OBSERVATIONAL COMPARISONS WITH
OTHER OPEN CLUSTERS
Perhaps as our northern cousins often ellude, observers are indeed
are impressed with the NGC 4755’s
beauty, but they always to be hesitant to claim it is the best in the
entire sky. An example of this appears in the recent book “Star Clusters”
by B.A. Archinal and S.J. Hynes (2004) who states on pg. 174;
“One of the most striking and
spectacular open clusters in the southern sky” This is a bold statement indeed,
but is it? Perhaps among the southern clusters I would find this
almost agreeable, however, there are many other clusters have their
own unique characteristics and particular charm. Two clusters that
are possible usurpers to the ‘crown
jewels’ of all clusters, is the almost
forgotten NGC 3114 / The Gem Cluster(10027-6006) and
the larger and more extensive scattered sister Carina cluster NGC
3532 / The Football Cluster (11055-5844). Both of these
clusterd contain many blue stars interspersed with a few prominent
yellow, orange or red stars. Often there own individual brilliance is
only foreshadowed and particularly lessened by the fact that they are
much larger — requiring only small apertures and/or very low
magnifications to encompass all the stars in the telescopic
field. Below is a general selection of must see
open clusters which can be observed during autumn (in the southern
hemisphere) in March or April. I have observed all these clusters and
eventually I hope to write some short text on all of them. A fuller
version of this selection appears in these pages, giving the Fifty Best Clusters in both the Southern and
Northern and skies, and the list of all 100
Selected Clusters. (Also see the more general discussion
can be seen in
the section apecificaly on Open Clusters, if required.)
BEST SOUTHERN OPEN STAR CLUSTERS
--------------------------------------------------------------------------------
NGC No. Other Names RA (2000) Dec Con Mag Size Class *n Source
hh mm.m dd mm v ′
--------------------------------------------------------------------------------
NGC 2287 Cr 118 M41 06 46.0 -20 45 CMa 4.5 38 I 3 r 80 Lynga
* NGC 2362 Cr 136 Mel 65 07 18.7 -24 57 CMa 3.8 6 I 3 r 60 Archinal
NGC 2422-78 Cr 152 07 36.6 -14 29 Pup 4.4 29 I 3 m 30 Lynga
NGC 2439 Cr 158 Mel 74 07 40.8 -31 42 Pup 6.9 10 II 3 r 80 Lynga
NGC 2437 Cr 159 M46 07 41.8 -14 49 Pup 6.1 27 II 2 r 100 Lynga
NGC 2451 Cr 161 07 45.4 -37 57 Pup 2.8 45 II 2 m 40 Lynga
NGC 2477 Cr 165 Mel 78 07 52.2 -38 32 Pup 5.8 27 I 2 r 160 Lynga
* NGC 2516 Cr 172 Mel 82 07 58.0 -60 45 Car 3.8 29 I 3 r 80
NGC 2547 Cr 177 Mel 84 08 10.2 -49 14 Vel 4.7 20 I 3 r 80 Lynga
NGC 2546 Cr 178 08 12.4 -37 37 Pup 6.3 40 III 2 m 40 Lynga
* IC 2391 Cr 191 o Vel 08 40.3 -52 55 Vel 2.6 49 II 3 m 30 Lynga
NGC 2660 Cr 193 Mel 92 08 42.7 -47 12 Vel 8.8 4 I 1 r 70 Lynga
IC 2395 Cr 192 08 42.5 -48 08 Vel 4.6 7 II 3 m 40 Lynga
Cr 203 Tr 10 08 47.8 -42 30 Vel 5.0 14 II 3 m 40 Lynga
* NGC 3114 Cr 215 Mel 98 10 02.7 -60 06 Car 4.2 35 II 3 r 171 Lynga
NGC 3228 Cr 218 10 21.4 -51 43 Vel 6.0 18 II 3 p 15 Lynga
IC 2581 Cr 222 10 27.4 -57 38 Car 4.3 7 II 2 p n 25 Lynga
* NGC 3293 Cr 224 Mel 100 10 35.8 -58 14 Car 4.7 5 I 3 r n 93 Lynga
* NGC 3324 Cr 225 10 37.3 -58 40 Car 6.7 5 I 3 r n 44
* IC 2602 Cr 229 Mel 102 10 43.0 -64 24 Car 1.6 49 I 3 r 60 Lynga
Cr 230 Tr 14 10 44.0 -59 33 Car 5.5 5 44 Lynga
