NGC 3293 is another Carina open cluster that is of a similar age to the more famous open cluster known as the Jewel Box / NGC 4755. This wonderfully attractive cluster has an associated bright nebulosity being Gum 30 covering some 28.3 × 17.4 arcmin - but sometimes this given designation is wrongly NGC 3293 In fact, NGC 3293 is only the cluster. Gum 30 appears roughly triangular shape object, whose aligned longest axis is roughly north to south. Ninety-three (93) components have been identified, with the cluster’s brightness has the total magnitude is 4.7, or 8.5 magnitude based on the 5th brightest star. The Trumpler class has been established as I 3 r n.
The cluster also happens to lie within a bright portion of the Milky Way, being placed some 1.6° NW (PA 312°) from the very centre of the bright 2° across Eta Carina Nebula / NGC 3372 (10440-5930). This great region of star-bearing nebulosity finds near its centre another relatively young open cluster Tr15 / Cr 231 (10h 44.7m -59° 21′) which is small and moderately bright, but contains only about twenty (20) stars.
Cluster NGC 3293 is best found 40′S from the 4.5 magnitude orange K3/K4II star, HIP 51849 / SAO 238222 / HD 91942 (10h 35.6m -57° 33′). Alternatively, it can be found about 32′ NW of sister cluster, NGC 3324 (10h 37.3m -58° 39′).
The CLUSTER STARS
The brightest cluster star lies 2.1′ NNW (PA 333°) from the observed cluster’s centre. This is the bluish-white 6.4v (6.7V) magnitude V513 Car/ HIP 51857/ SAO 238225/ HD 91943 (10357-5712)- an ACYG-type variable star whose brightness varies by only 0.04 magnitudes. From the B-V of +0.042 and spectral class B0.5Ib, this is likely the most luminous star in NGC 3293 - equal in overall nature to the brightest star in the Jewel Box cluster (HIP 62894) at the apex of the A-shape.
One star is a red-orange supergiant that particularly stands out like kappa Crucis in the Jewel Box. Lying 1.4°S SW (204°) of the cluster’s heart, this 7.1 magnitude star does not appear in Hipparcos Catalogue but only in the Tycho one; as T 8613:1280:1. Listed also as V361 Car / SAO 238228 (10357-5815) this star predominately sticks out from the rest of the other blue to white coloured stars and does dominate the entire cluster. The B-V is +1.443 and is placed at 10h 35m 44s -58° 14′ 42″.
V361 Car is also an irregular LC variable star that varies between 7.09V and 7.57V. This ideally placed star for variable star amateurs has numerous comparison stars, whose magnitude variations are obvious. Some have stated that estimating the true magnitude of V361 Car was difficult because of the colour differences - a classic example of the so-called Purkinje Effect.
The third prominent 6.5 magnitude white star at the very centre of the cluster. This is 6.4v magnitude HIP 51866 / SAO 238230 / HD 91969 which, like HIP 51857 above, is not a known variable. An accurate position is 10h 35m 49s -58° 13′ 27″, and the star has the B-V of +0.009 and with the given spectral class of B0Ia.
EARLY HISTORY of NGC 3293
Abbe Nicholas Louis de la Callé from the Cape of Good Hope discovered NGC 3293 in 1752, and listed the cluster (L II 8) —
“Small heap of 4 small stars to an lozenge.”
However, his early positions of this cluster remains in some dispute. Lacaillé gives the 1st Jan 1752 positions as 10h 26m 32s -56° 56′ 05″ which is in 2000 co-ordinates is 10h 35m 57s -58° 12′ 49″. This is only about 1.2′ WSW (PA 239°) from the centre of NGC 3293 - quite good considering the equipment he used. It is certainly more precise than Dunlop’s later placement, if it is indeed NGC 3293 and not NGC 3324 as concluded below;
Kenneth Glyn-Jones in “Search for the Nebulae” Pub. Alpha Academic (1975) pg.48 wrongly states;
“Class II Object 8 : Not identified: nearest object is 40′ north of Lacaillé’s position. This is IC 2581, Clust. diameter 5′, mag. 5.2”
Dunlop states this as being the next to observe it, and stating he saw the cluster on four separate occasions during 1827. Here he describes DUN 321 / Δ321),as;
“A very small cluster of very bright small stars; round figure, above 4′ diameter; rich in the extremely small stars resembling faint nebula”
Dunlop’s 1827 position for NGC 3293 is 10h 27m 13s NPD 32° 37′ which translates into today’s 2000 co-ordinates of 10h 33m 45s 58° 16′ 27″ and is 17.3′E (PA 81°). This is centred only some 1.2′ ENE (PA 68°) on the cluster NGC 3324 and its bright nebula, IC 2599. His descriptions of both the open clusters of NGC 3293 and 3324 cannot be easily matched with his given positions.
