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NGC 6618 (15,101 of 18,816)


oc gc pln bn dn gx gxcl ast aka lost




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Swan Nebula

NGC 6618, Ced 161, LBN 60, Sh 2-45, Horseshoe Nebula, Lobster Nebula, Omega Nebula, Checkmark Nebula, Messier 17, Bennett 108, Swan Nebula, h 2008, GC 4403

RA: 18h 20m 47.1s
Dec: −16° 10′ 18″

Con: Sagittarius
Ch: MSA:1367, U2:294, SA:15


(reference key)

Type: bright nebula

Mag: B=?, V=?

Size: 20′ x 15′
PA: ?

Image gallery

Sketches  (4)

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Photos  (14)

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Historical observations

de Cheseaux

Discovered by de Cheseaux in 1764, he described it as "A nebula which has never been discovered; it has a shape quite different from the others; it has the perfect form of a ray or the tail of a comet, 7' long and 2' wide. Its sides are exactly parallel and well terminated. The centre is whiter than the edges.. It makes an angle of 30 degree with the meridian." A few weeks later it was seen by Messier,

Messier, Charles

A few weeks after being spotted by de Cheseaux, it was independently discovered by Messier (1764 June). He described it as "a train of light without stars, 5' or 6' in extent, in the shape of a spindle, a little like that in Andromeda's belt, but the light is very faint. In a good sky, seen very well with a 3.5 foot telescope..."

William Herschel (c.1784)

William Herschel observed it in 1784 with his newly completed 18.7-inch f/13 speculum telescope. He called it "a wonderful, extensive Nebulosity of the milky kind. There are several stars visible in it, but they can have no connection with that nebulosity and are, doubtless, belonging to our own system scattered before it." In the Appendix to the 1912 'Scientific Papers of Sir William Herschel' this object is described as "1783, July 31, 20 feet telescope, a very singular nebula; it seems to be the link to join the nebula in Orion to others, for this is not without a possibility of being stars. I think a great deal more of light and a much higher power would be of service. 1784, June 22. A wonderful nebula. Very much extended, with a hook on the preceding side; the nebulosity of hte milky kind; several stars visible in it, but the yseem to have no connection with the nebula, which is far more distant. I saw it only through short intervals of flying clouds and haziness; but the extent of the light including the hook is above10' I suspect besides, that on the following side it goes on much father and diffuses itself towards the north and south. It is not of equal brightness throughout, and has one or more places where the milky nebulosity seems to degenerate into the resolvable kind; such a one is that just following the hook towards the north. Should this be confirmed on a very fine night, it would bring on the step between these two nebulosities which is at present wanting and would lead us to surmise that this nebula is a stupendous stratum of immensely distant fixed stars, some f whose branches come near enough to us to be visible as resolvable nebulosity, while the rest runs on to so great a distance as only to appear under the milky form."

Lassell, W. (1866)

Bibcode: [1866MmRAS..36....1L]

Sketched and described.

Admiral Smythe (1884)

Admiral Smythe described it as "a magnificent arched and irresolvable luminosity occupying more than a third of the area in a splendid group of stars, principally from 9 to 11 magnitude, reaching more or less all over the field..."

Barnard, E.E. (1908)

See: Barnard, E.E. (1908) Some notes on nebulae and nebulosities. Astr. Nachr., No.4239.


Published comments

Bailey, S.I. (1908)

"! nebula, irregular, mottled, nuclei; the Horse Shoe, or Omega, nebula."

Bailey, S.I. (1908) A catalogue of bright stars and nebulae. Ann.Harv.Coll.Obs., 60(8), 199.

Remarks, p.218: "the spectrum is gaseous."

Duncan, J.C. (1920)

Duncan, J. C. (1920) Bright nebulae and star clusters in Sagittarius and Scutum photographed with the 60-inch reflector. Astrophys. J., 51, 4-12.

Doig, P. (1925)

Doig, P. (1925) Notes on the nebulae and clusters in Webb's 'Celestial Objects for Common Telescopes' (Sixth edition, Vol.ii). Part IV. M.N.R.A.S., 36(2), 58.

Doig, P. (1926)

Doig, P. (1926) "A Catalogue of Estimated Parallaxes of 112 Nebulae, Open clusters and Star Groups", Vol 36 (4), p 107-115.

"nebulosioty and stars." He gives the approx. diameter as 26x20 arcmin.

Vorontsov-Velyaminov, B. (1925/1926)

Vorontsov-Velyaminov, B. "Integral magnitudes of south star clusters", Astron. Nach. 228, 325. Comparing the brightness of the cluster with the extrafocal images of stars, he estimates the magnitudes as 7.55.