Cr 228 10 44.0 -60 05 Car 4.4 14 98 Lynga
Cr 233 Tr 16 10 45.2 -59 43 Car 5.0 10 n 90 Lynga
* NGC 3532 Cr 238 Mel 103 11 05.5 -58 44 Car 3.0 55 II 3 r 150 Lynga
* NGC 3572 Cr 239 11 10.3 -60 15 Car 6.6 6 II 3 m n 35 Lynga
NGC 3766 Cr 248 Mel 107 11 36.3 -61 37 Cen 5.3 12 I 3 r 100 Lynga
* NGC 4755 Cr 264 Mel 114 12 53.6 -60 21 Cru 4.2 10 I 3 r 218
NGC 5281 Cr 276 Mel 120 13 46.6 -62 55 Cen 5.9 5 I 3 m 40 Lynga
NGC 5460 Cr 280 Mel 123 14 07.6 -48 18 Cen 5.6 24 I 3 m 40 Lynga
NGC 6067 Cr 298 Mel 140 16 13.2 -54 11 Nor 5.6 12 I 3 r 100 Lynga
NGC 6124 Cr 301 Mel 145 16 25.6 -40 39 Sco 5.8 29 I 3 r 100 Lynga
NGC 6193 Cr 310 16 41.3 -48 46 Ara 5.2 14 II 3 p n 14 Lynga
* NGC 6231 Cr 315 Mel 153 16 54.2 -41 50 Sco 2.6 14 I 3 p 93 Archinal
NGC 6242 Cr 317 Mel 155 16 55.5 -39 28 Sco 6.4 9 I 3 m 23 Lynga
NGC 6281 Cr 324 Mel 161 17 04.8 -37 53 Sco 5.4 8 II 2 p 70 Archinal
IC 4651 Cr 327 Mel 169 17 24.7 -49 55 Ara 6.9 12 II 2 r 80 Lynga
* NGC 6405 Cr 341 M6 17 40.7 -32 16 Sco 4.2 30 II 3 r 80 Lynga
* NGC 6475 Cr 354 M7 17 53.9 -34 47 Sco 3.3 80 I 3 r 80 Lynga
NGC 6494 Cr 356 M23 17 56.9 -19 01 Sgr 5.5 27 II 2 r 150 Lynga
NGC 6530 Cr 362 18 04.5 -24 21 Sgr 4.6 14 II 2 m n 113 Lynga
IC 4725 Cr 382 M25 18 31.7 -19 07 Sgr 4.6 32 I 3 m 30 Lynga
NGC 6705 Cr 391 M11 18 51.1 -06 16 Sct 5.8 13 I 2 r 682 Lynga
--------------------------------------------------------------------------------
BEST NORTHERN OPEN STAR CLUSTERS
--------------------------------------------------------------------------------
NGC No. Other Names RA (2000) Dec Con Mag Size Class *n Source
hh mm.m dd mm v ′
--------------------------------------------------------------------------------
NGC 884 Cr 25 Mel 14 02 22.3 +57 08 Per 6.1 29 I 3 r 150 Lynga
NGC 869 Cr 24 Mel 13 02 19.1 +57 08 Per 5.3 29 I 3 r 200 Lynga
Pleiades M45 03 47.5 +24 06 Tau 1.2 110 I 3 r n 100 Sky Cat
Hyades 04 26.9 +15 52 Tau 0.5 330 II 3 m 380 Archinal
NGC 2632 M44 08 40.1 +19 59 Cnc 3.7 95 II 3 r 50 Archinal
--------------------------------------------------------------------------------
* Praesepe or the Beehive Cluster
* Description available. More will be added as
time permits.
GENERAL CLUSTER DESCRIPTIONS
Of the southern clusters, three have some similar
characteristics, but differ much in their comparative ages. 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. However, there a quite a few examples of clusters with
similar ages and a few that are even younger.
Below 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
these three 15′×15′ STScI images of clusters of
similar size.)
NGC 3324
NGC 3324 / “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 magnitude star. The brightest star is the O6 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 5.0 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 1827, 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!”
Steve Crouch’s in his column “Southern Deep Sky” (Southern Astronomy Mar/ Apr 1994
p.55) says NGC 3324 is;
“...is a fairly large
nebula in a rich star field on the north-western side of Eta Carinae.
The most striking thing that I find about this nebula is its peculiar
shape, having a smooth and well defined edge on the northern side but
being very irregular on the opposite side. The shape is, of course,
clearer with a nebula filter.”
