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Ptolemy's Cluster

Lacaille II.14, Messier 7, NGC 6475, Cl Collinder 354, C 1750-348, Cl VDBH 254, Ocl 1028.0, COCD 420, Ptolemy's Cluster, h 3710, GC 4340

RA: 17h 53m 48s
Dec: −34° 47′ 0″

Con: Scorpius
Ch: MSA:1437, U2:377, SA:22

Ref: SIMBAD, Collinder (1931), DAML02, Archinal&Hynes (2003)

(reference key)

Type: open cluster, 13r

Mag: B=3.45, V=3.3

Size: 80′
PA: ?

Image gallery

Photos  (6)

Select a photo and click the button to view

Historical observations

Lacaille (1755)

Lacaille observed it at the Cape of Good Hope in 1751, and included it in his 1755 catalogue as Class II No. 14. In his half-an-inch 8x telescope he saw it as a "group of 15 to 20 stars in a square."

Messier, Charles (1764)

Messier, in May 1764, described M7 as "a cluster considerably larger than the preceding [M6]. It appears to the naked eye as a nebulosity; it is situated a short distance from the preceding, between the bow of Sagittarius and the tail of Scorpius. Diameter 30'."

William Herschel

In the Appendix to the 1912 'Scientific Papers of Sir William Herschel' this object is described as "1783, July 30. About 20 small stars (Only seen once)."

John Herschel (1847) Cape Observations

Observed by Sir John Herschel at the Cape of Good Hope with an 18-inch f/13 speculum telescope. He recorded it as "a brilliant coarse cluster class VIII of about 60 stars 7-8 .. 12m, which fills field. Irregular figure. A star 8m taken." On a second occassion he called it "Cluster VIII Very fine and brilliant; stars of very large and mixed magnitudes. Fills field."

Published comments

Burnham's Celestial Handbook

M7 is a large and brilliant group, easily seen with the naked eye, and "one of the few clusters which can be thoroughly appreciated in a good pair of field glasses" writes Burnham. He continues: "It is mentioned in the catalogue of Ptolemy, and in the 16th century Latin translation of the Almagest appears as "Girus ille nebulosus", the reference probably including both M7 and M6. The Arabian name "Tali al Shaulah" is the equivalent of the Latin translation of Ulug Beg's title: "Stella nebulosa quae sequitur aculeum Scorpionis [The Cloudy Ones which Follow the Sting]" Hevelius includes M7 in a list published in 1690, and it appears again in W. Derham's short catalogue of "nebulous stars" in 1730.

Bailey, S.I. (1908)

"! cluster, fairly condensed"

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

Raab, S. (1922)

Raab, S. (1922) A research on open clusters. Lund Medd. Astron. Obs. Ser. II, 28, 1.

Discussed, based of F-A plates.

Doig, P. (1925)

Journal BAA, 36(3), Dec, p91

thin cluster, about 130' diam., centre consists of about 15 B*.s

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.

"open cluster of bright stars in field at least 1 degree square." He gives the approx. diameter as 90 arcmin.

Trumpler, R.J. (1928)

Trumpler (Lick Obs Bul, Vol 14, No. 420) gives the diameter as 50' and the class as 1 3 m.

Sulentic & Tifft (1973)

The RNGC (Sulentic and Tifft 1973) notes that this is a 3.5 mag open cluster.

Modern observations

(unknown)

The cluster is seen projected on a background of numerous faint and distance Milky Way stars, while the bright stars of the group are close to naked-eye visibility. On Lowell Observatory 13-inch telescope plates the bright central portion of the cluster just fills a 30' field; the total apparent diameter is possibly about 50'....M7 contains 80 stars brighter than 10th magnitude in a field 1.2 in diameter. The group as a whole resembles Praesepe (M44) in Cancer, though somewhat smaller, and would certainly be as well known if it were further north. Incidentally, this is the southernmost object in the catalogue of Messier."

