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Eye of the Scorpion

Lacaille I.9, Messier 4, NGC 6121, C 1620-264, GCl 41, Bennett 75, Eye of the Scorpion, GC 4183

RA: 16h 23m 35.41s
Dec: −26° 31′ 31.9″

Con: Scorpius
Ch: MSA:1397, U2:336, SA:22

Ref: SIMBAD, Archinal&Hynes (2003), SEDS

(reference key)

Type: globular cluster

Mag: B=8.13, V=7.12

Size: 36′
PA: ?

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Sketches  (4)

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Photos  (6)

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Historical observations

Cheseaux

This beautiful globular cluster was probably discovered by Cheseaux in 1746. He described it as "close to Antares...white, round and smaller than the preceding ones. I do not think it has been found before..."

Lacaille (1755)

Lacaille included it in his 1755 catalogue as Class I No. 9. In his half-an-inch 8x telescope he saw it as "a small comet nucleus."

Messier, Charles

Messier observed in it May 1764, describing it as a "cluster of very small stars; with an inferior telescope it appears more like a nebula; this cluster is situated near Antares and on the same parallel. Observed by M. de la Caille and reported in his catalogue... Diameter 2.5'"

William Herschel (c.1784)

In the Philosophical Transactions, 1814, Herschel described it as "a rich cluster of considerably compressed small stars surrounded by many straggling ones. It contains a ridge of stars running through the middle from sp to nf. The ridge contains 8 or 10 pretty bright stars. All the stars are red. The curious construction of this cluster is sufficiently accounted for by the bright stars in what is called a ridge; the small stars accumulated about it having somewhat the appearance of the shelving sides of the ridge. The observed red colour was probably owing to the low situation of the object." In the Philosophical Transactions, 1818, William Herschel wrote: "."1783, 10 feet telescope. All resolved into stars. I can count a great number of them, while others escape the eye by their minuteness. 1783, small 20 feet telescope. All resolved into stars. 1784, 20 feet telescope. The cluster contains a ridge of stars in the middle, running from sp to nf."

Admiral Smythe

Admiral Smyth called it "a compressed mass of small stars with outliers and a few small stellar companions in the field. It is elongated N-S and has the aspect of a large pale granulated nebula, running up to a blaze in the centre."

Published comments

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.

Melotte, P.J. (1915)

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

Burnham's Celestial Handbook

Burnham calls it "a rather loose cluster, showing no great central condensation, and begins to show resolution into stars in a good 4-inch refractor. The brightest detail and first to appear is a curious central 'bar' or chain of 11th magnitude stars, some 2.6' in length, oriented toward PA 12 ." This bar was noted by Herschel in 1783, who described it as a "ridge of 8 or 10 pretty bright stars running from the middle to N.f." The NGC description reads "Cluster, 8 or 10 stars in line, with 5 stars, clearly resolved into stars." Burnham adds that "with larger apertures, many other chains of stars are discerned around the edges of the cluster, flowing outward in the form of curved loops and streams. The central bar makes the cluster appear quite oblate in small instruments, but long exposure photographs show that the outer distribution is very nearly spherical...In a star count reaching to magnitude 19.3, slightly over 10,000 star images have been detected in M4; the number of fainter stars will probably never be known."

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 7.84.

Bailey, S.I. (1908)

"! globular cluster, fairly condensed, irregular"

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

Remarks, p.218: "an interesting globular cluster; the separation of the stars into groups of widely different magnitudes is especially well marked."

Sulentic & Tifft (1973)

The RNGC (Sulentic and Tifft 1973) notes that this is a 7.5 mag globular cluster.

The mean blue magnitude of the 25 brightest stars, excluding the 5 brightest, is 13.21.

