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RA: 17h 40m 18s
Dec: −32° 12′ 0″
Ch: MSA:1416, U2:377, SA:22
Ref: SIMBAD, Collinder (1931), DAML02, Archinal&Hynes (2003)
Type: open cluster, 23r
Mag: B=4.48, V=4.2
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This fine open cluster lies some 5 north of the tail of Scorpius and is one of the largest and brightest clusters, most suitable for small telescopes.
Burnham notes that the discovery of M6 is usually credited to de Cheseaux in 1746, "though the cluster is a definite naked eye object, and appears to be mentioned, along with M7, in the catalogue of Ptolemy. They seem to be the "Girus ille nebulosus" of the 1551 edition of the Almagest, and also appear in Ulug Beg's star catalogue as the "Stella nebulosa quae sequitur aculeum Scorpionis [The Cloudy Ones which Follow the Sting]" De Cheseaux, however, was probably the first to identify M6 as "a very fine star cluster"."
Burnham regards it as one of the most attractive clusters for very small instruments, calling it "a completely charming group whose arrangement suggests the outline of a butterfly with open wings. The main portion of M6 just fills a 25' field, and the cluster is at its best in a good 6 or 8-inch glass with wide-angle oculars. Visually, the brightest member is the golden K-type giant which highlights the NE wing-tip; this is the semi-regular variable BM Scorpii..."
In 1752 it was seen by Lacaille who included it in his 1755 catalogue as Class III No. 12. In his half-an-inch 8x telescope he saw it as a "remarkable group of faint stars in parallel lines making a diamond 20-25 minutes diameter filled with nebulosity."
Messier, in 1764, found it "a cluster of small stars between the bow of Sagittarius and the tail of Scorpius. To the naked eye it resembles a nebula without a star, but even a small telescope reveals it as a cluster of small stars. Diameter 15'."
Flammarion saw here "stars of 7-10 mag very dispersed and arranged in a remarkable pattern... three starry avenues leading to a large square."
In the Philosophical Transactions, 1814, Herschel described it as "a cluster of stars of various sizes containing several lines that seem to be drawing to a centre like a forming cluster." In the Appendix to the 1912 'Scientific Papers of Sir William Herschel' this object is described as "1783, July 30. 20 feet, lower power. I counted above 50 stars; it contains the greatest variety of magnitudes of any nebula I recollect. The compound eyepiece shows more of them variously scattered and intermixt."
Observed by Sir John Herschel at the Cape of Good Hope with an 18-inch f/13 speculum telescope. He recorded it as "Chief star 7m of a fine, L discrete cluster of stars 10..11m; one star is 7m, one 7-8m. Fills field; VIII class."
Bailey, S.I. (1908) A catalogue of bright stars and nebulae. Ann.Harv.Coll.Obs., 60(8), 199.
"! cluster, fairly condensed"
Bailey, examining a Bruce plate (Harvard Annals, Vol 72, No 2), describes it as "Milky Way, coarse cluster, pretty bright stars, several hundred, diameter 30'."
A catalogue of star clusters shown on Franklin-Adams chart plates. Mem.R.A.S., 60(5), 175-186.
Raab, S. (1922) A research on open clusters. Lund Medd. Astron. Obs. Ser. II, 28, 1.
Discussed, based of F-A plates.
Journal BAA, 36(3), Dec, p91
Nearly circular cluster, well defined, mainly of B*s which are chielfy in one half of the cluster.
Doig, P. (1926) "A Catalogue of Estimated Parallaxes of 112 Nebulae, Open clusters and Star Groups", Vol 36 (4), p 107-115.
"nearly circular open cluster of bright stars." He gives the approx. diameter as 35 arcmin.
Trumpler (Lick Obs Bul, Vol 14, No. 420) gives the diameter as 26' and the class as 2 3 m.
The RNGC (Sulentic and Tifft 1973) notes that this is a 4.5 mag open cluster.
Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "4.6M; 25' diameter; bright and medium rich; cluster N6416 (8.7M; 20' diameter) sparse and dim <1 degree E; cluster N6404 (10.6M 5' diameter) compressed and dim 1 degree to S; see VADSS-183."
Harrington notes that "most observers can immediately spot two stellar wings spreading out from the groups more densely packed body. ... of the 80 stars that make up the butterfly, more than a third are bright enough to be seen with 7x50 binoculars. Observers viewing through giant glasses may be able to count 50 suns, while 6- to 8-inch telescopes reveal all of the cluster members. The brightest star within M6 is BM Sco, a blazing orange irregular variable that fluctuates between 6.8 and 8.7 in about 850 days."
Donald J. Ware:"The Butterfly Cluster. This fine open cluster is large, about 25' in diameter and contains over a hundred bright and relatively bright stars. It is called the Butterfly Cluster because some observers see the shape of a butterfly formed by the stars. This cluster is visible to the naked eye as a faint patch of light."