Harrington, Phil

Harrington writes that Messier's description "seems a bit sterile for such a magnificent group. My 7x50 wide-angle binoculars create a three-dimensional effect as many of the brighter stars appear to float in front of fainter points of light. Colors abound in M7, with several stars tinted yellow and blue. The brightest is a G-type star of 6th mag lying close to the group's centre."

Hartung, E.J. (1968) Astron.Obj.South.Tel

Hartung notes that M7 is "a remarkable sight in a large field with its structure of quadrant and straight lines. With outliers it is more than 40' wide, and very effective for small telescopes. The fine orange star S.p. is the very close pair Struve 342. This close double was discovered in 1897 by T.J.J. See, and Hartung notes that it seems elongated but not truly resolved with a 12-inch telescope.

Bortle, John (1976)

John Bortle (Webb Society Quarterly Journal, January 1976) using 10x50 binoculars, estimates the visual magnitude as 2.8.

Tom Lorenzin

Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "5M; 1 degree diameter! 50-plus 7 thru 11M members; easy naked-eye object; GLOB N6453 (11M; 1' diameter) 30' to WNW from N6475's center; M-7 looks like M-6 but twice as large; each's brightest star is deep orange."

Ware, Donald J

"This beautiful open cluster is almost a degree in diameter, so either very low powers or binoculars should be used to optimally view it. This loosely concentrated cluster is easily visible to the naked eye, but should you observe it through a telescope, be sure to look for NGC 6453. This is a small, faint globular cluster seemingly imbedded in M-7's western edge."

Gramer, Lew (IAAC)

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.

Melotte, P.J. (1915)

A catalogue of star clusters shown on Franklin-Adams chart plates. Mem.R.A.S., 60(5), 175-186.

Steve Coe

Steve Coe, observing with a 17.5" f/4.5 at 100X, notes: " (M 7) Very, very bright, very, very large, not compressed, many faint members at 60X. This huge cluster is easily naked eye, even on poor nights. I can resolve 8 to 10 of the brightest members in 10X50 binoculars. It is at its best in an RFT. Using a 4.25" f/4 at 16X there is enough room around the cluster to frame it in the Milky Way and there are 40 stars resolved with this modest scope.; 6" f/6 Dugas Very, very bright, very, very large, not compressed, 49 stars counted, including a nice orange 9th mag star on the SW edge of the cluster. Several delicate pairs and groupings, best view ever of this cluster.

Ed Finlay

1992 May 22

Ed Finlay, observing with a Meade 4-inch ED APO refractor from Johannesburg, 1992 May 22, notes "open cluster easily seen in 10-x50 binoculars. Nice at 35x but beautiful at 184x."

Rui Henriques

1997 July 06

10x50 tripod-mounted. 1997-07-06. triangle shaped glow with a distict x shaped pattern of stars in middle; wide field of stars 60'; stars more or less the same size; nebulosity of unresolved stars in the background; two darker patches each side of the x formation (OXO) [Rui Henriques]

Contemporary observations

Auke Slotegraaf

1983

Observing from Stellenbosch, 1983, I saw this object as an excellent, bright and loose cluster, easily visible. A rough sketch with my 2-inch refractor shows 21 prominent stars.

1997 July 9

1997 July 9, Wednesday, 20:00 - 22:00 Jonkershoek. 11x80's tripod-mounted. Moderate conditions. Glorious cluster. One orange star. Dark nebulae near, esp. Barnard 283.

Kerneels Mulder

2008 October 31

Date and Time: 31 October 2008, 21:35
Location: Prince Albert (Western Cape, South Africa).
Instrument: Orion XT10 10" f/4.7 Dobsonian Reflector
Eyepieces: 10mm (120x, 26′ FOV), 25mm (48x, 1 FOV)
Sky Conditions: Clear. Seeing: 5/10. Transparency: Average
Windy, waxing crescent moon

M7 (Ptolemy's cluster): Very easy to find. Visible naked eye as a faint patch close to the tail of Scorpius.

