Editing Star formation (section)

==Observations==
[[Image:Orion Nebula - Hubble 2006 mosaic 18000.jpg|thumb|left|The [[Orion Nebula]] is an archetypical example of star formation, from the massive, young stars that are shaping the nebula to the pillars of dense gas that may be the homes of budding stars.]]
Key elements of star formation are only available by observing in [[wavelength]]s other than the [[Visible-light astronomy|optical]]. The protostellar stage of stellar existence is almost invariably hidden away deep inside dense clouds of gas and dust left over from the [[giant molecular cloud|GMC]]. Often, these star-forming cocoons known as [[Bok globule]]s, can be seen in [[silhouette]] against bright emission from surrounding gas.<ref>{{cite journal | bibcode=1947ApJ...105..255B | author=B. J. Bok | author2=E. F. Reilly | name-list-style=amp | title=Small Dark Nebulae | journal=Astrophysical Journal | date = 1947 | volume = 105 | pages=255 | doi=10.1086/144901 }}<br />{{cite journal | doi=10.1086/185891 | title=Star formation in small globules – Bart BOK was correct | date=1990 | author=Yun, Joao Lin | journal=The Astrophysical Journal | volume=365 | pages=L73 | last2=Clemens | first2=Dan P. | bibcode=1990ApJ...365L..73Y| doi-access=free }}</ref> Early stages of a star's life can be seen in [[infrared astronomy|infrared]] light, which penetrates the dust more easily than [[visible-light astronomy|visible]] light.<ref>{{cite journal | doi= 10.1086/376696 | arxiv=astro-ph/0306274 | title= GLIMPSE. I. An ''SIRTF'' Legacy Project to Map the Inner Galaxy | date= 2003 | author= Benjamin, Robert A. | journal= Publications of the Astronomical Society of the Pacific | volume= 115 | issue= 810 | pages= 953–964 | last2= Churchwell | first2= E. | last3= Babler | first3= Brian L. | last4= Bania | first4= T. M. | last5= Clemens | first5= Dan P. | last6= Cohen | first6= Martin | last7= Dickey | first7= John M. | last8= Indebetouw | first8= Rémy | last9= Jackson | first9= James M. | last10=Kobulnicky | first10=Henry A. | last11=Lazarian | first11=Alex | last12=Marston | first12=A. P. | last13=Mathis | first13=John S. | last14=Meade | first14=Marilyn R. | last15=Seager | first15=Sara | last16=Stolovy | first16=S. R. | last17=Watson | first17=C. | last18=Whitney | first18=Barbara A. | last19=Wolff | first19=Michael J. | last20=Wolfire | first20=Mark G. | bibcode=2003PASP..115..953B| s2cid=119510724 | display-authors=8 }}</ref>  
Observations from the [[Wide-field Infrared Survey Explorer]] (WISE) have thus been especially important for unveiling numerous galactic protostars and their parent [[star cluster]]s.<ref name=wright>{{cite web|url=http://wise.ssl.berkeley.edu/ |title=Wide-field Infrared Survey Explorer Mission |publisher=NASA}}</ref><ref name=ma2013>Majaess, D. (2013). [http://adsabs.harvard.edu/abs/2013Ap&SS.344..175M ''Discovering protostars and their host clusters via WISE''], ApSS, 344, 1 ([http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=J%2Fother%2FApSS%2F344%2E175 ''VizieR catalog''])</ref>  Examples of such embedded star clusters are FSR 1184, FSR 1190, Camargo 14, Camargo 74, Majaess 64, and Majaess 98.<ref name=ca2015>Camargo et al. (2015). [http://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:1406.3099 ''New Galactic embedded clusters and candidates from a WISE Survey''], New Astronomy, 34</ref>
[[File:Star-forming region S106 (captured by the Hubble Space Telescope).jpg|thumb|Star-forming region S106.]]
The structure of the molecular cloud and the effects of the protostar can be observed in near-IR [[extinction (astronomy)|extinction]] maps (where the number of stars are counted per unit area and compared to a nearby zero extinction area of sky), continuum dust emission and [[rotational transition]]s of [[Carbon monoxide|CO]] and other molecules; these last two are observed in the millimeter and [[radio astronomy|submillimeter]] range. The radiation from the protostar and early star has to be observed in [[infrared|infrared astronomy]] wavelengths, as the [[extinction (astronomy)|extinction]] caused by the rest of the cloud in which the star is forming is usually too big to allow us to observe it in the visual part of the spectrum. This presents considerable difficulties as the Earth's atmosphere is almost entirely opaque from 20μm to 850μm, with narrow windows at 200μm and 450μm. Even outside this range, atmospheric subtraction techniques must be used.

