Some of these swarms of old stars are Outer-Halo members of the Large Magellanic Cloud and are far away from it in the southern sky, appearing as "isolated" objects.
Reticulum cluster
R.A. 04 36 09.0 Dec. -58 51 30.0 (J2000.0)
Once there, I observed the field at low magnification, knowing in advance that this is a faint globular for this kind of telescope. After identifying the star GSC-8515-1355 (magnitude 12) and the pair TYC 8515-1611-1 and GSC-8515-0765 (visual magnitudes 10.7 and 11.8 respectively) I focused on the small region between that stars (center of the field of view on Figure 3) to try to glimpse this extragalactic globular. Only a few faint stars are visible there at this magnification, surely foreground stars of our Milky Way galaxy.
At higher magnification (63x), the view is similar. The same stars are visible (labelled with blue circles in Figure 3). Even higher power (118x) did not show more than that faint stars scattered in the area. The faint star labelled with a red circle could be very barely glimpsed using averted vision at this last magnification.
As a final remark, this globular cluster is a faint object for 8-inch telescope. Owners of bigger mirrors should be glimpse this member of the Large Magellanic Cloud when observing from a dark sky site.
ESO 121-SC03 lies at a projected angular separation of ∼ 10◦ from the LMC centre. It was first studied in detail by Mateo et. al. (1986). David Stevenson in his book "The Complex Lives of Star Clusters" (2015) estimates the age of this globular in around 9 billions years.
At 42x, the field where this object, also known as KMHK2 1591, is situated looks interesting with some stars forming fine shapes. The brightest star in the 1◦ field of view is the 6.4 magnitude SW Pictoris. The chain of four stars (indicated with blue lines in Figure 4), which contains the 6.9 magnitude star HD 41451 at its eastern end, and the stars forming a sort of crown (center of the field in Figure 4) helped to find the zone where this faint and obscure member of the Large Magellanic Cloud lies. This 14 magnitude cluster with angular dimension 2.0 x 2.0 arc minutes is, without a doubt, a faint object for an 8-inch telescope. Obviously at this low magnification nothing was visible in the field.
According with the paper "Photometry of Magellanic Cloud clusters with the
Advanced Camera for Surveys - II. The unique LMC
cluster ESO 121-SC03" by A. D. Mackey et al. (2006), this cluster may lie 20 per cent closer to us than does the centre of the LMC.
At 78x, ESO 121-SC03 is not visible after observing carefully under a dark and steady sky. I observed it again from another place, higher in Los Andes mountains, but I had the same results after using 118x. In both nights and from both sites, something seems to be there when observing with averted vision, but it is hard to assure that the cluster can be glimpsed. As I always say, if you are not sure you saw any object, you must say "I did not see it".
The brightness of this extragalactic object reserves the view only for bigger instruments. Higher magnification (e.g. 160x, 222x) gave the same results, no object visible in the field.
John Frederick William Herschel discovered NGC 1466 in 1834, a globular cluster with a magnitude of 11.4 situated in constellation Hydrus. It is listed, along with NGC 1841, 2257, and Reticulum, as one of the oldest generation globular (10 Gyr) in our nearby galaxy according to B. E. Westerlund in his book "The Magellanic Clouds" (Cambridge University Press, 1997).
With roughly 8◦ of separation with the Large Magellanic Cloud in the sky, this member of our satellite galaxy is visible at 42x in a field that offers a beautiful image. The 6.3 magnitude HD 24188 is, by far, the brightest star in the 1◦ field, visible close to the cluster. This makes easy to find NGC 1466. At this low power, NGC 1466 looks like a slightly defocused star situated between the stars HD 24115 (visual magnitude 9) and the fainter GSC-9156-0534 (visual magnitude 12). At this magnification, the globular cluster reminds me a round planetary nebula. With averted vision, the core of NGC 1466 appears somewhat brighter.
At 78x it is possible to get a good view of the cluster and the stars surrounding it. NGC 1466 appears obvious in the field of view of the telescope. Even with direct vision the cluster shows some "granularity". Applying averted vision, at least two stars can be discerned.
