Omega Nebula

The Omega Nebula, also known as the Swan Nebula, Checkmark Nebula, and the Horseshoe Nebula[1][2] (catalogued as Messier 17 or M17 or NGC 6618) is an H II region in the constellation Sagittarius. It was discovered by Philippe Loys de Chéseaux in 1745. Charles Messier catalogued it in 1764. It is located in the rich starfields of the Sagittarius area of the Milky Way.

Omega Nebula
Emission nebula
H II region
ESO-The Omega Nebula-phot-25a-09-fullres
Composite images obtained with the 3.58-metre NTT at La Silla Observatory.
Credit: ESO
Observation data: J2000 epoch
Right ascension 18h 20m 26s[1]
Declination−16° 10′ 36″[1]
Distance5,000-6,000 ly
Apparent magnitude (V)+6.0[1]
Apparent dimensions (V)11 arcmins
Physical characteristics
Radius11 ly
DesignationsM17, Swan Nebula,[1] Sharpless 45, RCW 160, Gum 81


Omega Nebula M17
Picture of M17 captured in narrowband by amateur astronomer Chuck Ayoub

The Omega Nebula is between 5,000 and 6,000 light-years from Earth and it spans some 15 light-years in diameter. The cloud of 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.[3] The total mass of the Omega Nebula is an estimated 800 solar masses.[4]

It is considered one of the brightest and most massive star-forming regions of our galaxy.[3] Its local geometry is similar to the Orion Nebula except that it is viewed edge-on rather than face-on.[5]

The open cluster NGC 6618 lies embedded in the nebulosity and causes the gases of the nebula to shine due to radiation from these hot, young stars; 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 spectral type O, plus over a thousand stars in formation on its outer regions.[3] It is also one of the youngest clusters known, with an age of just 1 million years.[6]

The luminous blue variable HD 168607, located in the south-east part of the Omega 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 Sagittarius nebulosity is said to resemble a barber's pole.[7]

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:[2]

The figure of this nebula is nearly that of a Greek capital omega, Ω, somewhat 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:[2]

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.[2]


The VST captures three spectacular nebulae in one image

Omega Nebula (left), Eagle Nebula (center), and Sharpless 2-54 (right).[8]

The star formation region Messier 17

Gas and dust clouds measure about 15 light-years across.[9]

Omega Nebula

Section of the nebula.

VST image of the spectacular star-forming region Messier 17 (Omega Nebula)

First released VLT Survey Telescope image shows the star-forming region Messier 17.

TPSMV8P284 Fig 1

A drawing of the nebula by John Herschel in 1833.

TPSMV8P285 Fig 2

Herschel's second drawing in 1837.

TPSMV8P294 Fig 7

A sketch by Trouvelot in 1875.

This video gives us a close-up view of the rose-coloured star forming region Messier 17.

See also


  1. ^ a b c d e "NGC 6618". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2006-11-16.
  2. ^ a b c d Holden, Edward S. (January 1876). "The Horseshoe Nebula in Sagittarius" . Popular Science. 8: 269–281.
  3. ^ a b c Povich, M. S.; Churchwell, E.; Bieging, J.H.; Kang, M.; et al. (2009). "The Extended Environment of M17: A Star Formation History". The Astrophysical Journal. 696 (2): 1278–1306. arXiv:0902.3280. Bibcode:2009ApJ...696.1278P. doi:10.1088/0004-637X/696/2/1278.
  4. ^ "Messier 17". SEDS. 2007-08-13. Retrieved 2011-03-09.
  5. ^ . Broos, P. S.; Feigelson, E. D.; Townsley, L.K.; Getman, K.V; et al. (2007). "The Young Stellar Population in M17 Revealed by Chandra". The Astrophysical Journal Supplement Series. 169 (2): 353–385. arXiv:astro-ph/0612590. Bibcode:2007ApJS..169..353B. doi:10.1086/512068.
  6. ^ Hanson, M. M.; Howarth, I.D.; Conti, P.S. (1997). "The Young Massive Stellar Objects of M17". The Astrophysical Journal. 489 (2): 698–718. Bibcode:1997ApJ...489..698H. doi:10.1086/304808.
  7. ^ Steven Coe (2007). Nebulae and How to Observe Them. Springer Science & Business Media. p. 116. ISBN 978-1-84628-729-9.
  8. ^ "VST Captures Three-In-One". Retrieved 16 June 2017.
  9. ^ "A Cosmic Rose With Many Names". Retrieved 24 September 2015.