[[Image:NASA-FlameNebula-NGC2024-20140507.jpg|thumb|Young stars (purple) revealed by X-ray inside the [[Flame Nebula|NGC 2024]] star-forming region.<ref name= Getman14>{{Cite journal | last = Getman | first = K. | display-authors = etal | year= 2014| title = Core-Halo Age Gradients and Star Formation in the Orion Nebula and NGC 2024 Young Stellar Clusters | journal = Astrophysical Journal Supplement | volume = 787 | issue =2 | pages = 109 | doi = 10.1088/0004-637X/787/2/109| bibcode = 2014ApJ...787..109G|arxiv = 1403.2742 | s2cid = 118503957 }}</ref>]]
[[X-ray astronomy|X-ray]] observations have proven useful for studying young stars, since X-ray emission from these objects is about 100–100,000 times stronger than X-ray emission from main-sequence stars.<ref name=Preibisch05>{{Cite journal | last = Preibisch | first = T. | display-authors = etal | year= 2005| title = The Origin of T Tauri X-Ray Emission: New Insights from the Chandra Orion Ultradeep Project | journal = Astrophysical Journal Supplement | volume = 160 | issue =2 | pages = 401–422 | doi = 10.1086/432891| bibcode = 2005ApJS..160..401P|arxiv = astro-ph/0506526 | s2cid = 18155082 }}</ref> The earliest detections of X-rays from T Tauri stars were made by the [[Einstein Observatory|Einstein X-ray Observatory]].<ref name= Feigelson81>{{Cite journal | last1 = Feigelson | first1 = E. D. | last2 = Decampli| first2=W. M. | year= 1981| title = Observations of X-ray emission from T-Tauri stars | journal = Astrophysical Journal Letters | volume = 243 | pages = L89–L93 | doi = 10.1086/183449| bibcode = 1981ApJ...243L..89F}}</ref><ref name= Montmerle83>{{Cite journal | last = Montmerle | first = T. | display-authors = etal | year= 1983| title = Einstein observations of the Rho Ophiuchi dark cloud - an X-ray Christmas tree | journal = Astrophysical Journal, Part 1 | volume = 269 | pages = 182–201 | doi = 10.1086/161029| bibcode = 1983ApJ...269..182M}}</ref> For low-mass stars X-rays are generated by the heating of the stellar corona through [[magnetic reconnection]], while for high-mass [[O-type star|O]] and early B-type stars X-rays are generated through supersonic shocks in the stellar winds. Photons in the soft X-ray energy range covered by the [[Chandra X-ray Observatory]] and [[XMM-Newton]] may penetrate the interstellar medium with only moderate absorption due to gas, making the X-ray a useful wavelength for seeing the stellar populations within molecular clouds. X-ray emission as evidence of stellar youth makes this band particularly useful for performing censuses of stars in star-forming regions, given that not all young stars have infrared excesses.<ref name=feigelson13>{{Cite journal | last = Feigelson | first = E. D. | display-authors = etal | year= 2013| title = Overview of the Massive Young Star-Forming Complex Study in Infrared and X-Ray (MYStIX) Project | journal = Astrophysical Journal Supplement | volume = 209 | issue =2 | pages = 26 | doi = 10.1088/0067-0049/209/2/26| bibcode = 2013ApJS..209...26F|arxiv = 1309.4483 | s2cid = 56189137 }}</ref> X-ray observations have provided near-complete censuses of all stellar-mass objects in the [[Orion Nebula|Orion Nebula Cluster]] and [[Taurus Molecular Cloud]].<ref name=Getman05>{{Cite journal | last = Getman | first = K. V. | display-authors = etal | year= 2005| title = Chandra Orion Ultradeep Project: Observations and Source Lists | journal = Astrophysical Journal Supplement | volume = 160 | issue =2 | pages = 319–352 | doi = 10.1086/432092| bibcode = 2005ApJS..160..319G|arxiv = astro-ph/0410136 | s2cid = 19965900 }}</ref><ref name=Gudel07>{{Cite journal | last = Güdel | first = M. | display-authors = etal | year= 2007| title = The XMM-Newton extended survey of the Taurus molecular cloud (XEST) | journal = Astronomy and Astrophysics | volume = 468 | issue =2 | pages = 353–377 | doi = 10.1051/0004-6361:20065724| bibcode = 2007A&A...468..353G|arxiv = astro-ph/0609160 | s2cid = 8846597 }}</ref>