Cederblad, S. (1946) [VII/231]

Ced 161 (NGC 6618)

Position (1900): RA 18 15, Dec - 16 13

Star: Cl (Mp=7.7:, SpT=B0, B)

Spectrum of nebula: emission spectrum (observed)

Classification: Nebulous cluster (Nebulous envelop of intricate structure, eg. NGC 2175)

Size: 46'x37'

Notes: "NGC 6618 = GC 4403 = h 2008 = M 17 = "The Omega nebula" = "The Horseshoe nebula") = "The Swan nebula". Disc. de Cheseaux about 1748 (119). FA 86. (30, 74, 85, 88 Pl 55, 93 Pl 31 Pl 34, 114, 158, 174, 191, 194, 216, 228, 234, 260, 349, 482, 486, 520, 573, 578, 603 Pl 38, 630 Pl 6 Pl 7 Pl 8, 631, 674, 791). R. The brightest part of the nebula has also been catalogued as -16 4820 = HD 168520. HD stars within the nebula : -16 4817 = HD 168486. -16 4821 = HD 168521. -16 4827 = HD 168569. -16 4828 = HD 168585. -16 4836 = HD 168701/2. The two brightline stars -16 4829 = HD 168607 = Boss 25035, and -16 4830 = HD 168625 = Boss 25037 are situated in the immediate vicinity of the nebula, and they may be partly responsible for the nebular light."

Sharpless (1959)

A catalogue of H-II regions. Astrophys.J.Suppl.Ser., 4, 257-279.

"M17. Two detached portions apparently separated by foreground dark lane."

Sulentic & Tifft (1973)

The RNGC (Sulentic and Tifft 1973) notes that this is a 7.0 mag cluster associated with nebulosity.

Burnham's Celestial Handbook

Burnham notes that the most prominent feature of the nebula is "the long bright comet-like streak across the north edge; on the west end a curved hook gives the nebula a resemblance to a ghostly figure 2, with the bright streak forming the base. It requires only the slightest use of the imagination to transform this pattern into the graceful figure of a celestial swan floating in a pool of stars..." Burnham notes that Sir William Herschel saw the swan's heads as the curving top of the Greek Omega, while other observers have seen it as a horseshoe or a two hooked bar of nebulosity.

Photo index

Photo Index by Jim Lucyk: Deep Sky #3 Su83 p10, Deep Sky Monthly 8/80 p12.

Modern observations

Whitman, A. (1998)

Seeking summer's dark nebulae. Sky&Telescope, Aug, 114-118.

Walter Scott Houston

In 1978 Houston wrote: "This nebula covers an irregular area about 46' by 37' just north of the small starcloud in Sagittarius. M17 can be seen in most finder scopes, and it becomes a marvelous object in a 10-inch at 80x.

Bushnall, Darren

Darren Bushnall (Hartlepool, Cleveland) observing with a 8.5-inch f/6, writes in the The Webb Society Nebulae and Clusters Section Report No. 11, January 1993: "Visible as a bright V-shape of nebulosity at low power. At medium power, dark irregular patches seen at the centre. The UHC filter dramatically enhances it, the H-beta filter kills it."

Harrington, Phil (1990)

Harrington, P. (1990) An observer's guide to diffuse nebulae � II. Sky&Telescope, July, 97.

Tom Lorenzin

Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "6M; 35' x 45' extent; impressive "SWAN" swimming WNW-ward, head to the S; 11M stars at neck and beak; N-filter yields mottled appearance; brightest portion is wing folded over back; a.k.a. "OMEGA" and "CHECK-MARK" and "DUCK" (Tectron's Tom Clark); rivals M-42 for detail; very faint nebulosity (the swan's wake?) extends to E."

Ware, Donald J

"The Omega or Swan Nebula. Probably my personal favorite example of nebulosity, M-17 is seen as a curving check mark relatively bright nebulosity, with many dark mottlings seen, especially along its straighter portion. As with most astronomical objects, the longer this nebula is studied, the more detail becomes apparent. A UHC filter is helpful, but definitely not necessary with this celestial gem."

Mitsky, Dave (IAAC)

(IAAC) Obj: M17 (NGC 6618) - Inst: 12.5" f/6.5 Cave equatorial Newtonian


Observer: Dave Mitsky

Your skills: Intermediate (some years)

Date/time of observation: 6/18/98 05:40 UT

Location of site: Naylor Observatory http://www.msd.org/obs.htm (Lat 40.1d N, 76.9d W, Elev 570')

Site classification: Exurban

Sky darkness: ~5.0 Limiting magnitude

Seeing: 8 1-10 Seeing Scale (10 best)

Moon presence: None - moon not in sky

Instrument: 12.5" f/6.5 Cave equatorial Newtonian

Magnification: 65x, 83x

Filter(s): O-III

Object(s): M17 (NGC 6618)

Category: Emission nebula.

