Editing Extragalactic astronomy

{{short description|Study of astronomical objects outside the Milky Way Galaxy}}
[[Image:Hubble deep field.jpg|thumb|right|250px|Galaxies in the 1995 [[Hubble Deep Field]]]]
'''Extragalactic astronomy''' is the branch of [[astronomy]] concerned with objects outside the [[Milky Way]] galaxy. In other words, it is the study of all astronomical objects which are not covered by [[galactic astronomy]].

The closest objects in extragalactic astronomy include the [[galaxy|galaxies]] of the [[Local Group]], which are close enough to allow very detailed analyses of their contents (e.g. supernova remnants, [[Stellar kinematics#Stellar associations|stellar associations]]). As instrumentation has improved, distant objects can now be examined in more detail and so extragalactic astronomy includes objects at nearly the edge of the observable universe.<ref>{{Cite web|url=https://www.as.arizona.edu/extragalactic-astronomy|title=Extragalactic Astronomy|website=as.arizona.edu|access-date=2020-04-18}}</ref> Research into distant galaxies (outside of our local group) is valuable for studying aspects of the universe such as galaxy evolution<ref>{{Cite web|url=https://physics.missouri.edu/research/astrophysics-and-general-relativity|title=Astrophysics and General Relativity {{!}} Physics and Astronomy|website=physics.missouri.edu|access-date=2020-04-18|archive-date=2020-08-11|archive-url=https://web.archive.org/web/20200811010518/https://physics.missouri.edu/research/astrophysics-and-general-relativity|url-status=dead}}</ref> and [[Active galactic nucleus|Active Galactic Nuclei (AGN)]] which give insight into physical phenomena (e.g.  [[Supermassive black hole|super massive black hole]] accretion and the presence of dark matter<ref>{{Cite web|url=https://sites.krieger.jhu.edu/astronomy/extragalactic-astronomy/|title=Extragalactic Astronomy|website=Center for Astrophysical Sciences|date=7 May 2014 |language=en-US|access-date=2020-04-18}}</ref>). It is through extragalactic astronomy that astronomers and physicists are able to study the effects of [[General relativity|General Relativity]]<ref>{{Cite journal|last1=Collett|first1=Thomas E.|last2=Oldham|first2=Lindsay J.|last3=Smith|first3=Russell J.|last4=Auger|first4=Matthew W.|last5=Westfall|first5=Kyle B.|last6=Bacon|first6=David|last7=Nichol|first7=Robert C.|last8=Masters|first8=Karen L.|last9=Koyama|first9=Kazuya|last10=van den Bosch|first10=Remco|date=2018-06-22|title=A precise extragalactic test of General Relativity|journal=Science|language=en|volume=360|issue=6395|pages=1342–1346|doi=10.1126/science.aao2469|pmid=29930135|arxiv=1806.08300|bibcode=2018Sci...360.1342C|s2cid=49363216|issn=0036-8075}}</ref> such as [[Gravitational lens|gravitational lensing]]<ref>{{Cite web|url=http://hubblesite.org/contents/articles/gravitational-lensing|title=Gravitational Lensing|website=HubbleSite.org|language=en|access-date=2020-04-18}}</ref><ref>{{Cite web|url=https://www.astronomynotes.com/relativity/s4.htm|title=Einstein's Relativity|website=www.astronomynotes.com|access-date=2020-04-18}}</ref> and [[Gravitational wave|gravitational waves]], that are otherwise impossible (or nearly impossible) to study on a galactic scale.

A key interest in extragalactic astronomy is the study of how galaxies behave and interact through the [[universe]]. Astronomer's [[methodologies]] depend — from theoretical to observation based methods.[[File:NGC2207+IC2163.jpg|thumb|right|[[NGC 2207 and IC 2163|NGC 2207]] (the bigger galaxy to the left) and [[NGC 2207 and IC 2163|IC 2163]] (the smaller galaxy to the right) as seen by the Hubble Space Telescope.]]

Galaxies form in various ways. In most cosmological [[N-body simulation|''N''-body simulations]], the earliest galaxies in the cosmos formed in the first hundreds of millions of years.<ref>{{cite journal |last1=Bromm |first1=Volker |last2=Yoshida |first2=Naoki |title=The First Galaxies |journal=Annual Review of Astronomy and Astrophysics |date=2011 |volume=49 |issue=1 |pages=373–407 |doi=10.1146/annurev-astro-081710-102608|arxiv=1102.4638 |bibcode=2011ARA&A..49..373B }}</ref>

These primordial galaxies formed as the enormous reservoirs of gas and dust in the early universe collapsed in on themselves, giving birth to the first stars, now known as Population III Stars.<ref>{{cite book | arxiv=1209.2509 | doi=10.1007/978-3-642-32362-1_3 | chapter=The First Stars | title=The First Galaxies | series=Astrophysics and Space Science Library | date=2013 | last1=Glover | first1=Simon | volume=396 | pages=103–174 | isbn=978-3-642-32361-4 | s2cid=117350129 |editor1=Tommy Wiklind |editor2=Bahram Mobasher |editor3=Volker Bromm }}</ref> These stars were of enormous masses in the range of 300 to perhaps 3 million [[solar masses]]. Due to their large mass, these stars had extremely short lifespans.

== Famous examples ==

* [[Hubble Deep Field]]
* [[LIGO]]'s detection of [[Gravitational wave|gravitational waves]]
* [[Chandra Deep Field South]]

== Topics ==
*[[Active galactic nucleus|Active Galactic Nuclei]] (AGN), [[Quasar]]s
*[[Dark matter|Dark Matter]]
*[[Galaxy cluster]]s, [[Supercluster]]s
*[[Intergalactic star]]s
*[[Intergalactic dust]]<ref name="dust1">{{citation|author1=M. E. Bailey|author2=D. A. Williams|title=Dust in the universe: the proceedings of a conference at the Department of Astronomy, University of Manchester, 14-18 December 1987|url=https://books.google.com/books?id=UPA8AAAAIAAJ&pg=PA509|year=1988|publisher=CUP Archive|isbn=978-0-521-35580-3|page=509}}</ref>
*The [[observable universe]]
*[[Radio galaxy|Radio galaxies]]
*[[Supernova]]e
*[[Extragalactic planet]]

==See also==
* [[Andromeda–Milky Way collision]]
* [[Galaxy color–magnitude diagram]]
* [[Galaxy formation and evolution]]
* [[Observational cosmology]]

==References==
{{reflist}}

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[[Category:Extragalactic astronomy| ]]
[[Category:Astronomical sub-disciplines]]
[[Category:Physical cosmology]]
[[Category:Edwin Hubble]]