Harris, W.E. (1997)

RA 16 23 35.5 (2000) Dec -26 31 31 Integrated V magnitude 5.63 Central surface brightness, V magnitudes per square arcsecond 17.88 Integrated spectral type F8 Central concentration, c = log(r_total/r_core); a 'c' denotes a core-collapsed cluster 1.59 Core radius in arcmin .83. ["Catalog Of Parameters For Milky Way Globular Clusters", compiled by William E. Harris, McMaster University. (Revised: May 15, 1997; from http://www.physics.mcmaster.ca/Globular.html; Harris, W.E. 1996, AJ, 112, 1487) ]

Hogg

According to Hogg, the total diameter is 22.8', integrated photographic magnitude is 7.41 and the average magnitude of the 25 brightest stars is 13.1.

Modern observations

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

Hartung writes "it is crowded with stars running to a broad haze at the centre, across which is a bar of brighter stars. The outliers are in curved arms, forming a marked concentric pattern reaching to 12' across and visible with care even with a 3-inch." Because of its proximity to Antares, M 4 is very easy to find since both objects are visible in the same binocular field.

Harrington, Phil (1986)

Harrington, P. (1986) An observer's guide to globular clusters. Sky&Telescope, Aug, 198.

At times the naked eye can just catch sight of it

Bennett, Jack

Bennett observed it with a 5-inch short-focus refractor, including it in his list of cometary objects as number 75. His coded description describes it as a circular or slightly elliptical extended object, fully or partially resolved into stars under a higher magnification, with a larger aperture or in very good conditions.

Clarke, W.P. (1993)

William P. Clarke (San Diego, California, USA) writes in the The Webb Society Nebulae and Clusters Section Report No. 11, January 1993: "Observed [NGC 6121 & NGC 6144] with 20x80 binoculars. NGC 6121 (M4) just shows some traces of resolution. NGC 6144, its smaller neighbour, is a small hazy spot north of the line joining Antares and M4. The field is an excellent binocular subject."

Tom Lorenzin

Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "5.9M; 26' diameter; like M-13 but less so; bright belt of stars girds cluster through center in a N-S direction; GLOB N6144 (11M; 6' diameter) 1 degree to NE; 11M star just W of center; Ced-130 is faint nebular cloud surrounding 3M Sigma SCO, 1 degree to NNW of N6121 (M-4)."

Moore, S. (1992)

Stewart Moore (Fleet, Hampshire, UK), observing with a 12-inch f/5, writes in the The Webb Society Nebulae and Clusters Section Report No. 10, July 1992: "A very impressive cluster. Easily resolved and stands magnification well. One particularly bright star visible on the S.W. edge. Unlike other reported observations, I found that M.4 appeared perfectly round. A pleasing starfield. Due to low haze this cluster was not a naked eye object."

Steve Coe

Steve Coe, observing with a 17.5" f/4.5 at 100X, notes: "(M 4) Very bright, very large, irregularly round and much brighter in the middle at 135X. There is a central bar that can be seen even in the 8X50 finderscope and long chains of stars surround this cluster and wind their way out into the Milky Way.; 36" f/5 TSP 96 S+T 7/10 20mm Nagler Amazing spray of stars it is spectacular, as you would expect. Several of the chains are curved, but are parallel to each other, so they appear like old Roman stadium seats. Little pairs all over the place, several stars are light orange in color. I counted 76 stars in NE quad. A terrific view of a real showpiece.; 6" f/6 Dugas Rd. 5/10 seeing, a little twinkly; 6/10 contrast. with 8.8mm EP very bright, very large, irregularly round, the "bar" feature obvious, with several chains of stars, even at just 100X. The curved chains form a "stadium" row of seats. In moments of good seeing, there is a silvery background of stars seen. I counted 32 stars resolved.

Ware, Donald J

Donald J. Ware:"As this fine globular cluster lies only about 1.5 degrees due west of the bright star Antares, this is one of the easiest of the Messier objects to find. This loosely concentrated globular is about 15' in diameter, and is easily resolved. Visually, this object is distinctive in that about 8-10 relatively bright stars appear to form a bar right through its center. These stars help in giving the appearance that the cluster is slightly elongated."