Photographically, the cluster appears to me ‘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.
NGC 3114
Mel 98 / Cr 215 / C1001-598
Position :10h 02.7m
-60° 06′
NGC 3114 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 (Δ297);
“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, that is about 4th magnitude and
subtends some 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 results seem
still currently preferred.
NGC 3293
Cr 224 / Mel 100 / Rb 85 / Ld 153 / C1133-579
Position
: 11h 35.8m -58° 14′ (U427)
NGC 3293 is another Carina open cluster that is of a similiar age
to the Jewel Box.
This cluster has an associated bright
nebulosity being Gum 30 covering some
28.3′×17.4″, that is wrongly given as also NGC 3293.
NGC 3293 is only the cluster. Gum 30 appears roughly triangular in
shape whose longest axis is almost perfectly aligned north-south.
Some ninety-three (93) components have been presently identified. In
brightness, the total magnitude is 4.7, while the estimated Trumpler
class is I 3 r n.
NGC 3293 is best found 40′S from the 4.5 magnitude orange
K3/K4II star, HIP 51849 / SAO 238222 / HD 91942 (10356-5733). The
cluster also happens to lie in quite a bright portion of the Milky
Way, being some 1.6° (PA 312°) from the very centre of the
bright Eta Carina nebula, NGC 3372 (10440-5930) which cover
2° of sky. The Eta Carina Nebulae is a pproximately centred on
another relatively young open cluster Tr 15 / Cr 231
(10447-5921). This small and moderately bright but contains only
about twenty stars. Alternatively, this cluster can be found about
32′NW of fellow cluster, NGC 3324 (10373-5839).
The brightest star lies 2.1′ NNW (PA 333°) from the
observed cluster’s centre. This is the
bluish-white 6.4v (6.7V) magnitude HIP 51857 / SAO 238225 / HD 91943
(10357-5781) From the B-V of +0.042 and spectral class B0.5Ib, this
is the most luminous star in NGC 3293 — equal in ovrall nature
to the brightest star in the Jewel Box cluster (HIP 62894) at the
apex of the A-shape.
Likely the Jewel Box there is also one
red-orangery supergiant that stands out. Lying 1.4°SSW
(204°), this is 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 prodominately
sticks out from the rest of the oher blue to white coloured stars and
does dominates the entire cluster. The B−V is +1.443 and is
placed at 10h 35m 44s -58° 14′ 42″. V361 Car is also
an irrgular LC variable star which varies between 7.09V and 7.57V.
This staris ideally placd for amateurs todo variable star observation
with numerous comparison stars. The magnitude variations are clearly
obvious, though it was certainly difficult to estimate a true
magnitude because of the colour diferences — a classic example
of the so-called Purkinje Effect.
A third prominent 6.5 magnitude
white star at the very centre ofthe cluster. This is 6.4v magnitude
HIP 51866 / SAO 238230 / HD 91969 which, like HIP 51857 above, is not
a variable. An accurate position is 10h 35m 49s -58° 13′ 27″, and the
star has a B−V of +0.009 and with the given spectral class of
B0Ia.
The most recent age for NGC 3293 is 10.3 m.yr as stated in Dias,
W.S., et al. “New Catalog of
Optically Visible Open Clusters and Candidates”, A.&A., 389, 871 (2002).
Distance is estimated to be 2.3 kpc.
Observational Descriptions
NGC 3293 was discovered by Abbe Nicholas Louis de la Callé
from the Cape of Good Hope in 1752, who listed the cluster (L II
8);
“Small heap of 4
small stars to an lozenge.”
The 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 coordinates 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 is stated as being the next to observed the cluster which
stating he saw acluster on four seperate occassions 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 centered only some 1.2′ENE (PA 68°) on the cluster
NGC 3324 and its bright nebula, IC 2599. Both these clusters (NGC
3293 and 3324) can be matched with Dunlop’s description but NOT in the given position.
Something is simply wrong here, and so we cannot conclude that
Δ321 is NGC 3293.
John Herschel only
describes the cluster through his small refractor describing it 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 Herschel in his
relentless pursuit of Dunlop double stars and deep-sky objects.
Another problem is also with Lacaillé’s position, which Herschel certainly had in
his possession during this main southern survey. This problem is
simply perpetuated in many references. For a cluster which is
so comparable with the Jewel Box - of which we have copious notes.