Constellation: Sagittarius

Data: mag 6.0 size 46.0' x 37.0'

Position: RA 18:20.8 DEC -16:11


M17 has many nicknames: the Checkmark Nebula, the Horseshoe Nebula, Omega Nebula, and the Swan Nebula. It is one of the finest emission nebulae in the heavens. An open cluster that is comprised of 30 to 40 stars is associated with the nebula.

M17 is some 5900 light years away and is about 75 light years in size.

An O-III filter improved the view of M17 and increased somewhat the amount of outlying nebulosity seen at 65x. M17 is an outstanding object in almost any instrument. To me it most closely resembles an elongated number 2, a checkmark, or a swan.


Shaffer, Alan (IAAC)

Observer: Alan Shaffer (e-mail: milkyway@gte.net, web: http://www.geocities.com/CapeCanaveral/3693/)

Instrument: 25-inch Dobsonian reflector Location: Mt. Pinos, California, US

Light pollution: none Transparency: excellent Seeing: excellent

Time: Sat Jun 7 10:00:00 1997 UT Obs. no.: 144

The Swan was my next target at Mt. Pinos. Again, I used the O-III filter and a low power. Throught the 25" and the 10"

SCT, it looked as if I were on top of the nebula. Very bright but void of any color. I obsered it for @ 20 min. and picked out

a lot of detal. This nebula was picked up in my 50mm exposure of the Sagittarius constelation. Very nice.

Callender, John

Observer: John Callender

Instrument: 8-inch Dobsonian reflector Location: Carpinteria, CA, USA

Light pollution: light Transparency: good Seeing: fair

Time: Sun Jun 29 10:30:00 1997 UT Obs. no.: 172

The Omega (Swan) Nebula jumped out at me as I did a quick sweep through the summer Milky Way. It still startles (and pleases) me how my new 8-inch Dobsonian (the biggest telescope I've ever owned), when used in the fairly dark skies around here, allows me to really SEE such objects, and recognize their appearance from photographs, rather than merely detecting them as subliminal fuzzy patches, which was about all I could hope with my binoculars or 2.4-inch refractor from my former light-polluted home in Manhattan Beach. Anyway, the Swan was beautiful! I definitely need to come back to this region soon, and perform a detailed exploration. Perhaps a summer "bug hunt," to go with the winter one I performed back in February.

Steve Coe

Steve Coe, observing with a 13" f/5.6, notes: "(M 17) Very bright, large, irregularly elongated at 100X. This nebula has received a variety of names over the years. The Swan section is the brightest and most obvious section on the south side. Adding the UHC filter brings out a dimmer portion of the nebula to the northeast. The addition of this fainter part makes the entire nebula take on the shape of a Horseshoe. Putting in the UHC also shows off the fact that thin dark lanes cut across the bright nebula in several places, the filter raises the contrast between the bright and dark areas. M 17 is one of my favorites, there is lots of detail to see at a variety of power and filter combinations. It is just naked eye on a good night and the finder or a pair of binoculars will show the nebula afloat in a rich Milky Way field."

Contemporary observations

Auke Slotegraaf


With a 2-inch refractor, M 17 shows as a straight-edged nebulosity broadening towards its western extent. No hook feature is seen. Just northeast of the nebula, in the same low-power field, is a loose collection of faint stars with about 6 brighter members. This is the associated open cluster, Collinder 377, which is very loose, moderately rich (40 members) and contains both bright and faint stars. The cluster does not, however, appear as closely linked with the nebulosity as does, say, NGC 6530 with the Lagoon Nebula.


In a 15.5-inch at 220X, this beautiful nebula shows clearly as a check mark, with the long part of the tick more prominent than the shorter arm. To the north-east of the longer bar lies a collection of stars forming a loose open cluster. Ensconced within the short bar are two bright stars, and from this bar extends a fainter curved-back bough of nebulous matter, ending near two bright stars.

1995 May 30

1995-05-30: 11x80.Technopark. 23:00 SAST. Hazy sky. Orange star to the north-west. The nebula appears as a V-shaped nebulous wedge with a feeble open cluster attached.