Gramer, Lew (IAAC)

(IAAC) Obj: M4, ngc6144, Cr302, B42, VdB302, Sh2-9 - Inst: 5" f/5 refractor From: Lew Gramer (lewkaren@tiac.net) Date: Tue, 15 Jul 97 14:21:36 -0400 Sender: owner-netastrocatalog@latrade.com Observer: Lew Gramer Your skills: Intermediate Date and UT of Observation: 1997-07-4/5, 04:40 UT Location: Savoy, MA, USA (42N, elev 700m) Site classification: rural Limiting magnitude: 7.2 (zenith) Seeing: 5 of 10 - mediocre Moon up: no Instrument: 5" f/5 Jaegers refractor on altaz mount Magnification: 18x (3.7o field), 25x (2o field) Filters used: None Object: M4, ngc6144, Cr302, B42 (VdB302 and Sh2-9 NOT seen) Category: Globulars (amid large Open Cluster and Dark Nebula) Constellation: Sco Data: mags 5.4, 9.1, 1.0 size 35', 9.3', 8o RA/DE: 16h25m -26o Description:

M4 was a magnificent sight in a rich-field scope under these tar-pool black skies! Visibly elongated N-S and 20' long by about 15' wide, M4 was relatively homogeneous in brightness across its hazy face, and stunning even at this low power. Unlike in nastier urban skies, the globular shows definite signs of resolution, especially on it's N and S extensions, and is far more intriguing and complex than the mere "spot" which I'm used to seeing in Boston! n6144 can be seen in the same spectacular, star-studded 2o eyepiece field, as a fairly faint "dot", looking in fact very much like M4 does under worse skies! With 18x, the addition of mag. 1 alpha (Antares), mag. 3 sigma Sco, and the many field stars near the center of huge open cluster "Collinder 302" (which takes up much of the constellation Sco!), make this a field of view nearly unsurpassed as far as RFTs on dark Summer nights go! One interesting thing I noted at the time was that many of the field stars seemed concentrated toward the M4 (S) end of the 18x Panoptic field. Later chart examination shows in fact a large dark nebula (Barnard 42) interposed onto this area from Ophiuchus to the NE. Finally and not surprisingly, no hint of the reflection nebulosities Van der Bergh 107 or Sharpless 2-9 (around alpha Sco and sigma Sco respectively) could be seen at either power, using any vision trick!

(IAAC) Obj: M4 - Inst: Naked eye, 7x50 binoculars From: Lew Gramer(lewkaren@tiac.net) Date: Thu, 17 Jul 97 12:06:13 -0400 Observer: Lew Gramer Your skills: Intermediate Date and UT of Observation: 1997-07-4/5, 03:00 UT Location: Savoy, MA, USA (42N, elev 700m) Site classification: rural Limiting magnitude: 7.1 (zenith) Seeing: 4 of 10 - medium good, intermittent cumulus Moon up: no Instrument: Naked eye, 50mm Simmons binoculars Magnification: 1x, 7x Filters used: None Object: M4 Category: Globular cluster Constellation: Sco Data: mag 5.9 size 26' RA/DE: 16h24m -26o32m Description:

Just visible to the naked as a slightly unusual star in the thorax of the Scorpion, M4 was also readily found in binocs, as the S apex of a pretty triangle whose other vertices are alpha (Antares) & sigma Scorpii. To binocs under dark skies, M4 is a BRIGHT, loose "ink spot" of white light, with a 15' halo which surrounds a broadly concentrated 15' core. No hint of resolution can be seen, but the cluster lies amid a stunning field of stars.

Callender, John (IAAC)

Observer: John Callender (e-mail: jbc@west.net, web: http://www.west.net/~jbc/); Instrument: 8-inch Dobsonian reflector Location: Carpinteria, CA, USA; Light pollution: light Transparency: fair Seeing: good; Time: Sat Jul 5 07:20:00 1997 UT Obs. no.: 192

Easy at 49x, a diffuse glow with a sprinkling of foreground stars, especially a north/south line of stars that seemed to bound the east side of the cluster, giving it a semi-circular look. The north/south line of stars was even more prominent at 122x and 244x, since the background glow disappeared at those magnifications.