This single failure is likely the whole cause of the neglect from
visual observers een today. Burnham’s
Celestial Handbook says both oddly and non-descriptly, NGC 3293 is
an; “Open
cluster, bright, rich, 8′ diameter, about 50 stars 6..13th mag,
with dark nebulosity to the south.” E.J. Hartung comments in
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 diferent colours. R is about 900pc.”
” There is much to comment
on this AOST1 description. The concentric structure he refers too
does not seem me visible in either 20cm or 30cm — the same
aperture that Hartung uses. Secondly none of the nebulosity is
described at all, which is quite visible in dark skies using 30cm and
averted vision. Thirdly Hartung fails to mention anything about the
surrounding nebulosities at all. Another small problem is the
distance, which is now estimated to be two-and-a-half times further
at 2.3 kpc. Also within AOST1’s
pages 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. Overall I straight
away thought this is a worthy and true rival to the Jewel Box.
In AOST2 this 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 abright red
supergiant which contrasts nicely with the other cluster members. It
is a fine object in even the smallest telescopes, and is about
2800pc. distant. ” 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.” While South African
observer V. Hirsch in the see reference above correctly
describes; “Seems
like a globular in the finder”
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” ...Ed.]” In April 2005 Australian Sky and
Telescope (Frew, D., “The Eta
Carinae Region” A.S&T.,
1,4, p.60-61 (2005)), David Frew describes NGC 3293 as; “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′ 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 the
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 perpetuate it any future. 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.” Even in
the very recent (2004) book by Archinal, B., Hynes, S. “Star Clusters”, Catalogue pg.41-110 Pub. by Willmann-Bell.
(2004), NGC 3293 doesn’t even rate a
mention in the ‘Extended Notes’ !
[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, appearing as a
faint but largish haze.]
Double Stars in NGC 3293
Desig. Pos.(2000) Mag Mag Sep PA Yr Sp
BRT 3145 (10355-5816) 10.6 12.8 4.3 40 1924
DAW 52 BC (10357-5814) 9.5 12.5 2.3 35 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 10 --
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
Variables in NGC 3293
Desig Pos.(2000) Max Min Type P(d)
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: --
Mel 105
Cr 246 / Rb 80 / C1111-632
Position : 11h 19.7m -69°
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′ arcmin, 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, distance is estimated at 2.1 to 2.2kpc,
but the extinction by interstellar absorption being so high towards
this region of space, makes the quoted results uncertain. Frandsen,
Dreyer and Kjeldsen (A&A., 215, 287 (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 -’.
NGC 6231
NGC 6231 / Mel 53 / Cr315
16h 54.0m -41°
47′
NGC 6406 / M6
NGC 6405 / The Butterfly Cluster / M6 / Mel 178 / C1736-321
(17401-3213)is one of two of the bright naked-eye open star clusters
in the southern region of Scorpius. This most beautiful of open
clusters was first noted in Ptolemy’s
‘Almagest’ as a lucid spot. Edmond Halley listed it as
a “nebula” No.20 in his observations from an
expedition to the island of St.Helena in the South Atlantic Ocean for
his ‘Southern Star
Catalogue’. De Cheseaux, however,
was the first to identify its stellar nature in 1745. Lacaille found
it again in 1752, while Messier listed the cluster on the same night
as when found for M7 on the 23rd May, 1764. Flammarion described its
shape as being like; "“...three
starry avenues leading to a large square.”
Diameter is some 20' to 25'min.arc. translating to the physical
size of 8 to 12 ly. By the Shapley classification it is type ‘e', or by the Trumpler system as III 2 p.
Total visual magnitude is 4.2, though later assessments place it
fainter, at 5.0 to 5.6. To the naked-eye, it lies almost centrally
within the Milky Way, and although this is true, the magnitude
extinction is about 0.5.
An estimation of age is about 51 million years, though some have
quoted (like the IAU reference) ages between 75 and 100 million
years. Stellar numbers show about eighty (80) stars between spectral
classes B5 and A2, with 28 stars above 10th magnitude.
Distance is estimated to be 460 pc. or 1 500 ly. (400 pc. or 1 300
pc. in AOST II), though earlier catalogues and other references give
values somewhere between 490pc. to 600pc. (I.e. Sky Catalogue 2000.0
states 600 ly.) The best estimation is more likely nearer to the 500
pc. mark.