External links

Works related to The Horseshoe Nebula in Sagittarius at Wikisource

Coordinates: Sky map 18h 20m 26s, −16° 10′ 36″

1745 in science

The year 1745 in science and technology involved some significant events.

Bob Moses (musician)

Bob Moses (born January 28, 1948) is an American jazz drummer.

Carina–Sagittarius Arm

The Carina–Sagittarius Arm (also known as Sagittarius Arm or Sagittarius–Carina Arm, labeled -I) is generally thought to be a minor spiral arm of the Milky Way galaxy. Each spiral arm is a long, diffuse curving streamer of stars that radiates from the galactic center. These gigantic structures are often composed of billions of stars and thousands of gas clouds. The Carina–Sagittarius Arm is one of the most pronounced arms in our galaxy as a large number of HII regions, young stars and giant molecular clouds are concentrated in it.The Milky Way is a barred spiral galaxy, consisting of a central crossbar and bulge from which two major and several minor spiral arms radiate outwards. The Carina–Sagittarius Arm lies between two major spiral arms—the Scutum–Centaurus Arm the near part of which is visible looking inward i.e. toward the galactic centre with the rest beyond the galactic centre and the Perseus Arm, similar in size and shape but locally positioned outward. It is named for its proximity to the Sagittarius and Carina constellations as seen in the night sky from Earth, in the direction of the galactic center.

The arm dissipates near its middle, shortly after reaching its maximal angle, viewed from our solar system, from the galactic centre of about 80°. Extending from the galaxy's central bar is the Sagittarius Arm (Sagittarius bar). Beyond the dissipated zone it is the Carina Arm.

Graal (album)

Graal (stylized as Gr44l; Polish: Holy Grail, pronounced: [ɡral]) is the second album by Polish rapper Tau and the last released under the pseudonym Medium. It was released as a double album on November 20, 2012 by Asfalt Records. Recording took place during 2012 at Studio Łukasza Kowalskiego in Kielce with DJ Deszczu Strugi in charge of mixing the album. As with his previous album Teoria równoległych wszechświatów, production and arrangement were overseen by Medium himself.The album was promoted with two singles: "Hologram" features a guest appearance by American rapper Gift of Gab, and "Skwer pod słońcem", which was accompanied by a promotional video clip featuring Medium's fans. Graal also features guest appearances by O.S.T.R., Bisz and Te-Tris. The album peaked at number 18 on the Polish OLiS albums chart On the album Medium addresses questions of patriotism, his attachment to Christian values, and his relationship with God. The rapper also criticizes the issues of globalization, homosexuality and political correctness. Although it was well-received, Graal also met with criticism, mainly for its lyrics which were considered too radical by most reviewers and listeners.

Gum catalog

The Gum catalog is an astronomical catalog of 84 emission nebulae in the southern sky. It was made by the Australian astronomer Colin Stanley Gum (1924-1960) at Mount Stromlo Observatory using wide field photography. Gum published his findings in 1955 in a study entitled A study of diffuse southern H-alpha nebulae which presented a catalog of 84 nebulae or nebular complexes. Similar catalogs include the Sharpless catalog and the RCW catalog, and many of the Gum objects are repeated in these other catalogs.

The Gum Nebula is named for Gum, who discovered it as Gum 12; it is an emission nebula that can be found in the southern constellations Vela and Puppis.

HD 168607

HD 168607 (V4029 Sgr) is a blue hypergiant and luminous blue variable (LBV) star located in the constellation of Sagittarius, easy to see with amateur telescopes. It forms a pair with HD 168625, also a blue hypergiant and possible luminous blue variable, that can be seen at the south-east of M17, the Omega Nebula.

