Editing Astronomy (section)

=== Extragalactic astronomy ===

[[File:grav.lens1.arp.750pix.jpg|thumb|This image shows several blue, loop-shaped objects that are multiple images of the same galaxy, duplicated by the [[gravitational lens]] effect of the cluster of yellow galaxies near the middle of the photograph. The lens is produced by the cluster's gravitational field that bends light to magnify and distort the image of a more distant object.]]
{{Main|Extragalactic astronomy}}
The study of objects outside our galaxy is a branch of astronomy concerned with the [[Galaxy formation and evolution|formation and evolution of galaxies]], their morphology (description) and [[Galaxy morphological classification|classification]], the observation of [[Active galaxy|active galaxies]], and at a larger scale, the [[Galaxy groups and clusters|groups and clusters of galaxies]]. Finally, the latter is important for the understanding of the [[large-scale structure of the cosmos]].<ref name=":0" />

Most [[galaxy|galaxies]] are organized into distinct shapes that allow for classification schemes. They are commonly divided into [[spiral galaxy|spiral]], [[elliptical galaxy|elliptical]] and [[irregular galaxy|Irregular]] galaxies.<ref>{{cite web|last = Keel|first = Bill|date = 1 August 2006|url=http://www.astr.ua.edu/keel/galaxies/classify.html|title = Galaxy Classification|publisher = University of Alabama|access-date =8 September 2006| archive-url= https://web.archive.org/web/20060901074027/http://www.astr.ua.edu/keel/galaxies/classify.html| archive-date= 1 September 2006 | url-status= live}}</ref>

As the name suggests, an elliptical galaxy has the cross-sectional shape of an [[ellipse]]. The stars move along [[randomness|random]] orbits with no preferred direction. These galaxies contain little or no interstellar dust, few star-forming regions, and older stars.<ref name=":0" />{{Rp|pages=877–878}} Elliptical galaxies may have been formed by other galaxies merging.<ref name=":0" />{{Rp|page=939}}

A spiral galaxy is organized into a flat, rotating disk, usually with a prominent bulge or bar at the center, and trailing bright arms that spiral outward. The arms are dusty regions of star formation within which massive young stars produce a blue tint. Spiral galaxies are typically surrounded by a halo of older stars. Both the [[Milky Way]] and one of our nearest galaxy neighbors, the [[Andromeda Galaxy]], are spiral galaxies.<ref name=":0" />{{Rp|page=875}}

Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical.<ref name=":0" />{{Rp|page=879}} About a quarter of all galaxies are irregular, and the peculiar shapes of such galaxies may be the result of gravitational interaction.<ref>{{Cite web |date=2016-08-08 |title=A lopsided lynx |url=https://esahubble.org/images/potw1632a/ |access-date=2023-03-17 |website=esahubble.org |publisher=[[European Space Agency]] |language=en |archive-date=9 July 2021 |archive-url=https://web.archive.org/web/20210709183618/https://esahubble.org/images/potw1632a/ |url-status=live }}</ref>

An active galaxy is a formation that emits a significant amount of its energy from a source other than its stars, dust and gas. It is powered by a compact region at the core, thought to be a supermassive black hole that is emitting radiation from in-falling material.<ref name=":0" />{{Rp|page=907}} A [[radio galaxy]] is an active galaxy that is very luminous in the radio portion of the spectrum, and is emitting immense plumes or lobes of gas. Active galaxies that emit shorter frequency, high-energy radiation include [[Seyfert galaxy|Seyfert galaxies]], [[quasar]]s, and [[blazar]]s. Quasars are believed to be the most consistently luminous objects in the known universe.<ref>{{cite web|url=http://imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html|title=Active Galaxies and Quasars|publisher=NASA|access-date=17 November 2016|archive-url=https://web.archive.org/web/20060831033713/http://imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html|archive-date=31 August 2006 }}</ref>

The [[large-scale structure of the cosmos]] is represented by groups and clusters of galaxies. This structure is organized into a hierarchy of groupings, with the largest being the [[supercluster]]s. The collective matter is formed into [[Galaxy filament|filaments]] and walls, leaving large [[Void (astronomy)|voids]] between.<ref name="evolving universe">{{cite book|author=[[Michael Zeilik]]|title=Astronomy: The Evolving Universe|edition=8th|publisher=Wiley|date=2002|isbn=978-0-521-80090-7}}</ref>