NGC 6405 contains several variables. Brightest is the obvious
yellow star BM Sco (HD 160371) which is a supergiant
semi-regular or SRd type variable. This is the only a few truly giant
variable sars known in any open star cluster. Magnitudes varies
between 6.8p and 8.7p in a average period of 815 days. Spectral data
reveals a K2.5 star whose luminosity class has been place more
recently between II and III, downgrading to a mere giant. It is
likely that its not an Ib type, as stated in Sky Catalogue 2000.0.
Within the cluster, the brightest star is 6.17 magnitude, while
the second star, HD 160202 is a known flare star that was
observed in 1965 to brighten by five magnitudes over a period of
twenty minutes.
NGC 6475 / M7
NGC 6475 / M7 / Mel 183 / C1750-348 (17536-3449) is another
naked-eye bright cluster in the southern part of Scorpius,
3.9°SW of the Butterfly Cluster / M6. Edmund Halley catalogued
it as “nebula” No.29 in 1677. Lacaillé was first to telescopically discover the cluster in 1755, but its first fame was its listing in the Messier catalogue in 1764, and is the furthest
southern object in the entire listing. The “nebulous” spot
was noted in Ptolemy’s ‘Almagest’. Although unusually the clusteer does not
have a common or proper name, M7 it is actually brighter than its
neighbour M6.
Telescopically this is an impressive object, worthy of
low-magnification and a wide-field eyepiece. Even in binoculars it is
wonderful. Members total about fifty (50) blue-white (B0-B5) stars,
with an apparent diameter of about 1°. Total visual magnitude of
the cluster is 3.2 to 3.3, with the brightest star of the cluster at
5.6 magnitude. It is type ‘e’, or ‘II 2
r’.
Like its nearby cousin M6, the modern distance of M7 is estimated
to be 400 pc. or 1 300 ly. This is some 60% further than the earlier
estimates, that were stating about 240 parsecs or 720 ly. Age has
been judged from the turn-off point of its colour-magnitude diagram
as 220 million years — being twice the age of the nearby M6.
More recent estimations between 1985 and 1992 suggest that its age is
younger. Accordingly, such as the IAU recommended data, the age maybe
as young as 128 million years.
Within the cluster is an bright orange-red star. This is the
double star SEE 342 (6.4 / 6.5 40° 244° (1996)),
that is unresolvable in telescope (1997) below about 0.6m. —
and this would even require optimum seeing.
V907 Sco / JSP 935 (17569-3445) is an EA/DM type eclipsing
variable star some 2.0° due west. Variations in brightness is
between 8.6 to 9.2 magnitude in a period of 3.776 days. Spectral type
is A0, indicating a surface temperature of 10 200K.
8.61V —
9.2 JDE : 2414862.585 Period: 3.776277 A0
V 907 Sco is also the
double star JSP 935 (17569-3445), which was discovered by Jessep but
not measured by him in 1911. Listed as 8.7v and 11.5v magnitude the
faint duo is separated by 9.1 along position angle 288° (1998),
which has continued to remain fixed. Spect: B9V
Surrounding Fields
NGC 6453 (17508-3536) is a smallish globular cluster hat
lies in Scorpius some 2.1° due east of M7. At 10.2v magnitude
its small diameter is only 7.6′. The Shapley type is class IV,
indicating that it is fairly condensed in moderately sized
telescopes.
17h 50m 51.8s -34° 35′ 55″
Mag:
10.2 Mag V(tip): 14.3 Mag V(HB): 17.5 Size: 7.6′
NGC 2362
NGC 2362 / Tau Canis Majoris Cluster / Mel 65 /
Cr136 (07187-2457) is a bright open cluster / nebula combination
surrounding the yellow orange star Tau Canis Majoris. This rather
elegant and tightly knitted group was discovered by Sir John Herschel
in 1831 is one of the youngest open cluster knowm, whose total star
count is about sixty (60).
Rev. Thomas Webb in his now famous “Celestial Objects the Common Telescopes”, makes no mention of this cluster, although
the actual star of Tau Canis Majoris is listed as the 30 CMa and as
the John Herschel triple, HJ 3948.
NGC 2362’s total magnitude is 3.8v,
while the size is merely 8′. The Trumpler classification of
this object is “1 3 p n”.
τ CMa / 30 CMa / HIP 35415 / SAO 173446 / PPM 252114 /
HD 57061 (0718) itself is a remarkable supergiant star. At 4.4
magnitude this bright white with perhaps with a tint of blue star has
a spectrum of ‘Oe’ — or the
later O9Ib, and shows several prominent emission lines. Tau CMa is
also the multiple HJ 3948 AB, AC and AD / FIN 313 Aa
(07187-2457).