48x: Very large open cluster visible over a wide area. Some concentration at the centre. 18 bright stars counted with numerous fainter stars surrounding them. Two rows of 4 stars in central region visible, oriented roughly E to W. Row of 7 stars at E edge of central region in an N to S direction. Fills 1 degree FOV including stars in the surrounding area.

Chris Vermeulen

2006 March 25

2006/3/25-26, 21h00-03h30

Sky Conditions: Poor: Cloudy

Quality of Observation: Moderate

Ngwenya Lodge

6" Dobsonian, 25mm & 10mm Eyepieces

Clearly visible to the naked eye, this cluster of stars appears much more spectacular through a telescope. It stands out like a diamond in the nightsky just below the tail of Scorpio. This beautiful open cluster attracts all the beauty and shines more magnificent than its companion M6. Looking at M7 you clearly experience the feeling of being in a different world.

Just as with M6 viewing M7 at 120x magnification does not do this cluster justice. To fully comprehend its splendour it is best viewed at 48x magnification.

Magda Streicher

(no date)

Alldays (22.50S, 20.12E, 770m).

12-inch f/10 SCT (95x)

Several chains scattered over a large field of view. M7 is a lovely cluster (more or less 50 stars seen). Displays very faint stars to fill in the space between the brighter stars. Few doubles can be seen imbedded in the middle section and the cluster can be seen with the naked eye. NGC 6453 cab be seen 37' towards the west. There are Three PK planetary nebula's to the NW spaced in a string formation very close to M7. About 50 stars, fine naked-eye cluster.

Gary Lillis

2007 July 18, 19:30 SAST

Walmer, Port Elizabeth

2.5-inch f/7.6 refractor (EP: 25mm 28x 45arcmin fov)

Conditions: Fine.

Ptolemy's Cluster. Very prominent and large cluster, consists of a chain of equally spaced prominent stars M6-M6.5 forming the main body PA 310 degrees and PA 270 degrees. Clump of coarse stars is within the body M8, larger clump of coarse stars North on the outskirts. Long "arm and leg-like" chain of brighter stars of about equal brightness M7, running west-east and north-south. Cluster is well concentrated and is separate from field stars. NGC 6475 has magnitude of M3.3, it is visible with the naked eye and is large enough, 80arcminutes, to find without much difficulty through binoculars, not visible through 24mm finder scope. 32 stars visible through telescope's eyepiece 28x 25mm.

Richard Ford

2010 ,July 3rd Saturday

Location:Perderberg.

Time:8:45pm.

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 stable with little interference.

Limiting Magnitude:4.5.

First Impression:Open Cluster.

Location:Scorpius.

Chart Number:No.18(Extract taken out of "Atlas of the Night Skt").

Size:26mm eyepiece:Field of View:57'/4.5=12.6'.

20mm eyepiece:Field of View:50'/4=12.5'.

12.6'+ 12.5'=25.1'.

25.1'/2=12.5'.

Size in Arc Minutes:12.5'.

Ratio:1:5.

Major Axis:12.5'.

12.5'/5=2.5'.

Minor Axis:2.5'.

Open Cluster is 12.5'*2.5'.

Brightness: Extremely Bright.

Brightness Profile:High Surface Brightness.

Challenge Rating:A stunning sight to observe this cluster in a dark sky under a large telescope.

Description

-----------

This open cluster is well detached and is well arranged into a bright chain of stars which are nearly the same brightness as each other.This cluster consists of well over 25 stars.This cluster has an irregular shape of bright stars, it is hard to describe what it looks like.There are plenty of areas of starless patches, although on the outskirts I have found a clump chain of bright stars.

Favourite lists

Lacaille's catalogue

The Messier objects

Dunlop's catalogue

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The Caldwell list

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