Star formation

2016-11-02T11:37:58Z

117.200.160.191: /* W75N(B)-VLA2 */ time to times

[[File:LH 95.jpg|thumb|right|250px|[[LH 95]] stellar nursery in [[Large Magellanic Cloud]]]]
[[File:Star-forming region.jpg|thumb|right|250px|Stellar cluster and [[Stellar nursery|star-forming region]] [[Messier 17|M 17]], the [[Omega nebula]]]]

'''Star formation''' means the earliest stages in a star's life. Dense regions of particles exist in [[molecular cloud]]s known as "stellar nurseries". The clouds collapse under [[gravitation]] into spheres of [[Plasma (physics)|plasma]] to form [[star]]s.

==Stellar nurseries==
[[Image:Eagle nebula pillars.jpg|thumb|230px|[[Hubble telescope]] image known as ''[[Pillars of Creation]]'' where stars are forming in the [[Eagle nebula]]]]

High density regions of the [[interstellar medium]] (ISM) form clouds, or ''[[nebula|nebulae]]'', where star formation takes place.<ref name=prialnik>{{cite book| last=Prialnik, Dina 2000. | title=An introduction to the theory of stellar structure and evolution| pages=195–212 | publisher=Cambridge University Press | isbn=0-521-65065-8| nopp=true }}</ref>

=== Dense nebulae ===
In the dense nebulae where stars are produced, much of the hydrogen is in the molecular (H2) form, so these nebulae are called [[molecular cloud]]s.<ref name=prialnik /> The largest such formations, giant molecular clouds, have densities of 100 particles per cm3, diameters of {{convert|100|ly|km|lk=on}}, and masses up to 6 million [[solar mass]]es,<ref>Williams J.P; Blitz L. & McKee C F. 2000. The structure and evolution of molecular clouds: from clumps to cores to the IMF. In ''Protostars and Planets IV'', 97.</ref> and an average interior temperature of 10 K. About half the total mass of the galactic ISM is found in molecular clouds.<ref>{{cite book| author=Alves J; Lada C. & Lad, E. 2001.| title=Tracing H2 Via infrared dust extinction| booktitle=Molecular hydrogen in space| publisher=Cambridge University Press | page=217| isbn=0-521-78224-4 }}</ref> In the [[Milky Way]] there are about 6,000 molecular clouds, each with more than 100,000 solar masses.<ref>{{cite journal| author=Sanders D.B.; Scoville N.Z. & Solomon P M. 1985.| title=Giant molecular clouds in the galaxy. II – Characteristics of discrete features | journal=Astrophysical Journal, Part 1| volume=289 | pages=373–387| doi=10.1086/162897 | bibcode=1985ApJ...289..373S}}</ref> The nearest nebula to the [[Sun]] where massive stars are being formed is the [[Orion nebula]], {{convert|1300|ly|km|abbr=on}} 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| year=2007 | author=Sandstrom, Karin M.| journal=The Astrophysical Journal | volume=667| pages=1161 | bibcode=2007ApJ...667.1161S | arxiv=0706.2361}}</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.; Gagné M. & Allen L E. 2008. Star formation in the ρ Ophiuchi molecular cloud| editor= In Bo Reipurth | title=Handbook of star forming regions, Volume II: The Southern Sky ASP Monograph Publications| arxiv=0811.0005| bibcode=2008hsf2.book..351W}}</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.<ref>{{cite journal| author=Khanzadyan T. ''et al'' 2002. Active star formation in the large Bok globule CB 34 | doi=10.1051/0004-6361:20011531| journal=Astronomy and Astrophysics | volume=383| issue=2| pages=502–518 | bibcode=2002A&A...383..502K}}</ref> The Bok globules are typically up to a light year across and contain a few [[solar mass]]es.<ref>{{cite book| first=Lee | last=Hartmann | year=2000| title=Accretion processes in star formation | page=4| publisher=Cambridge University Press | isbn=0-521-78520-0 }}</ref> They can be seen as dark clouds 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 | year=2004| page=43–44 | title=The origin of stars| publisher=Imperial College Press | isbn=1-86094-501-5 }}</ref>

=== Spiral galaxies ===
A [[spiral galaxy]] like the [[Milky Way]] contains [[star]]s, the remains of stars (stellar remnants) and a diffuse [[interstellar medium]] (ISM) of gas and dust. The interstellar medium consists of 10–4 to 106 particles per cm3 usually of about 70% [[hydrogen]] by mass, with most of the remaining gas consisting of [[helium]]. This medium is chemically enriched by trace amounts of heavier elements that were ejected from stars as they passed beyond the end of their [[main sequence]] lifetime.

=== Elliptical galaxies ===
In contrast to spirals, an [[elliptical galaxy]] loses the cold component of its interstellar medium within about a [[billion]] years. This hinders the galaxy from forming diffuse nebulae except through mergers with other galaxies.<ref>Dupraz C. & Casoli F. 1990. The fate of the molecular gas from mergers to ellipticals. In ''Dynamics of galaxies and their molecular cloud distributions''. Proceedings of the 146th Symposium of the International Astronomical Union, Kluwer.</ref>

== W75N(B)-VLA2 ==
This is a massive [[protostar]] about 300 times brighter than our [[Sun]]. Astronomers have caught a key stage in the birth of a very heavy star. They used two [[radio telescope]] views of the process 18 years apart. The young star is 4,200 [[light-years]] from Earth and is surrounded by a doughnut-shaped cloud of dust.

The first observation was in 1996. By 2015 its [[stellar wind]] changed from a compact [[spherical]] form to a larger, [[ionization|ionized]] [[ellipse|elliptical]] shape. This gave information about what happens as a massive star forms.<ref>Carrasco-Gonzalez C. ''et al'' 2015. Observing the onset of outflow collimation in a massive protostar. ''Science'' '''348''' (6230) p114. [http://www.sciencemag.org/content/348/6230/114]</ref><ref>[http://www.bbc.co.uk/news/science-environment-32168507 BBC News: Star's birth glimpsed 'in real time', 3 April 2015]</ref> Being able to observe its rapid growth is unique, according to Huib van Langevelde of [[Leiden University]], one of the authors of a study of the [[Astronomical object|object]].

== References ==
{{Reflist}}

[[Category:Stars| ]]

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Star formation