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{{Short description|Emission nebula in Sagittarius}}
{{Use dmy dates|date=May 2024}}
{{Infobox nebula
| name = Omega Nebula
| image = VST image of the spectacular star-forming region Messier 17 (Omega Nebula).jpg
| caption = [[Very Large Telescope|VLT]] Survey Telescope image of the star-forming region Messier 17
| credit = [[ESO]]
| type = H II region
| epoch = [[J2000]]
| ra = {{RA|18|20|26}}<ref name="simbad">
{{Cite simbad
| title=NGC 6618
| access-date=16 November 2006
}}</ref>
| dec = {{DEC|−16|10|36}}<ref name="simbad" />
| dist_ly = 5,000–6,000
| appmag_v = 6.0<ref name="simbad" />
| size_v = 11 [[Minute of arc|arcmins]]
| constellation = [[Sagittarius (constellation)|Sagittarius]]
| radius_ly = 11
| absmag_v =
| notes =
| names = [[Messier object|M]]17, Swan Nebula,<ref name="simbad" /> [[Sharpless catalog|Sharpless]] 45, [[RCW Catalogue|RCW]] 160, Gum 81
}}

The '''Omega Nebula''' is an [[H II region]] in the [[constellation]] [[Sagittarius (constellation)|Sagittarius]]. It was discovered by [[Philippe Loys de Chéseaux]] in 1745. [[Charles Messier]] catalogued it in 1764. It is by some of the richest starfields of the [[Milky Way]], figuring in the northern two-thirds of Sagittarius. This feature is also known as the '''Swan Nebula''', '''Checkmark Nebula''', '''Lobster Nebula''', and the '''Horseshoe Nebula''',<ref name="simbad" /><ref name="psm1876" /> and catalogued as '''Messier 17''' or '''M17''' or '''NGC 6618'''.

== Characteristics ==
The Omega Nebula is between 5,000 and 6,000 [[light-year]]s from [[Earth]] and it spans some 15 light-years in diameter. The cloud of [[interstellar medium|interstellar matter]] of which this [[nebula]] is a part is roughly 40 light-years in diameter and has a mass of 30,000 solar masses.<ref name=Povich2009/> The total [[mass]] of the Omega Nebula is an estimated 800 [[solar mass]]es.<ref>{{Cite web|title=Messier 17|url=http://www.messier.seds.org/m/m017.html|publisher=[[SEDS]]|access-date=21 July 2024}}</ref>

It is considered one of the brightest and most massive star-forming regions of our galaxy.<ref name="Povich2009" /> Its local geometry is similar to the [[Orion Nebula]] except that it is viewed edge-on rather than face-on.<ref name=Broos2007>.
{{Cite journal
|last1=Broos |first1=P. S. |display-authors=4 |last2=Feigelson |first2=E. D. |last3=Townsley |first3=L.K. |last4=Getman |first4=K.V |last5=Wang |first5=J. |last6=Garmire |first6=G.P. |last7=Jhiang |first7=Z. |last8=Tsuboi |first8=Y.
|date=2007
|title=The Young Stellar Population in M17 Revealed by Chandra
|journal=The Astrophysical Journal Supplement Series
|volume=169 |issue=2 |pages=353–385
|bibcode=2007ApJS..169..353B
|doi=10.1086/512068
|arxiv = astro-ph/0612590 |s2cid=119348031 }}</ref>

The [[open cluster]] NGC 6618 lies embedded in the nebulosity and causes the [[gas]]es of the nebula to shine due to [[radiation]] from these hot, [[young star]]s; however, the actual number of stars in the nebula is much higher – up to 800, 100 of [[spectral type]] earlier than B9, and 9 of [[O-type main-sequence star|spectral type O]],{{Citation needed|date=December 2015}} plus over a thousand stars in formation on its outer regions.<ref name=Povich2009>
{{Cite journal
|last1=Povich |first1=M. S. |display-authors=4 |last2=Churchwell |first2=E. |last3=Bieging |first3=J.H. |last4=Kang |first4=M. |last5=Whitney |first5=B. A. |last6=Brogan |first6=C.A. |last7=Kulesa |first7=C.A. |last8=Cohen |first8=M. |last9=Babler |first9=B.L. |last10=Indebetouw |first10=R. |last11=Meade |first11=M. |last12=Robitaille |first12=T.P.
|date=2009
|title=The Extended Environment of M17: A Star Formation History
|journal=[[The Astrophysical Journal]] |volume=696 |issue=2 |pages=1278–1306
|bibcode=2009ApJ...696.1278P
|doi=10.1088/0004-637X/696/2/1278
|arxiv = 0902.3280 |s2cid=399050 }}</ref> It is also one of the youngest clusters known, with an age of just 1 million years.<ref name=Hanson1997>
{{Cite journal
|last1=Hanson |first1=M. M. |last2=Howarth |first2=I.D. |last3=Conti |first3=P.S.
|date=1997
|title=The Young Massive Stellar Objects of M17
|journal=The Astrophysical Journal
|volume=489 |issue=2 |pages=698–718
|bibcode=1997ApJ...489..698H
|doi=10.1086/304808
|s2cid=123249459 |doi-access=free }}</ref>

