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RA: 17h 56m 56s
Dec: −19° 00′ 42″
Ch: MSA:1369, U2:339, SA:22
Ref: SIMBAD, Collinder (1931), DAML02, Archinal&Hynes (2003)
Type: open cluster, 22r
Mag: B=6.03, V=5.5
Discovered by Messier in June of 1764, he described it as a cluster 15' in diameter with stars "very close to one another."
In the Appendix to the 1912 'Scientific Papers of Sir William Herschel' this object is described as "1784, June 18, 20 feet telescope, a cluster of beautifully scattered, large stars, nearly of equal magnitude (visible in my finder), it extends much further than the field of the telescope will take in, and in the finder seems to be a nebula of a lengthened form extending to about half a degree."
Sir John Herschel found about 100 stars in the cluster, ranging from 9th to 13th mag. Burnham notes that the stars are scattered more or less evenly across the whole field, with very little concentration toward the centre. A number of the stars, however, appear to be arranged in curving arcs and chains, "which to the imaginative observers may suggest the outline of a Chinese temple, or perhaps some bit of oriental calligraphy." Flammarion saw near the centre a circle of six stars, with "nine stars in an arc" in the north-east portion of the cluster. K G Jones points out that "three or more" of the star chains appear to be potions of concentric arcs which are seemingly focused toward the bright 8th mag star on the north-east side of the cluster. Admiral Smythe described M23 as "A loose cluster, an elegant sprinkling of telescopic stars over the whole field under moderate magnification. The most clustering portion is oblique in a direction Sp. and N.f." C E Barns writes of the cluster as "a blazing wilderness of starry jewels." Burnham comments on a chain of faint stars running from the cluster out to a bright mag 6.5 white star in the northwest, some 18' from the cluster centre. He adds that the most pleasing views can be obtained with a 10" f/6 reflector working at 45 power.
Trumpler (Lick Obs Bul, Vol 14, No. 420) gives the diameter as 27' and the class as 1 2 r.
Raab, S. (1922) A research on open clusters. Lund Medd. Astron. Obs. Ser. II, 28, 1.
Discussed, based of F-A plates.
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 7.01
Doig, P. (1925) Notes on the nebulae and clusters in Webb's 'Celestial Objects for Common Telescopes' (Sixth edition, Vol.ii). Part II. M.N.R.A.S., 35(8), 280.
Doig, P. (1926) "A Catalogue of Estimated Parallaxes of 112 Nebulae, Open clusters and Star Groups", Vol 36 (4), p 107-115.
He gives the approx. diameter as 37 arcmin.
"cluster, fairly condensed"
Bailey, S.I. (1908) A catalogue of bright stars and nebulae. Ann.Harv.Coll.Obs., 60(8), 199.
The RNGC (Sulentic and Tifft 1973) notes that this is a 6.0 mag open cluster.
A catalogue of star clusters shown on Franklin-Adams chart plates. Mem.R.A.S., 60(5), 175-186.
Harrington notes that it "might just be visible to the naked eye on clear nights as a slightly brighter spot along the Milky Way. Through binoculars it appears as a nebulous glow spanning about half a degree. With low-power telescopes M23 is a beautiful sight. Some observers see the group as fan-shaped, though the allusion has always escaped me. In all, about 120 suns as bright as 9th mag reside in the group.
Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "7M; 30' diameter; 100-plus 9M members; irregular in overall shape but more or less even distribution of equal-M stars; not terribly well-detached from crowded background."
Steve Coe, observing with a 13" f/5.6, notes: "(M 23) Bright, large and not compressed. At 100X it has chains of stars that form a pattern like the seats of a sports arena. Rose Bowl Cluster?"
1994-06-04 23:00, Die Boord, 11x80. Good seeing. Wonderful open cluster, a large haze of many pinpricks of light, accompanied by a bright star to the northwest. Very noticeable even when sweeping. Although not nearly as luminous as nearby M7, this M23 has a different charm and is as interesting as its bright neighbour.
1997 April 21. Jonkershoek. 11x80 tripod-mounted. No moon. Spectacular cluster. Irregularly round. Bright star nearby. Uncountable swarm, random scatter of stars, no concentration. Looks like M44 with the naked eye.
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 massive ball of tiny stars, swarming over an area about 20' across. There's a bright star to the north-northwest but its clearly not a cluster member. Within the grouping itself, there's a single bright [8.5m[ star, eccentrically situated to the north-east; the rest are 9th mag and fainter, in a thronging mass. Reminds me of the well-resolved M4 in the 15.5-inch.
Sky Conditions: Clear
Quality of Observation: Good
6" Dobsonian, 25mm Eyepiece
Initially M23 was thought to be a globular cluster when observed with the naked eye, but on closer inspection with a 25mm eyepiece on a 6" Dobsonian it could clearly be distinguished from a globular cluster.
Best viewed through lower magnification, M23 is a magnificent sight, with a great number of bright stars of about mag. 6 making up the cluster. M23 is also neighboured by the likes of M6, M7, M8, M16, M17, M25 in the rich area of the Milky way.
Instrument:12-inch Dobsonian Reflector Telescope.
Sky Conditions:The fainter parts of the Milky Way are barely visible on the horizon.
Transparency of the Sky:Haziness only visible on the horizon.
Seeing:Atmosphere stable with little interference.
Object Type:Open Cluster.
First Impression:This comet looks like an open cluster.
Chart Number:No.12(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'.
19'+ 20'= 39'.
Size in Arc Minutes:19.5'.
Open Cluster is 19.5'* 6.5'.
Brightness Profile: From the central outskirts of this cluster it grows brighter.
Challenge Rating:Very Easy.
The stars in this open cluster look like bright diamonds and that this cluster is not separated.I have counted 150 stars within a fixed diameter.I found that the stars range from 4th to 5th magnitude and that the stars are nearly the same brightness as each other.However the stars are slightly concentrated towards each other.I have some starless patches in some areas of this cluster.
Observing site: Little Tycho Observatory
[17h 56m 48s, -19° 1' 0"] A cluster of ~40 stars, 9-11 mv, mostly in a 30' field.
The Messier objects
The Bennett objects
The Caldwell list
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