Observer: Lew Gramer; Your skills: Intermediate; Date and UT of Observation: 1997-07-4/5, 03:30 UT; Location: Savoy, MA, USA (42N, elev 700m); Site classification: rural; Limiting magnitude: 7.1 (zenith); Seeing: 5 of 10 - mediocre, increasing cumulus; Moon up: no; Instrument: Naked eye, 50mm Simmons binoculars; Magnification: 1x, 7x; Filters used: None; Object: M6, M7, ngc 6416; Category: Open clusters; Constellation: Sco; Data: mags 4.2, 3.3, 5.7 sizes 15', 80', 18'; RA/DE: 17h45m -32o;
Description: M6 and M7 travel together just off of the mainstream of the Summer Milky Way, in a fascinating clump of naked-eye haze patches NE of the many tail stars of Sco. M7 even seemed to "glitter" to the naked eye, with the promise of many resolved stars just on the edge of vision. The edges of both M6 and M7 are just visible in the same binocular field, centered just SW of the fainter open cluster n6416. This bino view is one of INCREDIBLE stellar complexity, with the well-resolved NW edges of M7 filling the lower-left (SE) edge, M6 appearing as a sparkling and MUCH smaller irregular blur peaking around the upper-right (NW) edge, and faint n6416 being fully visible as a sparkling hazy patch toward the middle from M6. Scanning up to M6, a yellowish mag 6 star rides its ENE edge. Bringing M7 into full view, the observer is first struck by a dizzying myriad of field and cluster stars, then by the unresolved haze of perhaps hundreds of stars lying beneath these. Soon however, "dark" areas of relatively less concentration become visible in these light clouds, especially to the NW and SE. Last noted: a striking pair of RED mag 7 stars, just S of the blur of 6416.
Steve Coe, observing with a 17.5" f/4.5 at 100X, notes: "(M 6) Very, very bright, very large, pretty rich, somewhat compressed at 60X. This cluster is easily naked eye and several of the brighter members can be seen in 10X50 binoculars. The shape has caused observers to see a butterfly in this cluster and I agree with that evaluation. There are even two delicate curved chains of stars that form "antennae". BM SCO is a variable star on the east side, it is a nice orange color.; 6" f/6 Dugas 63 stars resolved with 8.8mm EP. Great view with the 22mm, shows cluster and field around it, going to higher powers brings out lots of pretty faint members.
10x50 tripod-mounted. 1997-07-06. Amazingly bright; very rich and condensed; background glow from unresolved stars; 4 brightest stars form a rectangle lying on the SW-NE axis of the cluster [Rui Henriques]
1997 July 9, Wednesday, 20:00 - 22:00 Jonkershoek. 11x80's tripod-mounted. Moderate conditions. A bright band of stars. Making the outline are six stars forming two joined parallelograms, making a V-shaped wire-frame box. Inside are scattered many smaller stars. The eastern parallelogram is most densely populated and also had a bright orange star in the following tip.
Sky Conditions: Poor: Cloudy
Quality of Observation: Moderate
6" Dobsonian, 25mm & 10mm Eyepieces
A beautiful open cluster also visible to the naked eye. This cluster of bright stars resemble the image of a butterfly in flight with its wings spread wide open. M6 is truly a magnificent object to observe, one can really get lost in its beauty as it is one of the best open clusters for observation.
At high magnification (120x) it is not so impressive as much of the surrounding background is lost. M6 was best viewed at 48x magnification and with the background stars also in view made a very impressive image.
2007 July 30, 20:10 SAST
Walmer, Port Elizabeth
Conditions: Moon very near.
Butterfly Cluster. Noticeable cluster for its size 15arcminutes, just noticeable with naked eye, stands out well against background, making it easy to find. Dimmer than Messier 7, with an apparent magnitude of M4.2. Large prominent star in the eastern part M6, a few dimmer stars could be resolved with averted vision M7.5 in the western part of the cluster running north-to-south. The remainder of the cluster remains unresolved and luminous through binoculars. The cluster is well concentrated and consists mainly of dimmer unresolvable stars, field stars located west M 5.2 about four degrees, another south M5 about 4.5 degrees.
Telescope:12"-inch Dobsonian Reflector Telescope.
Eyepieces:26mm super wide field eyepiece.
20mm ultra wide angle eyepiece.
Sky Conditions:The fainter parts of the milky way are barely visible.
Transparency of the Sky:Haziness only visible on the horizon.
Seeing:Atmosphere with little interference.
Object Name:Butterfly Cluster.
First Impression:Open Cluster.
Chart Number:No.18(Extract taken out of "Atlas of the Night Sky").
Size:26mm eyepiece:Field of View: 57'/3=19'.
20mm eyepiece:Field of View: 50'/2.5'=20'
Size in Arc Minutes:19.5'.
Open Cluster is 19.5'*3.2'.
Brightness: Extremely bright.
Brightness Profile: High Surface Brightness.
Challenge Rating:Very easy to observe in light polluted skies,as well as extremely dark skies.
This open cluster is well detached and the stars in this open cluster is arranged into a shape of a Butterfly's wings.There are over 90 stars in this open cluster; it is hard to determine the full amount of stars. Most of the stars in this cluster have different spectral characteristics which mean that all the stars in this cluster nearly have the same brightness as each other. Around most of the stars in this cluster there are plenty of empty starless patches, although around the outskirts of this cluster there is a bright chain of stars.
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