HJ 3948Aa,B 1835 1999 12 86 90 8.0 8.1 4.42 10.2
HJ 3948Aa,C 1835 1999 8 73 79 15.0 14.5 4.42 11.2 O9II
HJ 3948Aa,D 1834 1999 9 73 77 85.0 84.4 4.42 8.22 O9II -007-005 -24 5180
FIN 313Aa 1951 1996 32 110 125 0.2 0.1 5.33 4.89 O9II -002-001 -24 5176
For the AB pair, the last measures made in 1876 show little
change, and the WDS05 gives the separation as 8.1″ along a near
due east of PA 79°. This remarkable star is certainly very
luminous, and the dubious parallax from the 1.02±0.71mas,
suggesting a distance of 1 kpc. / 1000 pc. Stellar proper motion is
also small and fairly dubious.
pmRA -1.82±0.45 pmDec
3.74± 0.59
Some 2.5′E (PA 93°) is the 10.5 magnitude variable
star, IM CMa (07189-2457). This is a suspected ellipsoidal
eclipsing binary or ELL: type whos brightness variaion change between
10.52b and 10.58b is an undetermined period. Little else is known
about the system.
One of the most delightful portions of the sky for the
amateur is the southern half of Canis Major, which contains varieties
of interesting and varied objects embedded in the southern Milky Way.
The northern region of this constellation is the brightest and
brilliant blue-white star of Sirius, the open star cluster of M41,
and assorted varieties of other faint objects. The south of the
constellation contains the ‘Y-shaped’
asterism of the other bright star stars, which includes Delta,
Epsilon, Eta and Omicron Canis Major. Surrounding this area is quite
fascinating as there are a number of very interesting objects that
are included in this article, what I call simply ‘The Dog’s
Tail’.
Some 23′N of Tau CMa is the white 5th magnitude star 29 CMa.
This star is the eclipsing variable star UW CMa.
The catalogue variation is brightness is between 4.84V and 5.33V,
and this occurs in a period of 4.393407 days (Epoch 2436185.358) The
spectrum of the two is O7 IA:fp + O8.
HJ 3948 Aa-D 07 18 42.0 -24 57 00 4.4 8.2 85.0 --- 75 -- 1834 1960 O9II
FIN 313 Aa 07 18 42.0 -24 57 00 5.1 5.1 0.2 --- 110 122 1951 1989 O9II
HJ 3948 Aa-B 07 18 42.0 -24 57 00 4.4 1.0 8.5 --- 89 -- 1876 1960
HJ 3948 Aa-C 07 18 42.0 -24 57 00 4.4 11.2 14.8 --- 79 -- 1876 1960 O9II
Doubles in the vicinity includes;
Desig Cmpnts RA Dec PMag SMag Sep1 Sep2 PA1 PA2 Yr1 Yr2 Sp
BRT 2893 07 18 48.0 -24 49 00 11.0 --- 4.7 --- 186 -- 1910 --
B 2140 07 18 48.0 -24 40 00 8.3 12.0 7.0 --- -- -- -- -- B2V
B 2141 07 19 00.0 -24 39 00 9.0 13.0 6.0 --- -- -- -- -- A0
B 133 07 19 12.0 -24 57 00 6.8 14.2 4.6 --- 99 -- 1926 -- B3IV
B 134 07 19 18.0 -24 53 00 9.5 12.5 1.0 --- 98 -- 1926 --
B 2142 07 19 54.0 -25 00 00 8.6 15.0 6.0 --- -- -- -- -- B6IV/V
IC 2602
IC 2602 / The Southern Pleiades / Mel 102 / Cr 229
(10430-6424) in Carina is an extended cluster covering 0.8° or
49.0′ and is the bright of the cluster in the southern skies
— and third in the rankings of the entire sky behind the
Plieades and Hyades. This conspicuous cluster has a total magnitude
of 1.6,and is centred around Theta Carinae. θ Car / HIP
52419 / SAO 251083 (10430-6424) is a richly blue coloured star of
2.8v (2.74V) magnitude. θ Car has a B-V of -0.220 with the
spectral class of B0Vp.
The remaining stars are start about magnitudes fainter, whose
current population is estimated to be around sixty (60) Trumpler
classed IC 2602 as ‘I 3 r’
Last Update : 17th July 2012
Southern Astronomical Delights ©
(2012)
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