Extragalactic astronomy

{{short description|Study of astronomical objects outside the Milky Way Galaxy}}

Image:Hubble deep field.jpg]]

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 galaxies of the Local Group, which are close enough to allow very detailed analyses of their contents (e.g. supernova remnants, 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.{{Cite web|url=https://www.as.arizona.edu/extragalactic-astronomy|title=Extragalactic Astronomy|website=as.arizona.edu|access-date=2020-04-18}} Research into distant galaxies (outside of our local group) is valuable for studying aspects of the universe such as galaxy evolution{{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}} and Active Galactic Nuclei (AGN) which give insight into physical phenomena (e.g. super massive black hole accretion and the presence of dark matter{{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}}). It is through extragalactic astronomy that astronomers and physicists are able to study the effects of General Relativity{{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}} such as gravitational lensing{{Cite web|url=http://hubblesite.org/contents/articles/gravitational-lensing|title=Gravitational Lensing|website=HubbleSite.org|language=en|access-date=2020-04-18}}{{Cite web|url=https://www.astronomynotes.com/relativity/s4.htm|title=Einstein's Relativity|website=www.astronomynotes.com|access-date=2020-04-18}} and 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 (the bigger galaxy to the left) and 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 simulations, the earliest galaxies in the cosmos formed in the first hundreds of millions of years.{{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 }}

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.{{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 }} 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.