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RA: 12h 42m 18s
Dec: −62° 59′ 0″
Ch: MSA:988, U2:451, SA:25
Ref: SIMBAD, Collinder (1931), DAML02, Archinal&Hynes (2003)
Type: open cluster, 22m
Mag: B=7.32, V=6.9
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Discovered by Sir John Herschel at the Cape of Good Hope with an 18-inch f/13 speculum telescope. He recorded it as "Cluster class VII. Stars 11..13th mag; about 6' long and 4' broad; has 10 stars 11th mag, and some 20 or 30 smaller. It occurs in the midst of the black space following Alpha Crucis, which is by no means void of stars."
Trumpler (Lick Obs Bul, Vol 14, No. 420) gives the diameter as 4.5' and the class as 2 2 p.
"cluster, coarse, with one 6th mag star involved."
Bailey, S.I. (1908) A catalogue of bright stars and nebulae. Ann.Harv.Coll.Obs., 60(8), 199.
A catalogue of star clusters shown on Franklin-Adams chart plates. Mem.R.A.S., 60(5), 175-186.
Table, p.177: "Clusters" noted by Bailey but not included in the Catalogue:
NGC 4609: A few stars only. Not considered a cluster.
(Sulentic and Tifft 1973) notes that this is a 9.0 mag open cluster.
"Cat. of Open Cl. south of -45° Decl.", Mem. 17 Mnt Stromlo Obs.
pair NE side V=9.5,12.9. br * = HR4830 = HD110432, V=5.31.
15cm - in *ry part of Coalsack NW of br *. 140x: 35 *s in 4' diam, brtr *s on
E side of region. brtst *, m9, is un= pair on NE side of brtr grp. BS,
ASV Journal Vol 24 No 3 June 1971: "visible in 2-inch 64x."
"Bertha" 12-inch f/4.8 Dobsonian (EP: 32mm, 25mm, 10mm, 6.3mm Plossls, 2x Barlow, 32mm Erfle)
Conditions: Clear, dark.
This cluster lies immediately north-west of the orange variable star BZ Crucis, within the boundaries of the Coalsack. Its brightest five members (V=9.5 to 10.5) make a very sharp triangle, 4' long, pointing northward. Its exact boundaries are unclear. All powers show it as a very coarse cluster of about 15 stars.
Stellenbosch (Paradyskloof rifle range)
6-inch f/8 Newtonian
Conditions: NELM 5.8 at the pole. Thin cloud in the distance.
Must be one of the easiest star clusters to find: just point the telescope at Acrux and wait. In the low power eyepiece the cluster is an obvious bowtie shaped collection of 9th magnitude stars with a bright unrelated star just to the southeast.
From "Observing log, 2004 March 24"
Rifle Range, 6-inch f/8
I wait patiently for the tube to cool down. Over the next while, I check the progress. Acrux drifts out of the field of view and after a few moments is replaced by a deepsky object. This little cluster, NGC 4609 (the Coalsack Cluster), must be one of the easiest to find. Just point at Acrux and wait. In the low power eyepiece - which I refer to as the sweeper - the cluster is an obvious bowtie shaped collection of 9th magnitude stars with a bright unrelated star just to the southeast.
1998-03-01/02, 6-inch f/8.6 Newtonian, Die Boord. 5.6 (naked eye), seeing average. Obvious, well-resolved cluster of stars, guarded by bright orange star 4 arcmin to the southeast. In the sweeper power, the cluster reveals about 15 stars, most being of similar magnitude, arranged in an elongated (1:4) rectangle 4.5' x 1' (1.8x in Or4) in size. Stunning contrast to Ho15.
1998-01-26/27, 6-inch f/8.6 Newtonian, Stellenbosch Rifle Range site. A poor cluster on the edge of the Coalsack. About 13 stars in a north-south elongated [1:3] grouping 1.5 by 4.5 arcmin large [1/5 K12.5mm fov]. It is well separated from the background and exhibits no particular concentration. Moderate brightness range, with half-dozen similar magnitude stars, the rest slightly fainter. At both the northern and south end of the grouping lies a small triangle of stars. Outside the cluster, to the southeast, lies an 8th mag star.
A pretty bright star on the border of the Coalsack is kept company by two open clusters, to its west is the Coalsack cluster, NGC 4609, and directly opposite on the east is Hogg 15.
1997 April 14, 02:00 - 04:00 Jonkershoek. 11x80's tripod-mounted. Soft fuzzy glow; could just be 2 or 3 very faint stars.
1994-02-10, Die Boord, 11x80 tripod-mounted. This cluster is readily seen with averted vision; however, looking directly at the bright star to its southeast, the cluster vanishes. Using averted vision, the cluster and star look like an artistic, stylized impression of a comet: the star forms the nucleus, and the cluster the tail. The part of the cluster nearest the star forms a prominent north-south bar, and the other cluster members extend away to the northwest forming an irregular triangle.
A 10-inch f/5 with an 18mm eyepiece shows this cluster as small, very poor, scattered but distinctly shaped. The seven brightest stars form an oblong bow-tie shape.
A 15.5-inch shows the cluster as having a distinct box shape, elongated North-South, with a member in the middle, flanked on its North and South by two triangular patches. About a dozen 9-11th mag stars make up this sparse cluster, which forms a nice contrasting pair with Hogg 15. The latter cluster is a bit smaller than the triangular patches mentioned above.
Meade 8-inch, 18-mm eyepiece.
18 April 1999
Easily seen, consists of about 12 bright stars. The stars closer the reddish one are brighter than those further away.
Alldays (22.50S, 20.12E, 770m).
12-inch f/10 SCT (95x)
An easily visible cluster with about 12 bright stars (including a reddish one) Those stars nearest the reddish one are clearly brighter than the more distant ones. Star forms nucleus with the other fainter stars the comet tail going to the north-west. Cluster lies in a starry part of the Coalsack dark nebula just north-west of 5mag HD 110432 Cluster look artistic style impression of a comet. Star forms the nucleus and the cluster the tail. Cluster nearest the star forms a prominent north-south bar, and the other cluster members extend away to the north-west forming an irregular triangle. At both the south and northern end of the grouping lies a small triangle of stars. This elongated shape is quite apparent to me I called it oblong bow-tie shape, distinct box shape and elongated rectangle. Still in Crux, the Open cluster NGC 4609 lies in a starry part of the Coal Sack dark nebula just 6' southeast from the reddish 5 Magnitude BZ Crucis. At RA12h 42.3 Dec -62° 58' and Magnitude 6.9 the Cluster looks like an artistic style impression of a comet. The star BZ Crucis forms the nucleus and the cluster the tail. The cluster members accompany the star, extend away and spray out to the northwest forming an irregular triangle. At both the south and northern end of the grouping lies a small triangle of stars. The part of the cluster nearest the star forms a prominent north-south bar, and the members extend away to the northwest. Auke called it oblong bow-tie shape, distinct box shape, and elongated rectangle. BZ is half way 5' between the cluster and Hogg 15
The Messier objects
The Bennett objects
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