Head, Marilyn

"10 Easy Globs!" by Marilyn Head (105 Owen Street, Newton, Wellington, Aotearoa / New Zealand; mhead@clear.net.nz)

"M4, just to the left and up a bit of Antares shows another surprise for here there is a clearly discerned straight band of stars lying diagonally across the central haze of a somewhat oval and loose cluster, the closest globular at 6,800 l.y."

Ed Finlay

1993 May 22

Ed Finlay, observing with a Meade 4-inch ED APO refractor from Johannesburg, 1993 May 22, notes "You have to look carefully for this one. Nothing like Omega Cen. Very dim at 35x. Almost impossible to see at 102x. Spent two previous nights searching for this but missed it because of its insignificance." Ed's eyes were not dark adapted for this observation.

1993 May 30

On May 30 he wrote: After 1.5 hours ... [I was] pleased to note that with my eyes adapted I could see M4 in my finderscope."

Contemporary observations

Auke Slotegraaf

1984 January

Observing with the 15.5-inch at 220x in January 1984. My notes read: "Globular strange: not round, but angular; one end is extended, forming a peak of stars, which is attached to a bar of stars running across the edge of the cluster. Leading from this bar to the inside of the cluster is an elliptical brightening, brighter than the rest of the cluster stars. Rather untidy, more tidy than NGC 6193, but not as tidy as Omega Cen."

1984, September

"Re-observed this interesting object with the mirror re-silvered. FANTASTIC! Moving the scope across the field at 220x, it is quite possible to loose the globular, and see it as a reasonably clumped collection of stars - not AT ALL like a globular! The cluster has a triangular brightening, with the base of the triangle consisting of a bar of stars. East of the triangle lies a short bow of stars, stars close together, very rough guess at four stars perhaps? Cluster well resolved, only slight nebulosity to suggest a triangular shape. Very interesting indeed!"

1982

Observing with a 15.5-inch at 220x, this most strange globular cluster appears more like a reasonably clumped open cluster, and sweeping over the area it is quite possible to loose all impression of a globular. Lacking the normal order of a globular cluster, the field appears untidy, strewn with stars grouping together haphazardly. On the eastern edge of the cluster, the stars extended outward into a peak, forming a pretty conspicuous V-shaped gathering of stars that point due east. Just east of this V a very conspicuous row of bright stars runs across the cluster from north to south. Due east of the southern termination of the chain is a pretty wide bright double. Many individual stars are seen in the globular, and the extreme western edge of the cluster forms many more subtle arcs and curves. The cluster spans just under a quarter of a degree.

1991

Observing with a 6-inch f/8.6 Newtonian at 108x, the bar of stars is readily seen.

1993 September 24

24/09/93: 11x80 binoculars, strong moonlight: "M4 and the bright orange-red Antares make a pleasing, contrasting pair in the four-degree binocular field. M4 appears as a large nebulous disc or coin with uncertain faded edges."

Magda Streicher

1997 April

Location: Camp Site: ( South 23 16 East 29 26 )

Sky conditions: clear fair about 6 magnitude.

Instrument: 8 inch Meade ( super wide-angle 18 mm. Eyepiece ).

Date: April 1997.

Showpiece globular cluster of well resolved bright stars. Stars bright and faint forming lines round and about that lies scattered towards the edge. Very gaseous field.

(no date)

12-inch f/10 SCT (EP: 2-inch 32mm SW 95x 42' fov; 2-inch 14mm UW 218x 23' fov)

Showpiece globular cluster of well resolved bright stars that is been scattered towards the edges. Three dimensional, it shows off a variety of star-strings that mingles with a slightly gaseous field of view. Striking the straight line of stars that cut across the middle part from northeast to southwest. A lovely string together with clumps of faint stars swings out on the south side and is been rounded off with a prominent double star that given M4 a slightly oval appearance. Slowly brightens towards a strong core.