The [[luminous blue variable]] [[HD 168607]], in the south-east part of the nebula, is generally assumed to be associated with it; its close neighbor, the blue [[hypergiant]] [[HD 168625]], may be too.

The Swan portion of M17, the Omega Nebula in the [[Carina–Sagittarius Arm|Sagittarius]] [[nebula|nebulosity]] is said to resemble a [[barber's pole]].<ref>{{Cite book |author=Coe |first=Steven |url=https://books.google.com/books?id=roXyxpcc9MsC&pg=PA116 |title=Nebulae and How to Observe Them |publisher=Springer Science & Business Media |year=2007 |isbn=978-1-84628-729-9 |page=116}}</ref>

== Early research ==
The first attempt to accurately draw the nebula (as part of a series of sketches of nebulae) was made by [[John Herschel]] in 1833, and published in 1836. He described the nebula as such:<ref name="psm1876">{{Cite journal | title = The Horseshoe Nebula in Sagittarius | first = Edward S. | last = Holden | author-link = Edward Singleton Holden | journal = [[Popular Science]] | volume = 8 |date=January 1876 | pages = 269–281| title-link = s:The Horseshoe Nebula in Sagittarius }}</ref>

{{Cquote|The figure of this nebula is nearly that of a [[Greece|Greek]] capital [[omega]], Ω, somewhat [[Distortion (optics)|distorted]], and very unequally bright. ... Messier perceived only the bright eastern branch of the nebula now in question, without any of the attached convolutions which were first noticed by my father. The chief peculiarities which I have observed in it are – 1. The resolvable knot in the eastern portion of the bright branch, which is, in a considerable degree, insulated from the surrounding nebula; strongly suggesting the idea of an absorption of the nebulous matter; and, 2. The much feebler and smaller knot at the northwestern end of the same branch, where the nebula makes a sudden bend at an [[acute angle]].}}

A second, more detailed sketch was made during his visit to South Africa in 1837. The nebula was also studied by [[Johann von Lamont]] and separately by an undergraduate at [[Yale College]], Mr Mason, starting from around 1836. When Herschel published his 1837 sketch in 1847, he wrote:<ref name="psm1876" />
{{Cquote|In particular the large horseshoe-shaped arc … is there represented as too much elongated in a vertical direction and as bearing altogether too large a proportion to [the eastern] streak and to the total magnitude of the object. The nebulous diffusion, too, at the [western] end of that arc, forming the [western] angle and base-line of the capital Greek omega (Ω), to which the general figure of the nebula has been likened, is now so little conspicuous as to induce a suspicion that some real change may have taken place in the relative brightness of this portion compared with the rest of the nebula; seeing that a figure of it made on June 25, 1837, expresses no such diffusion, but represents the arc as breaking off before it even attains fully to the group of small stars at the [western] angle of the Omega. … Under these circumstances the arguments for a real change in the nebula might seem to have considerable weight. Nevertheless, they are weakened or destroyed by a contrary testimony entitled to much reliance. Mr. Mason ... expressly states that both the nebulous knots were well seen by himself and his coadjutor Mr. Smith on August 1, 1839, i.e., two years subsequent to the date of my last drawing. Neither Mr. Mason, however, nor any other observer, appears to have had the least suspicion of the existence of the fainter horseshoe arc attached to the [eastern] extremity of Messier's streak. Dr. Lamont has given a figure of this nebula, accompanied by a description. In this figure [our Fig. 4], the nebulous diffusion at the [western] angle and along the [western] base-line of the Omega is represented as very conspicuous; indeed, much more so than I can persuade myself it was his intention it should appear.}}