The formation of individual stars can only be directly observed in the [[Milky Way|Milky Way Galaxy]], but in distant galaxies star formation has been detected through its unique [[Gas chromatography–mass spectrometry|spectral signature]].

Initial research indicates star-forming clumps start as giant, dense areas in turbulent gas-rich matter in young galaxies, live about 500 million years, and may migrate to the center of a galaxy, creating the central bulge of a galaxy.<ref>{{Cite web|title = Young Star-Forming Clump in Deep Space Spotted for First Time|website = [[Space.com]]|date = 10 May 2015|url = http://www.space.com/29333-star-forming-clump-discovery.html|access-date = 2015-05-11}}</ref>

On February 21, 2014, [[NASA]] announced a [http://www.astrochem.org/pahdb/ greatly upgraded database] for tracking [[polycyclic aromatic hydrocarbon]]s (PAHs) in the [[universe]]. According to scientists, more than 20% of the [[carbon]] in the universe may be associated with PAHs, possible [[PAH world hypothesis|starting materials]] for the [[Abiogenesis#PAH world hypothesis|formation]] of [[Life#Extraterrestrial|life]]. PAHs seem to have been formed shortly after the [[Big Bang]], are widespread throughout the universe, and are associated with new stars and [[exoplanet]]s.<ref name="NASA-201409221">{{cite web |last=Hoover |first=Rachel |title=Need to Track Organic Nano-Particles Across the Universe? NASA's Got an App for That |url=http://www.nasa.gov/ames/need-to-track-organic-nano-particles-across-the-universe-nasas-got-an-app-for-that/ |date=February 21, 2014 |work=[[NASA]] |access-date=February 22, 2014 |archive-date=September 6, 2015 |archive-url=https://web.archive.org/web/20150906061428/http://www.nasa.gov/ames/need-to-track-organic-nano-particles-across-the-universe-nasas-got-an-app-for-that/ |url-status=dead }}</ref>

In February 2018, astronomers reported, for the first time, a signal of the [[reionization]] epoch, an indirect detection of light from the earliest stars formed - about 180 million years after the [[Big Bang]].<ref name="NAT-20180228">{{cite journal |last=Gibney |first=Elizabeth |title=Astronomers detect light from the Universe's first stars - Surprises in signal from cosmic dawn also hint at presence of dark matter. |url=https://www.nature.com/articles/d41586-018-02616-8 |date=February 28, 2018 |journal=[[Nature (journal)|Nature]] |access-date=February 28, 2018 |doi=10.1038/d41586-018-02616-8 }}</ref>

