Editing Star formation (section)

===Interstellar clouds===

[[Spiral galaxy|Spiral galaxies]] like the Milky Way contain [[star]]s, [[stellar remnant]]s, and a diffuse [[interstellar medium]] (ISM) of gas and dust. The interstellar medium consists of 10<sup>4</sup> to 10<sup>6</sup> particles per cm<sup>3</sup>, and is typically composed of roughly 70% [[hydrogen]], 28% [[helium]], and 1.5% [[Metallicity|heavier elements]] by mass. The trace amounts of heavier elements were and are produced within stars via [[stellar nucleosynthesis]] and ejected as the stars pass beyond the end of their [[main sequence]] lifetime. Higher density regions of the interstellar medium form clouds, or ''[[diffuse nebulae]]'',<ref>{{cite web | first=C. R. | last=O'Dell | title=Nebula | work=World Book at NASA | url=http://www.nasa.gov/worldbook/nebula_worldbook.html | archive-url=https://web.archive.org/web/20050429002503/http://www.nasa.gov/worldbook/nebula_worldbook.html |  url-status=dead | archive-date=2005-04-29 | publisher=World Book, Inc. | access-date=2009-05-18 }}</ref> where star formation takes place.<ref name=prialnik>{{cite book
 | first=Dina | last=Prialnik | title=An Introduction to the Theory of Stellar Structure and Evolution
 | pages=195–212 | date=2000
 | publisher=Cambridge University Press | isbn=0-521-65065-8
 | no-pp=true }}</ref> In contrast to spiral galaxies, [[elliptical galaxy|elliptical galaxies]] lose the cold component{{definition needed|date=August 2023}} of its interstellar medium within roughly a billion years, which hinders the galaxy from forming diffuse nebulae except through  mergers with other galaxies.<ref>{{cite conference
 | author=Dupraz, C. | author2=Casoli, F. | author2-link = Fabienne Casoli
 | chapter=The Fate of the Molecular Gas from Mergers to Ellipticals | title=Dynamics of Galaxies and Their Molecular Cloud Distributions: Proceedings of the 146th Symposium of the International Astronomical Union
 | date=June 4–9, 1990 | location=Paris, France
 | publisher=Kluwer Academic Publishers
 | bibcode=1991IAUS..146..373D
}}</ref>

[[Image:Eagle nebula pillars.jpg|thumb|left|[[Hubble Space Telescope]] image known as ''[[Pillars of Creation]],'' where stars are forming in the [[Eagle Nebula]]]]
In the dense nebulae where stars are produced, much of the hydrogen is in the molecular (H<sub>2</sub>) form, so these nebulae are called [[molecular cloud]]s.<ref name=prialnik /> The [[Herschel Space Observatory]] has revealed that filaments, or elongated dense gas structures, are truly ubiquitous in molecular clouds and central to the star formation process. They fragment into gravitationally bound cores, most of which will evolve into stars. Continuous accretion of gas, geometrical bending{{Definition needed|date=August 2023}}<!-- Is this pure bending, for which there's an article? -->, and magnetic fields may control the detailed manner in which the filaments are fragmented. Observations of supercritical filaments have revealed quasi-periodic chains of dense cores with spacing comparable to the filament inner width, and embedded protostars with outflows.{{technical inline|date=August 2023}}<ref>{{cite journal |last1=Zhang |first1=Guo-Yin |last2=André |first2=Ph |last3=Men'shchikov |first3=A. |last4=Wang |first4=Ke |title=Fragmentation of star-forming filaments in the X-shaped nebula of the California molecular cloud |journal=Astronomy and Astrophysics |date=October 2020 |volume=642 |pages=A76 |doi=10.1051/0004-6361/202037721 |url=https://ui.adsabs.harvard.edu/abs/2020A%26A...642A..76Z/abstract |language=en |issn=0004-6361|arxiv=2002.05984 |bibcode=2020A&A...642A..76Z |s2cid=211126855 }}</ref>

Observations indicate that the coldest clouds tend to form low-mass stars, which are first observed via the infrared light they emit inside the clouds, and then as visible light when the clouds dissipate. Giant molecular clouds, which are generally warmer, produce stars of all masses.<ref>{{cite book | first=James | last=Lequeux | title=Birth, Evolution and Death of Stars | publisher=World Scientific | date=2013 | isbn=978-981-4508-77-3}}</ref> These giant molecular clouds have typical densities of 100 particles per cm<sup>3</sup>, diameters of {{convert|100|ly|km|lk=on}}, masses of up to 6&nbsp;million [[solar mass|solar masses ({{Solar mass}})]], or six million times the mass of Earth's sun.<ref>{{cite conference
 | author =Williams, J. P. | author2 =Blitz, L. | author3 =McKee, C. F.
 | chapter=The Structure and Evolution of Molecular Clouds: from Clumps to Cores to the IMF | page=97
 | title=Protostars and Planets IV | date=2000
 | bibcode=2000prpl.conf...97W
 | arxiv=astro-ph/9902246 }}</ref> The average interior temperature is {{convert|10|K|F|lk=on}}.