HD 168625

HD 168625 (V4030 Sagittarii) is a blue hypergiant and candidate luminous blue variable located in the constellation of Sagittarius easy to see with amateur telescopes. It forms a visual pair with the also blue hypergiant (and luminous blue variable) HD 168607 and is located to the south-east of M17, the Omega Nebula.

IRC -10414

IRC-10414 is a red supergiant and runaway star in the constellation Scutum, a rare case of a red supergiant with a bow shock.

Jack McDevitt

Jack McDevitt (born April 14, 1935) is an American science fiction author whose novels frequently deal with attempts to make contact with alien races, and with archaeology or xenoarchaeology. His two main series are the Alex Benedict series and the Priscilla Hutchins series.

McDevitt's first published story was "The Emerson Effect" in The Twilight Zone Magazine in 1981. Five years later, he published his first novel, The Hercules Text, about the discovery of an intelligently conceived signal whose repercussions threaten human civilization. This novel set the tone for many of McDevitt's following novels, which focused on making first contact. Frequently this theme is mixed with both trepidation before the unknown and a sense of wonder at the universe.

With The Engines of God (1994), McDevitt introduced the idea of a universe that was once teeming with intelligent life, but contains only their abandoned artifacts by the time humans arrive on the scene. Although it was initially written as a standalone novel, the main character of The Engines of God, pilot Priscilla Hutchins, has since appeared in seven more books, Deepsix (2001), Chindi (2002), Omega (2003), Odyssey (2006), Cauldron (2007), StarHawk (2013), and The Long Sunset (2018). The mystery surrounding the destructive "Omega Clouds" (which are introduced in The Engines of God) is left unexplored until Omega.McDevitt's novels frequently raise questions which he does not attempt to answer. He prefers to leave ambiguities to puzzle and intrigue his readers: "Some things are best left to the reader's very able imagination." The SF Site's Steven H Silver has written about this:

McDevitt has a (probably unintentional) tendency to give the impression that his novels will go in one direction and then take them in a different direction. Or possibly his background is so well thought out, that throw away lines, or subplots, or minor characters, have enough information behind them to make the reader want to see their story as much as the main plot of the book. While slightly annoying, this is, I've decided, a strength of McDevitt's writing since it shows the depth of his created worlds.

The novel Seeker won the 2006 Nebula Award for Best Novel, given by the Science Fiction and Fantasy Writers of America. He has been nominated for the Nebula Award sixteen times; Seeker is his only win.

List of diffuse nebulae

'This lists:

Diffuse nebula

Emission nebula

Reflection nebula


M17 or M-17 may refer to:


M17 road (Ireland)

M17 road (Bosnia and Herzegovina)

Highway M17 (Ukraine)

M-17 (Michigan highway)Aircraft

Myasishchev M-17, an early version of the Myasishchev M-55 reconnaissance aircraft

Miles M.17 Monarch, a 1936 British, light, touring aeroplane

M.17, a German World War 1 prototype aircraft, the basis for one of the two Fokker B.II (1916)Military equipment

M17 Half-Track, an anti-aircraft variant of the M5 Half-track

M17 rifle grenade used by the United States during World War II.

Mikulin M-17, a Soviet copy of a German aircraft engine

SIG Sauer M17 pistol, SIG Sauer P320 used by the United States armed forces

M17 gas mask, formerly used as a standard gas mask,for the United States Armed ForcesOtherHMS M17, First World War Royal Navy M15-class monitor

BMW M17, car engine in the M10 family

M17 agar, Lactococcus growth medium developed in 1971

M17 or Messier 17, a nebula also called the Omega Nebula, the Swan Nebula, or several other names