Something is simply wrong here, and we cannot conclude that Dunlop’s Δ321 is actually cluster NGC 3293.
John Herschel dismally describes the cluster through his small refractor as;
“A fine bright rich not very large cluster.”
Quite amazingly did not gaze upon NGC 3293 with his 46cm f/13 speculum mirrored telescope. Why this did not happen is a mystery, because he would have surely seen the nebulosity - not to mention such a glorious sight. I am also surprised that Dunlop’s quite accurate description was not followed up by John Herschelin his initial relentless pursuit of Dunlop double stars and deep-sky objects. Another problem is also with the positions of Lacaillé. Herschel certainly had in his possession his catalogue during this main southern survey, but some how this same issue is simply perpetuated in many of future references.
This single failure of any enthusiasm by John Herschel is likely the whole cause of the nearly unforgivable neglect from these modern visual observers. Today, NGC 3293 is comparable with its sister cluster the Jewel Box - of which we incidentally also have copious notes.
SOME MODERN OBSERVATIONAL DESCRIPTIONS
a.) Burnham’s Celestial Handbook says both oddly and nondescriptly, NGC 3293 is an;
“Open cluster, bright, rich, 8′ diameter, about 50 stars 6..13th mag, with dark nebulosity to the south.”
b.) E.J. Hartung comments in “Astronomical Objects for Southern Telescopes” : (AOST1);
“Marked concentric structure is known by this beautiful open star group which is somewhat irregularly round and about 5′ across; it is bright, with stars of different colours. R is about 900pc.”
There is really much to comment on this AOST1 description;
1) The concentric structure he refers too does not seem to me visible in either 20cm or 30cm - the same aperture that Hartung has used.
2) Hartung says nothing about the nebulosity, even though it is readily visible in dark skies in 30cm with averted vision.
3) Hartung fails to mention anything about the surrounding nebulosities at all. Another issue is the distance - now estimated to be two and-a-half times further at 2.3 kpc.
Within the pages of AOST1 as well are two black and white plates (Plates 3 and 4) taken by A.R. Hogg that show both the Jewel Box and NGC 3293 (Hartung Object Nos. 387 & 497). These are worthy comparisons to look at. If anything, I think the stars of NGC 3293 are even more stunning than the Jewel Box. I can remember comparing these two clusters in AOST1 when I first became interested in astronomy, immediately thinking that I must look at each in succession with my own eyes. I straightaway thought this is was a worthy and true rival to the more famous Jewel Box.
d.) In the Second Edition (AOST2) the information on this cluster is vastly improved. David Frew appropriately adds;
“Called the Gem Cluster by H.C. Russell, it is reminiscent of the Jewel Box (NGC 4755), containing a bright red supergiant which contrasts nicely with the other cluster members. It is a fine object in even the smallest telescopes, and is about 800pc. distant.”
e.) American J. Corder states in the Webb Society Deep Sky Observer’s Handbook; Vol.7 “The Southern Sky” pg.38; who sees NGC 3293 as;
10.5cm “Small, compact and easy cluster. 12 stars seen at 30×, Angularly round.”
f.) While South African observer V. Hirsch in the see reference above correctly describes;
“Seems like a globular in the finder”
g.) J. Graham Little wrote about NGC 3293 in his “Ten O’clock High” (Southern Astronomy, Mar/ Apr 1994 p.54-55);
“Another smallish but nice cluster,
once again visible through ’scopes of 6cm or more.
[It] has a diameter of about 6′… [and] the 50
stars being from mag. 6.5 to approx. 13.0. Visually this
is a fairly condensed little cluster which stands out well
from the background stars. Colours present are white, blue,
green and red. There is some faint nebulosity present, with
a patch of dark nebulosity to the south.
“[H.C. Russell] referred to this as the “Gem” cluster in an 1879 paper: it is now sometimes called the “Diamonds and Rubies Cluster””
h.) In April 2005 Australian Sky and Telescope (Frew, D., “The Eta Carinae Region” A.S&T., 1,4, p.60-61 (2005)). He usefully describes:-
“A beautiful bright open cluster… A good 6cm refractor will show the nature of the group, which in my opinion is the best in the southern skies for small telescopes. Note how the bright stars are crowded into an area only about 6&8242; across, and arranged almost concentrically. As a bonus V361 Carinae is found… and contrasts beautifully with the other cluster members. The star is similar in mass and luminosity to Betelgeuse.”