Sketches were also made by [[William Lassell]] in 1862 using his four-foot [[telescope]] at [[Malta]], and by M. Trouvelot from [[Cambridge, Massachusetts]], and [[Edward Singleton Holden]] in 1875 using the twenty-six inch Clark refractor at the [[United States Naval Observatory]].<ref name="psm1876" />

== Observations by SOFIA ==
[[File:Composite image of the Omega Nebula.jpg|thumb|Composite image of the Omega Nebula]]
In January 2020, the Stratospheric Observatory for Infrared Astronomy ([[Stratospheric Observatory for Infrared Astronomy|SOFIA]]) provided new insights into the Omega Nebula. SOFIA's composite image revealed that blue areas (20 microns) near the center indicate gas heated by massive stars, while green areas (37 microns) trace dust warmed by massive stars and newborn stars. Nine previously unseen protostars were discovered primarily in the southern regions. Red areas near the edges represent cold dust detected by the [[Herschel Space Telescope]] (70 microns), and the white star field was observed by the [[Spitzer Space Telescope]] (3.6 microns). These observations suggest that parts of the nebula formed separately, contributing to its distinctive swan-like shape.<ref>{{Cite web |title=SOFIA Reveals How the Swan Nebula Hatched |url=https://www.jpl.nasa.gov/images/pia23409-sofia-reveals-how-the-swan-nebula-hatched |access-date=2024-05-26 |website=NASA Jet Propulsion Laboratory (JPL) |date=January 7, 2020 |language=en-US}}{{source-attribution}}</ref>

== Gallery ==
<gallery class="center">
File:The VST captures three spectacular nebulae in one image.jpg|alt=Omega Nebula (left), Eagle Nebula (center), and Sharpless 2-54 (right).|Omega Nebula (left), Eagle Nebula (center), and Sharpless 2-54 (right)<ref>{{Cite web|title=VST Captures Three-In-One|url=https://www.eso.org/public/news/eso1719/|website=www.eso.org|access-date=16 June 2017}}</ref>
File:The star formation region Messier 17.jpg|Gas and dust clouds measure about 15 light-years across.
File:Omega Nebula.jpg|Section of the nebula
File:ESO-The Omega Nebula-phot-25a-09-fullres.jpg|Composite images obtained with the 3.58-metre NTT at La Silla Observatory
File:TPSMV8P284 Fig 1.jpg|A drawing of the nebula by [[John Herschel]] in 1833
File:TPSMV8P285 Fig 2.jpg|Herschel's second drawing in 1837
File:TPSMV8P294 Fig 7.jpg|A sketch by Trouvelot in 1875
File:A close looks at the star formation region Messier 17.webm|This video gives us a close-up view of the rose-coloured star forming region Messier 17.
</gallery>

== See also ==
* [[List of Messier objects]]
* [[Messier object]]
* [[New General Catalogue]]

==References==
{{Reflist}}

==External links==
{{Commons|Messier 17|Omega Nebula}}
* {{wikisource-inline|The Horseshoe Nebula in Sagittarius}}
* [http://www.messier.seds.org/m/m017.html Messier 17, SEDS Messier pages]
* [https://web.archive.org/web/20070930030807/http://www.spacetelescope.org/bin/images.pl?embargo=0&viewtype=standard&searchtype=freesearch&lang=en&string=M17 Omega Nebula at ESA/Hubble]{{WikiSky|name=The Omega Nebula}}
* [http://www.constellation-guide.com/omega-nebula-messier-17/ Omega Nebula (Messier 17) at Constellation Guide]

{{Messier objects}}
{{Sh2 objects}}
{{RCW}}
{{Gum catalogue}}
{{Ngc70}}

{{Portal bar|Astronomy|Stars|Outer space}}

{{Sky|18|20|26|-|16|10|36|6000}}

[[Category:Carina–Sagittarius Arm]]
[[Category:H II regions]]
[[Category:Messier objects]]
[[Category:NGC objects]]
[[Category:Sagittarius (constellation)]]
[[Category:Sharpless objects]]
[[Category:Astronomical objects discovered in 1745|?]]
[[Category:Articles containing video clips]]
[[Category:Star-forming regions]]

Omega Nebula - Revision history

imported>JPHC2003: Author name.