M17 or March 17, 2007 anti-war protest, organized by the ANSWER Coalition

Messier 18

Messier 18 or M18, also designated NGC 6613, is an open cluster of stars in the constellation Sagittarius. It was discovered by Charles Messier in 1764 and included in his list of comet-like objects. From the perspective of Earth, M18 is situated between the Omega Nebula (M17) and the Sagittarius Star Cloud (M24).This is a sparse cluster with a linear diameter of 8.04 pc, a tidal radius of 7.3 pc, and is centrally concentrated with core radius of 0.012 pc. It has a Trumpler class of II 3 p. The cluster is 33 million years old with an estimated mass of 188 M☉. It has one definite Be star and 29 B-type stars in total. There are three supergiant stars, all of class A or earlier. The brightest component (lucida), designated HD 168352, is a B-type giant star with a class of B2 III and a visual magnitude of 8.65.Messier 18 is 1,296 pc from the Earth and 6,830 pc from the Galactic Center. It is orbiting the Milky Way core with a period of 186.5 million years and an eccentricity of 0.02. This carries it to as close as 6.5 kpc to, and as far as 6.8 kpc from the galactic core. It passes vertically through the galactic plane once every 27.4 million years, ranging no more than 80 pc above or below.


A nebula (Latin for 'cloud' or 'fog'; pl. nebulae, nebulæ, or nebulas) is an interstellar cloud of dust, hydrogen, helium and other ionized gases. Originally, the term was used to describe any diffuse astronomical object, including galaxies beyond the Milky Way. The Andromeda Galaxy, for instance, was once referred to as the Andromeda Nebula (and spiral galaxies in general as "spiral nebulae") before the true nature of galaxies was confirmed in the early 20th century by Vesto Slipher, Edwin Hubble and others.

Most nebulae are of vast size; some are hundreds of light-years in diameter. A nebula that is barely visible to the human eye from Earth would appear larger, but no brighter, from close by. The Orion Nebula, the brightest nebula in the sky and occupying an area twice the diameter of the full Moon, can be viewed with the naked eye but was missed by early astronomers. Although denser than the space surrounding them, most nebulae are far less dense than any vacuum created on Earth – a nebular cloud the size of the Earth would have a total mass of only a few kilograms. Many nebulae are visible due to fluorescence caused by embedded hot stars, while others are so diffuse they can only be detected with long exposures and special filters. Some nebulae are variably illuminated by T Tauri variable stars.

Nebulae are often star-forming regions, such as in the "Pillars of Creation" in the Eagle Nebula. In these regions the formations of gas, dust, and other materials "clump" together to form denser regions, which attract further matter, and eventually will become dense enough to form stars. The remaining material is then believed to form planets and other planetary system objects.

Nebulae in fiction

Nebulae, often being visually interesting astronomical objects, are frequently used as settings or backdrops for works of science fiction.

RCW Catalogue

The RCW Catalogue (from Rodgers, Campbell & Whiteoak) is an astronomical catalogue of Hα-emission regions in the southern Milky Way, described in (Rodgers et al. 1960). It contains 182 objects, including many of the earlier Gum catalogue (84 items) objects.

The later Caldwell catalogue included some objects from the RCW catalogue. There is also some overlap with the Sharpless catalogue-2 (312 items), although that primarily covered the northern hemisphere, whereas RCW and Gum primarily covered the southern hemisphere.

The RCW catalogue was compiled by Alexander William Rodgers, Colin T. Campbell and John Bartlett Whiteoak. They catalogued southern nebulae while working under Bart Bok at the Mount Stromlo Observatory in Australia in the 1960s.

Sagittarius (constellation)

Sagittarius is one of the constellations of the zodiac. It is one of the 48 constellations listed by the 2nd-century astronomer Ptolemy and remains one of the 88 modern constellations. Its name is Latin for the archer, and its symbol is (Unicode U+2650 ♐), a stylized arrow. Sagittarius is commonly represented as a centaur pulling back a bow. It lies between Scorpius and Ophiuchus to the west and Capricornus and Microscopium to the east.

The center of the Milky Way lies in the westernmost part of Sagittarius (see Sagittarius A).