Chris Vermeulen

2007 August 18

2007/8/18, 21h02

Sky Conditions: Very Good

Quality of Observation: Good

Bill Hollenbach's Pad

6" Dobsonian, 25mm Eyepiece (48x magnification)

M4 is a beautiful globular cluster located to the west of Antares Heart of the Scorpion. Not visible to the naked eye, but through a 25mm eyepiece this significantly round globular cluster's not so very dense core is seen with stars close to each other and it glows brightly fading off towards its outer edges.

Carol Botha

2010 July 17

Location: Betty's Bay (3421'10" S / 1854'06" E)

Time: 22:00

Instrument:12" Dob F5. Eyepiece: 8mmUWA

Limiting magnitude: +5

Seeing: Very good. Transparency: Gibbous Moon

Many stars resolved. Core seems to be very slightly elongated to the E and less dense towards the S.

A prominent "soft S-shaped" string of stars cross the core from E - SW. There is a sense of opulence in abundance: the core seems to be draped in strings of pearls.

Richard Ford

2016, June, 4th

Location:Perdeberg.

Time:7:05pm.

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 globular clusters stars are very well resolved into 5th to 6th magnitude stars.In appearance this clusters stars are irregularly distributed and this globular cluster are slightly loosely concentrated towards each other.In overall this cluster looks like like an irregular snowball.This globular cluster measures 11.2'x 8'.Chart No:337,NSOG Vol.2.

2016, June, 4th

Location:Perdeberg.

Time:7:05pm.

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 globular clusters stars are very well resolved into 5th to 6th magnitude stars.In appearance this clusters stars are irregularly distributed and this globular cluster are slightly loosely concentrated towards each other.In overall this cluster looks like like an irregular snowball.This globular cluster measures 11.2'x 8'.Chart No:337,NSOG Vol.2.

2016, June, 4th

Location:Perdeberg.

Time:7:05pm.

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 globular clusters stars are very well resolved into 5th to 6th magnitude stars.In appearance this clusters stars are irregularly distributed and this globular cluster are slightly loosely concentrated towards each other.In overall this cluster looks like like an irregular snowball.This globular cluster measures 11.2'x 8'.Chart No:337,NSOG Vol.2.

2010 July 3rd

Location:Perdeberg

Time:7:25

Telescope:12"Dobsonian Reflector Telescope

Limiting magnitude: 4.5

Sky conditions:The fainter parts of the milky way are barely visible.

Eyepieces: 26mm Super Wide Field Eyepiece.

20mm Ultra Wide Angle Eyepiece.

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

Seeing: Atmosphere stable with little interference.

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

Size: 26mm Eyepiece: Field Of View: 57'/6=9.5'.

20mm Eyepiece: Field Of View: 50'/5.5=9.0'.

9.5+9.0'=18.5'.

18.5'/2=9.2'.

Size in Arc Minutes: 9.2'.

Size of nucleus vs.halo: 6/9.2'=0.6'.

5.5./9.2'=0.6'.

0.6'+0.6'=1.2'.

1.2'/2=0.6'.

Size Of Halo: 0.6'.

Ratio: 1:4.

Major Axis: 9.2'.

9.2'/4=2.3'.

Minor Axis:2.3'.

Globular Cluster is 9.2'*2.3'.

Brightness: Bright.

Brightness Profile: Medium Surface Brightness.

Challenge Rating: Fabulous to observe in a large telescope under a very dark sky.

Description

-----------

This globular cluster is well resolved into a large agglomeration of bright stars radiating slightly away from the central outskirts of the nucleus. The stars in this globular cluster is loosely concentrated towards the nucleus. I have found a clump chain of stars on the outskirts where plenty of starless patches are observed.

Tom Bryant

2008-07-03 00:00:00

Observing site: Pinnacles overlook

Telescope: C-8

[16h 23m 36s, -26 32' 0"] A large, bright, loose cluster. Approximately 50 stars were seen with direct vision. There is a distinct bar of stars, running through the center of the cluster, in position angle 10.

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