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Glossary term: Galaxy

Description: A galaxy is a large system of stars and other material components such as dark matter, gas and dust that is gravitationally bound, mostly autonomous with respect to other objects, and usually separated from its neighbors by tens 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 200 billion stars.

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Term and definition status: This term and its definition is still awaiting approval

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 Comments 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 Comments 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 Comments Attribution 4.0 International (CC BY 4.0) icons