Amazingly, the cluster also does not appear in the Jack Bennett Southern catalogue! When I first read the catalogue I was quite impressed with this neo-Messier Catalogue, but quickly dismissed it as disappointing for not containing NGC 3293 (and one or two others) since it has many other much fainter objects and some of lesser importance. Since this time I have never referred to Bennett’s Catalogue in any of my observational descriptions - mainly not wanting it to be perpetuated it any future.
i.) Mike Inglis in “Astronomy of the Milky Way : Observer’s Guide to the Southern Sky” (2004);
“One cluster that is perhaps best suited to smaller telescopes is NGC 3114. It has about 50 stars spread over a region 35 arcseconds in diameter. The total magnitude of the cluster is 4.2, and it is interesting to compare this cluster with the previous one, NGC 2516, to see which one you find the most pleasing. The cluster NGC 3293, also called the Gem Cluster, is a very nice group to look for (see Figure 3.6). It is bright but small, at about 6 arcseconds in diameter, and has a bright red supergiant star at its center which makes a nice contrast with the other members.”
Appallingly, even the recent book by Archinal, B., Hynes, S. “Star Clusters” (2004), Catalogue pg.41-110, published by Willmann-Bell, NGC 3293 does not even rate even the small mention in the “Extended Notes.”
I really do wonder how they miss this one!
[ A similar problem to this is faced with one of the Top 20 brightest open cluster, the magnificent NGC 2516. This cluster is even visible in the city skies, appearing as a faint but largish haze. ]
In van den Bergh and Herbst “Catalogue of Southern Stars Embedded in Nebulosity”; Astro.J., 80, 3 (1975) list two reflection nebulae involved with this cluster. The first, BHe 42A, has the illuminating star embedded within it, the blue surface brightness is moderate, it is very much brighter on red plates where it has a maximum diameter of 4′ (on blue plates it is only 1.6′ across).
The second nebula, BHe 42B, which lies just to the north and west, lies just outside the illuminating star. It’s blue surface brightness is moderate, it is distinctly brighter on the blue plates on which is has a maximum diameter of 1.6′.
Colin S. Gum, in “A Survey of Southern H II Regions” RAS Memoirs, 67, 155 (1955) also described the nebula. He describes Gum 30 as:-
“…a more or less detached outer condensation of the Carina complex. There is considerably clustering of B stars apparently associated with the nebulosity. The cluster NGC 3293 whose corrected position is 10h 32.0m -57° 43′ (1900) is situated towards the edge of the nebula and contains.”
He presents the luminous stars causing the nebulosity to glow, as:-
HD 91943 7.1 B0 9.8 mag HD 91968 6.9 B0 HD 91983 8.9 B HD 92007 9.1 B HD 92024 9.1 B HD 92044 8.5 B0Nearer the nebulosity’s centre are:
HD 91824 8.2 Oe5 12.0 HD 91850 9.1 B2 HD 30306 8 B
[ Here magnitudes in the second column are photographic and the number in the last column is the distance modulus. ]
“The mean of these two widely differing moduli available place the nebula at a distance of 1500 parsecs. This is equivalent to the distance of the whole Carina complex in which it appears to be associated.”
Gum gives the maximum diameter of the nebula, which appears roughly circular, as 40′. The intensity, or…
“…visibility in the particular section of the Milky Way in which the object occurs, and is rated as “moderately bright” on a scale of vf - f - mb - b - vb. In his scheme of classifying the large-scale structural features of nebulae, the nebula is rated a ‘II’, which corresponds to; “irregular in shape with dark matter, but the concentration towards the centre is less marked and the central intensity is much less than Class I.” ”
AGE and DISTANCE
The most recent age for NGC 3293 is 10.3 million years as stated in Dias, W.S., et al. “New Catalog of Optically Visible Open Clusters and Candidates”, A&A., 389, 871 (2002). Currently the distance is calculated around 2.3 kpc.