Sagittarius Star Cloud

The Sagittarius Star Cloud (also known as Messier 24 and IC 4715) is a star cloud in the constellation of Sagittarius, approximately 600 light years wide, which was discovered by Charles Messier in 1764. It is sometimes known as the Small Sagittarius Star Cloud to distinguish it from the Great Sagittarius Star Cloud located to the north of Gamma Sagittarii and Delta Sagittarii.The stars, clusters and other objects comprising M24 are part of the Sagittarius or Sagittarius-Carina arms of the Milky Way galaxy. Messier described M24 as a "large nebulosity containing many stars" and gave its dimensions as being some 1.5° across. Some sources, improperly, identify M24 as the faint cluster NGC 6603. The location of the Sagittarius Star Cloud is near the Omega Nebula (also known as M17) and open cluster Messier 18, north of M24. M24 is one of only three Messier objects that are not deep sky objects.M24 fills a space of significant volume to a depth of 10,000 to 16,000 light-years. This is the most dense concentration of individual stars visible using binoculars, with around 1,000 stars visible within a single field of view. The star cloud can be seen visible when the Milky Way itself is visible as well. Without the dust and gas that conceals the Milky Way, M24 holds a collection of numerous kinds of stars that are placed along the Milky Way and through the galaxy's obscuring band of interstellar dust.HD 167356 is the brightest star that is located within the Sagittarius Star Cloud, a white supergiant with an apparent magnitude of 6.05. This star is an Alpha-2 Canum Venaticorum variable, showing small changes in brightness as it rotates. There are three other stars located in M24 with visual magnitudes between 6.5 and 7.0.The star cloud incorporates two prominent dark nebulae which are vast clouds of dense, obscuring interstellar dust. This dust blocks light from the more distant stars, which keeps them from being seen from Earth. Lying on the northwestern side is Barnard 92, which is the darkest out of the two. Inside the field filled with stars, the nebula appears as an immense round hole that is devoid of any stars. American astronomer Edward Emerson Barnard discovered this dark nebula in 1913. Along the northeast side contains Barnard 93, which is less obvious and large as the other dark nebula. There are also other dark nebulae within M24, including Barnard 304 and Barnard 307. Both Barnard 92 and 93 have the most significant features shown in M24 due to them both blocking out several stars and being the most visually prominent.The Sagittarius Star Cloud also holds two planetary nebulae, M 1-43 and NGC 6567. Located within a spiral arm of the Milky Way, Messier 24 holds some similarities with NGC 206, a bright, large star cloud.


Sh2-54 is an extended bright nebula in the constellation of Serpens.In its core there are many protostars and many infrared sources; some of these sources, like IRAS 18151−1208, are most probably very young high-mass stars. The older star population in this region has an average age of 4-5 millions years, and its components are grouped in the open cluster NGC 6604.Sh2-54 belongs to an extended nebulosity that includes also the Eagle Nebula and the Omega Nebula. The young high-mass stars of this region constitute the Serpens OB1 and Serpens OB2 OB association.

Sharpless catalog

The Sharpless catalog is a list of 313 HII regions (emission nebulae), intended to be comprehensive north of declination −27°. (It does include some nebulae south of that declination as well.) The first edition was published in 1953 with 142 objects (Sh1), and the second and final version was published by US astronomer Stewart Sharpless in 1959 with 312 objects. Sharpless also includes some planetary nebulae and supernova remnants, in addition to HII regions.In 1953 Stewart Sharpless joined the staff of the United States Naval Observatory Flagstaff Station, where he surveyed and cataloged H II regions of the Milky Way using the images from the Palomar Sky Survey. From this work Sharpless published his catalog of H II regions in two editions, the first in 1953 with 142 nebula. The second and final edition was published in 1959 with 312 nebulae.Sharpless coordinates are based on the star catalogs Bonner Durchmusterung (BD) and Cordoba Durchmusterung (CD), but the second release was adjusted to the 1900 epoch.In the second release, some coordinates for southern hemisphere regions have an uncertainty over 1 minute of arc. This can make them difficult to find, so a revised catalog called BFS (Blitz, Fich and Stark) was released. BFS has 65 new regions and about 20 removals. Most of the removed items were taken out because they were the aforementioned nebula or remnants.The 312 items in Sharpless sometimes overlap with the 110 Messier objects (M), 7,840 objects in the New General Catalogue (NGC), the Caldwell catalogue (that itself is a "best of" from other catalogues, with 109 items), and the RCW catalog. Contemporary catalogs were Gum and RCW, but they mainly covered the southern hemisphere.

See also

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