An article published on October 22, 2019, reported on the detection of [[3MM-1]], a massive star-forming galaxy about 12.5 billion light-years away that is obscured by clouds of [[cosmic dust|dust]].<ref name="WilliamsLabbe2019">{{cite journal|last1=Williams|first1=Christina C.|last2=Labbe|first2=Ivo|last3=Spilker|first3=Justin|last4=Stefanon|first4=Mauro|last5=Leja|first5=Joel|last6=Whitaker|first6=Katherine|last7=Bezanson|first7=Rachel|last8=Narayanan|first8=Desika|last9=Oesch|first9=Pascal|last10=Weiner|first10=Benjamin|title=Discovery of a Dark, Massive, ALMA-only Galaxy at z ∼ 5–6 in a Tiny 3 mm Survey|journal=The Astrophysical Journal|volume=884|issue=2|year=2019|pages=154|issn=1538-4357|doi=10.3847/1538-4357/ab44aa|arxiv=1905.11996|bibcode=2019ApJ...884..154W|s2cid=168169681 |doi-access=free }}</ref> At a mass of about 10<sup>10.8</sup> [[solar mass]]es, it showed a star formation rate about 100 times as high as in the [[Milky Way]].<ref name="UAnews">{{Cite web|url=https://uanews.arizona.edu/story/cosmic-yeti-dawn-universe-found-lurking-dust|title=Cosmic Yeti from the Dawn of the Universe Found Lurking in Dust|author=University of Arizona|website=UANews|date=22 October 2019|language=en|access-date=2019-10-22}}</ref>

===Notable pathfinder objects===
*[[MWC 349]] was first discovered in 1978, and is estimated to be only 1,000 years old.
*VLA 1623 – The first exemplar Class 0 protostar, a type of embedded protostar that has yet to accrete the majority of its mass. Found in 1993, is possibly younger than 10,000 years.<ref name="AndreWard-Thompson1993">{{cite journal |last1=Andre |first1=Philippe |last2=Ward-Thompson |first2=Derek |last3=Barsony |first3=Mary |title=Submillimeter continuum observations of Rho Ophiuchi A - The candidate protostar VLA 1623 and prestellar clumps |journal=The Astrophysical Journal |volume=406 |year=1993 |pages=122–141 |issn=0004-637X |doi=10.1086/172425 |bibcode=1993ApJ...406..122A|doi-access=free }}</ref>
*[[L1014]] – An extremely faint embedded object representative of a new class of sources that are only now being detected with the newest telescopes. Their status is still undetermined, they could be the youngest low-mass Class 0 protostars yet seen or even very low-mass evolved objects (like [[brown dwarf]]s or even [[rogue planet]]s).<ref>{{cite journal |last1=Bourke |first1=Tyler L. |last2=Crapsi |first2=Antonio |last3=Myers |first3=Philip C. |display-authors=etal |title=Discovery of a Low-Mass Bipolar Molecular Outflow from L1014-IRS with the Submillimeter Array |journal=The Astrophysical Journal |volume=633 |issue=2 |pages=L129 |year=2005 |doi=10.1086/498449 |arxiv = astro-ph/0509865 |bibcode = 2005ApJ...633L.129B |s2cid=14721548 }}</ref>
*[[GCIRS 8*]] – The youngest known [[main sequence]] star in the [[Galactic Center]] region, discovered in August 2006. It is estimated to be 3.5 million years old.<ref name="GeballeNajarro2006">{{cite journal |last1=Geballe |first1=T. R. |last2=Najarro |first2=F. |last3=Rigaut |first3=F. |last4=Roy |first4=J.-R. |title=The K-Band Spectrum of the Hot Star in IRS 8: An Outsider in the Galactic Center? |journal=The Astrophysical Journal |volume=652 |issue=1 |year=2006 |pages=370–375 |issn=0004-637X |doi=10.1086/507764 |bibcode=2006ApJ...652..370G |arxiv=astro-ph/0607550|s2cid=9998286 }}</ref>