Double Stars in NGC 3293
Desig. Pos.(2000) Mag Mag Sep. PA Year Sp (WDS) A B BRT 3145 (10355-5816) 10.6 12.8 4.3 040 1924 DAW 52 BC (10357-5814) 9.5 12.5 2.3 035 1918 DAW 51 (10358-5812) 9.4 12.0 2.3 183 1918 B1 B 2251 Aa (10358-5814) 8.5 14.0 3.0 010 -- DAW 52 AB (10358-5814) 8.5 9.5 14.2 109 1918 B0 DAW 198 (10358-5815) 8.7 9.1 11.8 344 1918 B0 DAW 197 (10359-5813) 8.9 9.3 12.8 329 1918 B1
WDS07 BRT 3145 (10356-5816) 1924 1924 1 40 40 4.3 4.3 10.6 12.8 -008+007 -57 3492 103535.7 -581650 DAW 52AB (10358-5814) 1893 2000 7 109 108 15.0 14.2 8.5 9.5 B0.5V +000+013 -57 3506 D 103546.5 -581412 DAW 52BC (10358-5814) 1918 1918 1 35 35 2.3 2.3 9.5 12.5 -006+005 103548.5 -581416 B 2251Aa (10358-5814) 1 10 10 3. 3. 8.5 14. 103546.6 -581410 DAW 197 (10360-5812) 1895 2000 5 332 330 13.0 12.8 8.82 9.12 B1 -005+003 -005-002 -57 3524 D 103558.62-581232.4 DAW 198 (10360-5815) 1895 2000 5 344 345 11.7 11.9 8.7 9.1 B0.5III -57 3526 D 103558.9 -581420
BRT 3145 (10356-5816) is a pair 1924 1924 1 40 40 4.3 4.3 10.6 12.8 -008+007 -57 3492 103535.7 -581650
Variables in NGC 3293
Desig Pos.(2000) Max Min Type P(d) Mag. Mag. V400 Car (10348-5809) 9.76v -- BCEP -- V401 Car (10355-5812) 9.56V -- BCEP -- V402 Car (10355-5815) 9.91V 9.97V? GCAS -- V403 Car (10357-5813) 8.78V -- BCEP -- V361 Car (10357-5815) 7.09V 7.57V LC -- V412 Car (10358-5814) 9.85V 9.88V BCEP: -- V438 Car (10358-5815) 11.25V -- ELL: -- V404 Car (10358-5814) 9.21V 9.23V BCEP -- V405 Car (10358-5813) 9.29V -- BCEP -- V439 Car (10359-5814) 13.46V -- BE: -- V378 Car (10359-5815) 9.22V -- BCEP: 0.188 V440 Car (10359-5813) 9.14B -- BCEP -- V406 Car (10360-5812) 9.26V -- BCEP -- V379 Car (10360-5814) 8.21V 9.34V BCEP+E: 0.1753 V380 Car (10360-5815) 8.95V -- BCEP: 0.236: V381 Car (10361-5813) 9.04V 9.06V BCEP: 0.17: V441 Car (10362-5814) 13.51V -- ELL: --
NGC 3293 is a delightful open star cluster to observe regardless of the aperture used. It remains among the very best deep-sky objects that the southern skies has to offer - that is ofcourse if you can by-pass the even more brilliant draw-card of the vastly more popular Eta Carina Nebula.
01. Archinal, B., Hynes, S. “Star Clusters” (2004)
02. Burnham, R., “Burnham’s Celestial Handbook : Volume 1”, Pub. Dover Press (1978)
03. Corder, J. “Webb Society Deep Sky Observer’s Handbook; Vol.7 : “The Southern Sky” pg.38; Pub. Webb Society (1987)
04. Dias, W.S., et al. “New Catalog of Optically Visible Open Clusters and Candidates”, A&A., 389, 871 (2002).
05. Frew, D., “The Eta Carinae Region” A.S&T., 1, 4, p.60-61 (2005)
06. Frew, D., Malin, D. “Astronomical Objects for Common Telescopes” 2nd Ed. Cambridge University Press (1995)
07. Gum, C.S., “A Survey of Southern H II Regions”; RAS Memoirs, 67, 155 (1955)
08. Hartung, E.J., “Astronomical Objects for Common Telescopes” Cambridge University Press (1968)
09. Inglis, M., “Astronomy of the Milky Way : Observer’s Guide to the Southern Sky” (2004)
10. Jones, K.G.; “The Search for Nebulae”, Alpha Academic (1975)
11. Little, G.L., “Ten O’clock High” Southern Astronomy, Mar/ Apr p.54-55 (1994)
12. Lynga, G., “Catalogue of Open Cluster Data.”: Lund Observatory, Lund, Sweden. (1964)
13. van den Bergh and Herbst, “ Catalogue of Southern Stars Embedded in Nebulosity”; Astro.J., 80, 3 (1975)
14. Webb, Rev. T.W.; “Celestial Objects for Common Telescopes.” pg.237, Dover Press (1962)