Of the southern clusters, three have some similar characteristics, but differ much in their comparative ages. I have attached some written text on these clusters so that observers might be able to compare these for themselves. Figure B shows examples of the three 15'x15' STScI images of clusters of similar size.
The Second Jewel Box / Δ322 / Cr 225 / C1035-583
Position : 10h 37.3m -58° 39'
NGC 3324 is in Carina and is of similar size (5' arcmin) and age to NGC 4755. This cluster is 1.3° NW of Eta Carinae, and it is presently speculated to be part of the enormous nebulosity complex in this region. Overall, the cluster covers about 10', whose central 5' area is completely embedded in nebulosity (see Figure B). Adding all the stellar magnitudes, NGC 3324 would appear as a 6.9 mag star. The brightest star is the 06 spectral class 8.4 mag SAO 238269 / PPM 339253* / HD 92206 that appears on the edge of the nebulosity and causes the nebulosity to shine. It is also identified as the John Herschel pair HJ 4338. (Note: Megastar 4.0 says this star is 6.9 mag. This is clearly wrong, as the combined magnitude is 8.6.) The double star's components are listed as 8.9 and 9.6. and separated by 5.3" along PA 91°. HJ 4338's magnitudes to me seem slightly brighter than this, and I saw the colours were white and bluish. Since measures were first made in 1913, no change has been seen in the relative positions. Spectrally, the primary is an O6e or O5e class star, and it is likely this is the illuminator of the nebulosity.
Dunlop discovered this cluster in 1826, appearing as object (Δ322 in his catalogue list. Here he says:
“A star of 7th magnitude, involved in a faint nebula.”
The faint nebulosity makes it appear more like a small globular in 15cm. Larger telescopes begin to see some faint structure to the nebula. David Frew in "March Deep-Sky Objects" (Universe, 33, 3, p.9; March 1986) says of this nebulosity:
“This diffuse nebulosity is probably connected with the great Eta Carinae nebula, and hence lies about 7 000 ly away. It is fairly faint and amorphous, and surrounds the 8th magnitude double star h.4338. At least 15cm and rural skies are needed to see this one!”
Photographically, the cluster appears "horseshoe" shaped, whose exit points point due east. Regarding the nebulosity, NGC 3324 has few dark globules. The Trumpler classified NGC 3324 as '1 3 r n' and gave the first distance estimate of 3.3kpc, though latter-day values now favours about 3.8kpc. AOST2 gives the distance as 3kpc. Its age was first estimated to be some 2.2 million years, and this has slightly increased to about 3.5 to 3.6 million years. Overall, this is a nice cluster for medium to large apertures.
Mel 98 / Cr 215 / C1001-598
Position :10h 02.7m -60° 06'
This is another bright Carina open cluster that shows some variations of colour in the component 120-odd stars. It is 5° W of the Eta Carinae Nebula, and its 40' diameter appears very much larger in area than the Jewel Box. (If the Jewel Box were superimposed, it would only cover about twenty-five of the central stars of NGC 3114.) Overall this wonderful object is somewhat triangular shape, but some observers have also described NGC 3114 having six separate arms radiating from its core. Although similar in brightness to the Jewel Box (4.2 mag), NGC 3114 it is just visible in binoculars. At least 15cm telescope is required to show NGC 3114 with its hundred-odd stars shining in their full glory.
Dunlop said of this cluster ((D297);
“A beautiful cluster of stars, arranged in curvilinear lines intersecting each other, about 40' in diameter, extended south preceding, and north following.”
David Frew (Universe, 33, 3, p. 9; March 1986) says of NGC 3114:
“This is another large and conspicuous open cluster, about mag. 4 and 35' across. The brightest stars are about 8th magnitude, and are scattered over the face of the cluster, with no central gathering. The cluster is about 2 900 ly away.”
Quite near the centre of this cluster is the 6.18 mag HIP 49233 / SAO 237640 / PPM 357724. This is a white A6 II/III giant, and is by far the most luminous star of the cluster, and followed by about twenty-five 8th and 9th magnitude stars. It has been calculated that this cluster is 0.9kpc from us, and that the true diameter crosses about 8.5pc - about one star per parsec. Classed by Trumpler as '2 3 r -'. The age then estimated between 60 and 120 million years, though the higher values are still preferred.