1997 July 7

1997 July 7, Monday, 21:00 - 24:00 Jonkershoek. 11x80's tripod-mounted. Wonderful! Generally, a rectaingular area of nebulosity with cluster, well separated. The tick shape is clearly seen, and the long bar of the tick is sharply straight defined. Two dark lanes, /, cordon the cluster/neb. The tick runs NW-SE.

1998 July 31

Location: Jonkershoek Nature Reserve, Assegaaibosch Station

Date: 1998 July 31 / August 01, 01:00-02:40 SAST

11x80 tripod-mounted binoculars (9.5 mag stars at times not easy)

Sky conditions: Mediocre (transp. low, seeing average, dew) The skies are showing the effects of the combination of pollution (mainly from a nearby wood-processing plant) and a stable inversion layer, turning daytime skies grey-blue, and night skies ashen.

Wow! A narrow triangular wedge of nebulosity, almost oriented east-west, with a mottled starpatch just to the north. Beautiful.

Magda Streicher

1997 August 05

12-inch Meade, 40mm eyepiece, 53' fov. 1997-08-05, fair sky conditions "A beautiful bright gaseous nebula-cloud stretches irregularly in haziness. Brighter towards the middle and in a fine starfield. The whole structure and extended dark area with pin-point stars form a combination and resembles a goose-like figure to me." [Magda Streicher, Pietersburg 23-53S, 29-28E]

(no date)

12-inch f/10 SCT (EP: 2-inch 40mm SW 76x 53' fov; 2-inch 14mm UW 218x 23' fov)

A beautiful bright gaseous nebula-cloud stretches irregularly long and in a somewhat tri-angle southeast to northwest embedded in haziness. The whole structure, together with the extended dark area with pinpoint loose stars, forms a combination and resembles a goose-like figure to me. Even though I know this cluster and nebula well, there is always new facets that can be investigated. With higher power (218x) some dark blotches come alive. The broadest part is towards the north and is well defined against the background. The small but beautiful star group meets towards the west end of the goose, right in front of its face.

Richard Ford

2015, June, 21st



Sky Conditions:The fainter parts of the Milky Way are barely visible.Haziness only visible on the horizon.Atmosphere stable with little interference.

Instrument:12-inch Dobsonian.

This nebula has the shape of a swan swimming on a starry lake seen at both 57x and 75x.This nebula glows like a bright mist in the night sky with some stars in the region which range in brightness from 10th to 11th magnitude seen loosely in the dark patches of this nebula.In overall there are plenty of areas of dark lanes been noticed around this nebula.This nebula measures 4.7'x 3.3'.Chart No.317,NSOG Vol.2.

2010 May,15 Saturday


Instrument:12"Dobsonian Reflector Telescope.

Sky Conditions:The fainter parts of the Milky Way are barely visible.

Transparency of the Sky:Haziness only visible on the horizon.

Seeing:Atmosphere stable with little interference.

Limiting Magnitude:4.9.

First Impression:Nebula.



Chart Number:No.12(Extract taken out of "Atlas of the Night Sky").

Size:26mm Eyepiece:Field Of View:57'/4=14.2'

20mm Eyepiece:Field Of View:50'/3.5=14.2'.



Size in Arc Minutes:14.2'.

Brightness:Extremely Bright.

Brightness Profile:High Surface Brightness.

Challenge Rating:It is a fantastic sight to observe in a large telescope under very dark skies.



This nebula has the appearance of a swan swimming on a lake with bright stars radiating away from this nebula.In this nebula,I have found areas of uneven brightness around the outskirts of this nebula.I have found no dark lanes,although

I have found bright patches of nebulosity around the 4th to 5th magnitude stars.It takes the shape of a swan or a checkmark when observed.This nebula has plenty of dust lanes on the outskirts,while the central outskirts has some areas of even brightness.

Carol Botha

2009 -06 -07

Location:Betty's Bay

Time: 23:30

Telescope: 8" Dobsonian � f5. Eyepiece 15mm. FOV- 45�

Sky conditions: Seeing3/5 (slight cloud)

Apparent size: 30'x 30'

Actual dimensions: 46'x 46'(Cartes Du Ciel)

Object description:

Emission nebula in Sagittarius

I am sure this is one of the most beautiful sights in the heaven. I cannot see an Omega, Checkmark, Lobster or Horseshoe but a graceful Swan has just drifted by. Slight clouds forming but I�m determined to take a closer look at this object.

The nebula has a smooth appearance.

The �body� of the Swan is elongated E-W

Two bright stars visible in the �head�. One bright star to the E just above the �tail feathers�

The rest of the nebula is not clearly visible.

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