the Sun are dimmed by a factor of two for every 3,000 light-years. Thus, a star that is 6,000 light-years away in the plane of the Galaxy will appear four times fainter than it would otherwise were it not for the interstellar dust.

Another way in which the effects of interstellar dust become apparent is through the [polarization](https://www.britannica.com/science/polarization-physics) of background starlight. Dust is aligned in space to some extent, and this results in selective absorption such that there is a preferred plane of vibration for the light waves. The electric vectors tend to lie preferentially along the galactic plane, though there are areas where the distribution is more complicated. It is likely that the polarization arises because the dust grains are partially aligned by the galactic [magnetic field](https://www.britannica.com/science/magnetic-field). If the dust grains are [paramagnetic](https://www.britannica.com/science/paramagnetism) so that they act somewhat like a magnet, then the general magnetic field, though very weak, can in time line up the grains with their short [axes](https://www.britannica.com/dictionary/axes) in the direction of the field. As a consequence, the directions of polarization for [stars](https://www.britannica.com/science/star-astronomy) in different parts of the sky make it possible to plot the direction of the magnetic field in the Milky Way.

The dust is accompanied by [gas](https://www.britannica.com/science/interstellar-gas), which is thinly dispersed among the stars, filling the space between them. This interstellar gas consists mostly of [hydrogen](https://www.britannica.com/science/hydrogen) in its neutral form. [Radio telescopes](https://www.britannica.com/science/radio-telescope) can detect neutral hydrogen because it emits radiation at a wavelength of [21 cm](https://www.britannica.com/science/21-centimeter-radiation). Such radio wavelength is long enough to penetrate interstellar dust and so can be detected from all parts of the Galaxy. Most of what astronomers have learned about the large-scale structure and motions of the Galaxy has been [derived](https://www.britannica.com/dictionary/derived) from the radio waves of interstellar neutral hydrogen. The distance to the gas detected is not easily determined. [Statistical](https://www.britannica.com/science/statistics) arguments must be used in many cases, but the velocities of the gas, when compared with the velocities found for stars and those anticipated on the basis of the dynamics of the Galaxy, provide useful clues as to the location of the different sources of hydrogen radio emission. Near the [Sun](https://www.britannica.com/place/Sun) the average density of interstellar gas is 10−21 gm/cm3, which is the equivalent of about one hydrogen [atom](https://www.britannica.com/science/atom) per cubic centimetre.

Even before they first detected the emission from neutral hydrogen in 1951, astronomers were aware of interstellar gas. Minor components of the gas, such as [sodium](https://www.britannica.com/science/sodium) and [calcium](https://www.britannica.com/science/calcium), absorb light at specific wavelengths, and they thus cause the appearance of absorption lines in the spectra of the stars that lie beyond the gas. Since the lines originating from stars are usually different, it is possible to distinguish the lines of the interstellar gas and to measure both the density and velocity of the gas. Frequently it is even possible to observe the effects of several concentrations of interstellar gas between [Earth](https://www.britannica.com/place/Earth) and the background stars and thereby determine the kinematics of the gas in different parts of the Galaxy.

## Companion galaxies

[![Hear about the prediction of the Milky Way colliding with the Andromeda galaxy, which might happen in about four billion years](https://cdn.britannica.com/02/186502-138-1C58E2B5/overview-collision-galaxies-Milky-Way-Andromeda.jpg?w=800&h=450&c=crop)](/video/186502/overview-collision-galaxies-Milky-Way-Andromeda)

Hear about the prediction of the Milky Way colliding with the Andromeda galaxy, which might happen in about four billion yearsAn overview of the predicted collision of the Andromeda and Milky Way galaxies, expected to occur in some four billion years.(more)

[See all videos for this article](/place/Milky-Way-Galaxy/images-videos)

[![](https://cdn.britannica.com/47/21247-050-3EBE4755/composite-NGC-1850-cluster-Large-Magellanic-Cloud-1850.jpg?w=300)](https://cdn.britannica.com/47/21247-050-3EBE4755/composite-NGC-1850-cluster-Large-Magellanic-Cloud-1850.jpg)

[globular cluster NGC 1850 in the Large Magellanic Cloud](https://cdn.britannica.com/47/21247-050-3EBE4755/composite-NGC-1850-cluster-Large-Magellanic-Cloud-1850.jpg) Most of the globular cluster NGC 1850 consists of yellow stars; the bright white stars are members of a second, open cluster about 200 light-years beyond NGC 1850. This picture is a composite of images taken by the Hubble Space Telescope. (more)

The [Magellanic Clouds](https://www.britannica.com/topic/Magellanic-Cloud) were recognized early in the 20th century as companion objects to the Galaxy. When American astronomer [Edwin Hubble](https://www.britannica.com/biography/Edwin-Hubble) established the extragalactic nature of what we now call galaxies, it became plain that the Clouds had to be separate systems, both of the irregular class and more than 100,000 [light-years](https://www.britannica.com/science/light-year) distant. (The current best [values](https://www.britannica.com/dictionary/values) for their distances are 163,000 and 202,000 light-years for the Large and Small Clouds, respectively.) Additional close companions have been found, all of them small and inconspicuous objects of the dwarf elliptical class. The nearest of these is the [Sagittarius](https://www.britannica.com/place/Sagittarius-constellation) dwarf, a galaxy that is falling into the Milky Way Galaxy, having been captured tidally by the Galaxy’s much stronger [gravity](https://www.britannica.com/science/gravity-physics). The core of this galaxy is about 90,000 light-years distant. Other close companions are the well-studied [Carina](https://www.britannica.com/topic/Carina-constellation), Draco, Fornax, [Leo I](https://www.britannica.com/biography/Leo-I-Roman-emperor), [Leo II](https://www.britannica.com/biography/Leo-II-Roman-emperor), Sextans, Sculptor, and [Ursa Minor](https://www.britannica.com/place/Ursa-Minor) galaxies, as well as several very faint, less well-known objects. Distances for them range from approximately 200,000 to 800,000 light-years. The grouping of these galaxies around the Milky Way Galaxy is mimicked in the case of the [Andromeda Galaxy](https://www.britannica.com/place/Andromeda-Galaxy), which is also accompanied by several dwarf companions.