About half the total mass of the [[Milky Way]]'s galactic ISM is found in molecular clouds<ref>{{cite book
 | author=Alves, J. | author2=Lada, C. | author3=Lada, E.
 | chapter=Tracing H<sub>2</sub> Via Infrared Dust Extinction
 | title=Molecular hydrogen in space | date=2001 | publisher=Cambridge University Press 
| page=217 | isbn=0-521-78224-4
 }}</ref> and the galaxy includes an estimated 6,000 molecular clouds, each with more than {{Solar mass|100,000}}.<ref>{{cite journal
 | author=Sanders, D. B. | author2=Scoville, N. Z. | author3=Solomon, P. M.
 | title=Giant molecular clouds in the Galaxy. II – Characteristics of discrete features 
 | journal=Astrophysical Journal, Part 1
 | volume=289 | date=1985-02-01 | pages=373–387
 | doi=10.1086/162897 | bibcode=1985ApJ...289..373S
}}</ref> The nebula nearest to the [[Sun]] where massive stars are being formed is the [[Orion Nebula]], {{convert|1300|ly|km}} away.<ref>{{cite journal | doi=10.1086/520922
 | title=A Parallactic Distance of <math>389^{+24}_{-21}</math> Parsecs to the Orion Nebula Cluster from Very Long Baseline Array Observations
 | date=2007 | author=Sandstrom, Karin M.
 | journal=The Astrophysical Journal | volume=667 | issue=2
 | pages=1161 | bibcode=2007ApJ...667.1161S | arxiv=0706.2361
 | last2=Peek | first2=J. E. G. 
 | last3=Bower | first3=Geoffrey C. 
 | last4=Bolatto | first4=Alberto D.
 | last5=Plambeck | first5=Richard L.
 | s2cid=18192326 }}</ref> However, lower mass star formation is occurring about 400–450 light-years distant in the [[Rho Ophiuchi cloud complex|ρ Ophiuchi cloud complex]].<ref>{{cite book
 | author=Wilking, B. A. | author2=Gagné, M. | author3=Allen, L. E.|author3-link=Lori Allen (astronomer)
 | chapter=Star Formation in the ρ Ophiuchi Molecular Cloud
 | editor=Bo Reipurth | title=Handbook of Star Forming Regions, Volume II: The Southern Sky ASP Monograph Publications
 | arxiv=0811.0005 | bibcode=2008hsf2.book..351W | year=2008
 }}</ref>

A more compact site of star formation is the opaque clouds of dense gas and dust known as [[Bok globule]]s, so named after the astronomer [[Bart Bok]]. These can form in association with collapsing molecular clouds or possibly independently.<ref>{{cite journal
 | author=Khanzadyan, T. | author2=Smith, M. D.
 | author3=Gredel, R. | author4=Stanke, T.
 | author5=Davis, C. J. | doi=10.1051/0004-6361:20011531
 | title=Active star formation in the large Bok globule CB 34
 | journal=Astronomy and Astrophysics | volume=383 | issue=2
 | pages=502–518 |date=February 2002 | bibcode=2002A&A...383..502K| doi-access=free
 }}</ref> The Bok globules are typically up to a light-year across and contain a few [[solar mass|solar masses]].<ref>{{cite book
 | first=Lee | last=Hartmann | date=2000
 | title=Accretion Processes in Star Formation | page=4
 | publisher=Cambridge University Press | isbn=0-521-78520-0 }}</ref> They can be observed as dark clouds silhouetted against bright [[emission nebula]]e or background stars. Over half the known Bok globules have been found to contain newly forming stars.<ref>{{cite book | first=Michael David | last=Smith | date=2004
 | pages=43–44 | title=The Origin of Stars
 | publisher=Imperial College Press | isbn=1-86094-501-5 }}</ref>
[[File:ALMA witnesses assembly of galaxy in early Universe (annotated).jpg|thumb|Assembly of galaxy in early Universe.<ref>{{cite web|title=ALMA Witnesses Assembly of Galaxies in the Early Universe for the First Time|url=http://www.eso.org/public/news/eso1530/|access-date=23 July 2015}}</ref>]]