Cr 246 / Rb 80 / C1111-632
Position : 11h 19.7m -63° 29'
Mel 105 is the third Carina southern cluster suitable for comparison. Mel 105 in reality is likely an older version of NGC 4755 and can be found 5.9' W from the Carina-Centaurus border. Its position is similar in declination to the multiple star Alpha (α) Crucis, some 7.5° further west. This 8.5 mag cluster has around seventy stars enclosed in its compact 4' arcminutes, and is contained in a pretty and starry field.
Small telescopes between 7.5cm and 10cm will see this object looking more like a slightly ovoid globular than an open cluster, and I still almost consider Mel 105 more as a "globular mimic" (See Figure B). (I have listed some thirteen other clusters that I consider mimics). Apertures between 20cm and 25cm, and using low powers, will continue to see the cluster as unresolved. Increasing the magnification begins to see some stars. In 20cm. I could see perhaps thirty tiny points, and I was certain for the want of more telescopic aperture, I would have seen more stars. The threshold magnitude must be about 13th. Most of the stars I was seeing were certainly 13th and 14th. I still wondered why I had never read or seen much amateur information on this wonderful cluster.
Presently, the distance is estimated at 2.1 to 2.2kpc, but extinction by interstellar absorption being so high towards this region of space makes the quoted results uncertain. Frandsen, Dreyer and Kjeldsen (Astron. & Astroph, 215, 287-304 (1989)) estimate from the CCD photometric data obtained from the 1.5-metre telescope at La Silla. Chile, an age of 150 million years. (Sky Catalogue 2000.0 suggests 59 million years, but this is likely too low.) Trumpler classes it as ' I 2 p -', though recently it was downgraded to 'I 3 m -'.
From the brightness, compactness and mass range of each of the component stars, there is no doubt that the Jewel Box is one of the youngest of clusters. Comparatively, the youngest of all is likely the magnificent open cluster NGC 6231 / Mel 53 / Cr315 (16h 54.0m -41° 47') placed on the back of the Scorpion's tail. This cluster is merely 3.2 million years old. Next in line is the famed naked-eye cluster known as the "Double Cluster of Perseus", NGC 884 / Mel 4 (02h 25.0m +57° 07') and NGC 869 / Mel 3 (02h 19.1m +57° 08'). Both are aged 3.4 and 5.6 million years, respectively.
Burnham (1955) also says, when comparing the Jewel Box with Perseus' famed "Double Cluster ":
“The Perseus group also resembles NGC 4755 in the possession of red supergiants, evidently objects of large mass which have already evolved away from the main sequence and are now nearing the point of ending the hydrogen burning reaction which powers the majority of stars. The bright blue stars of NGC 4755 will presumably begin their own evolutionary expansion in the near astronomical future.”
However. Burnham's comparison with the famed cluster is actually taken from Bidelman (1943), and possibly even from Johnston and Morgan (1955). After seeing the "Double Cluster" for myself in 1997, NGC 4755 is certainly far more exquisitely beautiful.
NGC 4755 follows all these clusters in age somewhere between 6 and 8 million years, sharing its commensurate age with many Trumpler clusters within the Eta Carinae Nebula / NGC 3322. (e.g. Tr 14, Tr 15 and Tr 16). Slightly older than this is NGC 2362 / Tau Canis Majoris Cluster / C0716-248 (07h 18.8m -24° 57') at 25 million years, followed by the brightest and best known open cluster of the Pleiades / M45 (03h 47.5m +24° 06') at 78 million years old.
Overall, what NGC 4755 does have in common with all these clusters are compactness and a similar Trumpler Class of Type 'I 3 r -'. However, unlike some other young clusters, we see no nebulosity. On the last point, to me, the literature seems notably scant explaining why.
“Southern Astronomical Delights” © Andrew James (2002) Sydney, Australia