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Glossary term: Galaxy

Description: A galaxy is a system of stars and other material components such as dark matter, gas, and dust that is gravitationally bound, and usually separated from its neighbors by hundreds of thousands of light years. Galaxies come in various different shapes and sizes. The smallest galaxies can have a few thousand stars, while the largest can have tens of trillions. "The Galaxy" or "Galaxy" with a capital letter usually refers to our home galaxy, the Milky Way, which has around 100–400 billion stars.

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Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".

Related Media


The Hubble Ultra Deep Field showing around 10,000 galaxies of various ages, sizes, shapes and colours.

Hubble Ultra Deep Field

Caption: This awe-inspiring image referred to as the Hubble Ultra-Deep Field (HUDF), was obtained using the Hubble Space Telescope (HST), by combining 800 exposures from 400 orbits of the HST, which equates to 11.3 days of total exposure time. The image shows nearly 10,000 galaxies and was taken in the direction of a patch of sky with the least amount of stars from the Milky Way galaxy in the field of view. The region of sky that the HST observed corresponds to 1/10 the angular size of the Full Moon, which is roughly equal to approximately a 1 millimeter-sized object placed 1 meter away. Every object in the image, except for the bright points with the crosshairs, are galaxies. As a consequence of the speed of light being a constant in a vacuum, the more distant an object, the further back in time we are observing. Therefore, the light from some of the galaxies in the HUDF image is from when the Universe as only a few hundred million years old. The HUDF image takes us through on a journey through space, and also in time.
Credit: NASA, ESA, and S. Beckwith (STScI) and the HUDF Team credit link

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons


Two spiral galaxies embracing in their early stages of merger with distortions on the smaller galaxy visible

Spiral Galaxy Merger

Caption: This image shows two interacting spiral galaxies located approximately 150 million light-years away. Two spiral galaxies are involved in this dance, creating visible distortions in their spiral arms due to the gravitational pull. What is starting as an embrace in this picture, will end in the merger of those two galaxies into one, most likely elliptical, galaxy.
Credit: ESO credit link

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons


This galaxy has a large bulge surrounded by a ring of dust creating an image similar to the mexican hat with the same name

Sombrero galaxy

Caption: This prominent galaxy M 104 is often called the Sombrero galaxy due to its resemblance to the Mexican hat. This look is created by the thin disk oriented nearly edge-on, which appears illuminated by the dominant bulge. Both, the disk and bulge contain stars, but the stars in the bulge tend to be older, while the disk also harbors large amounts of dust and gas creating shadows. M 104 is located in the constellation Virgo approximately 50 light-years away. While some of the dots visible in the picture are foreground stars and others background galaxies, the majority are actually globular clusters, dense massive clusters of stars, associated with the Sombrero Galaxy.
Credit: ESO/P. Barthel credit link

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons

Related Diagrams


Leo resembles a lion standing on the ecliptic (which runs ESE to WNW) with its nose pointed northwest.

Leo Constellation Map

Caption: The zodiac constellation Leo and its surrounding constellations. Starting from the top of the diagram and going clockwise, these are Leo Minor, Cancer, Sextans, Hydra, Crater, Virgo and Ursa Major. The brightest star in Leo, Regulus, lies almost exactly on the ecliptic (shown here as a blue line): the path the Sun appears to take across the sky over the course of a year. The Sun spends the period from mid August to mid September in Leo. The other planets in the Solar System can often be found in Leo. Leo spans the celestial equator and is thus part of it is visible at some time in the year from all of planet Earth with some of the constellation obscured for the most arctic and antarctic regions of the world. Leo is most visible in the evenings in the northern hemisphere spring and southern hemisphere autumn. Several objects can be seen in Leo, including M65 and M66 – two galaxies in the Leo Triplet, a trio of galaxies including NGC 3628, not listed here. In addition, M96, a spiral galaxy, can be seen as a fuzzy object using a small telescope, and Messier 105, an elliptical galaxy. Each of these objects are labelled on the map as red ellipses. The y-axis of this diagram is in degrees of declination and with north as up and the x-axis is in hours of right ascension with east to the left. The sizes of the stars marked here relate to the star's apparent magnitude, a measure of its apparent brightness. The larger dots represent brighter stars. The Greek letters mark the brightest stars in the constellation. These are ranked by brightness with the brightest star being labelled alpha, the second brightest beta, etc., although this ordering is not always followed exactly. The dotted boundary lines mark the IAU's boundaries of the constellations and the solid green lines mark one of the common forms used to represent the figures of the constellations. Neither the constellation boundaries nor the lines joining the stars appear on the sky.
Credit: Adapted by the IAU Office of Astronomy for Education from the original by IAU/Sky & Telescope

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons

Related Activities


Glitter Your Milky Way

Glitter Your Milky Way

astroEDU educational activity (links to astroEDU website)
Description: Explore the Milky Way and characteristics of galaxies using glitter drawing.

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons

Tags: Art , Creativity , Hands-on , Handcraft
Age Ranges: 6-8 , 8-10
Education Level: Middle School , Primary
Areas of Learning: Fine Art focussed
Costs: Medium Cost
Group Size: Group
Skills: Communicating information

Coma Cluster of Galaxies

Coma Cluster of Galaxies

astroEDU educational activity (links to astroEDU website)
Description: The basics of galaxy classification, using Hubble Space Telescope images.

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons

Tags: Coma Cluster
Age Ranges: 14-16 , 16-19 , 19+
Education Level: Secondary , University
Areas of Learning: Guided-discovery learning
Costs: Low Cost
Duration: 1 hour
Group Size: Group
Skills: Analysing and interpreting data , Communicating information , Constructing explanations , Planning and carrying out investigations

Living in the Milky Way

Living in the Milky Way

astroEDU educational activity (links to astroEDU website)
Description: Build a model of the Milky Way to discover what our galaxy contains.

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons

Tags: Hands-on , Model
Age Ranges: 6-8 , 8-10
Education Level: Primary
Areas of Learning: Problem-solving , Social Research
Costs: Medium Cost
Duration: 1 hour 30 mins
Group Size: Group
Skills: Asking questions , Communicating information , Developing and using models