106x seems to be a good magnification to observe NGC 1466, appearing very clear in the field of view as a round hazy patch. Some members of this cluster are easily discerned.
Even higher magnification, like 148x, is a good power to see this cluster under a very dark sky. You can see a nebulous patch and some stars within when using averted vision..
NGC 2257
R.A. 06 30 13.86 Dec. -64 19 32.2 (J2000.0)
NGC 2257 (also ESO 87-24, KMHK 1756) was discovered in 1834 by John Frederick William Herschel. NGC 2257 appears of particular interest since
according to Stryker 1983, Nemec Hesser and Ugarte 1985
(NHU) and Walker 1989 (W89) it is probably the oldest LMC cluster. This object is situated about 8◦5 northeast of the LMC bar in projection on the sky.
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| The Large Magellanic Cloud and its remote globular clusters. |
A conspicuous "nebulosity" shows up high in the sky as soon as it becomes dark, after sunset, on any summer day in the Southern Hemisphere. That "nebulosity", actually a nearby galaxy, the Large Magellanic Cloud (LMC), is surrounded by some faint constellations, namely Dorado, Reticulum, Hydrus, and Mensa. You can enjoy the view of these constellations from a dark sky site. Now, did you know that those constellations host globular clusters that are members of our satellite galaxy? Of course you will need a telescope, at least an 8-inch diameter mirror, to see some of them.
This article deals with 5 globular clusters that
are situated far from the central bar of the Large Magellanic Cloud, thus
appearing as isolated stellar systems in the southern sky, in the south
celestial pole neighborhood.
From any place on planet Earth south of -31◦ of latitude, this five objects never set, being thus what is known in astronomy as circumpolar objects. However, the summer months in the Southern Hemisphere (i.e. December, January, and February) is the optimum season to see them at their highest.
The Large Magellanic Cloud (LMC) is unique in containing massive star clusters at all stages of evolution. Reticulum, NGC 1841, and NGC 1466, three of the objects included in this article, have at times been considered to be Milky Way globulars (e.g. Webbink 1985). Bengt E. Westerlund, in his book "The Magellanic Clouds" (1997), states that these clusters are all outside the sky-projected tidal radius, but still considered to be LMC members.
NGC 1841
R.A. 04 45 23.4 Dec. -83 59 56.6 (J2000.0)
R.A. 04 45 23.4 Dec. -83 59 56.6 (J2000.0)
This
is an extragalactic globular cluster situated in constellation Mensa,
discovered in 1836 by John Frederick William Herschel. In
projection on the sky, this cluster is situated 14◦5 south of the center of
the Large Magellanic Cloud, close to
the border with the southernmost constellation Octans (see map above). In fact,
NGC 1841 is not too far from the south polar star Sigma (σ) Octantis (roughly 6◦5). NGC 1841 could
have been formed in a relatively isolated fragment of the proto-LMC, or it
could have been part of an independent system now disrupted. Further
support to the latter hypothesis could be the fact that NGC 1841 is the
farthest cluster from the LMC center (∼
10 kpc) (paper "The relative ages of LMC old clusters, and the case of NGC
1841" by Ivo Saviane, Alfred Rosenberg, Giampaolo Piotto, &
Antonio Aparicio, 2002). Evidence is provided that NGC 1841 is younger than the rest of LMC globular clusters.
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| Figure 1 |
I observed this remote globular cluster from a dark sky site, with a clear and steady sky that night. Although several sources give a magnitude of 14.1 for this object (which would make it a target out of reach of an 8-inch telescope), it could be glimpsed through an 8-inch telescope working at 42x like a very faint and hazy patch of light showing a smooth brightness. This supports my opinion that this cluster should be brighter than that magnitude. The value of 11.4 given by the Simbad Astronomical Database seems to be much more accurate. It is visible through an 8-inch (20cm.) telescope according with Ernst Johannes Hartung`s book "Astronomical Objects for Southern Telescopes: With an Addendum for Northern observatories". Averted vision slightly improves the detection of it at low magnification. The star HD 34017 (visual magnitude 9.1, spectral type F3V) is, along with HD 31132 (magnitude 8.8), one of the brightest
stars in the 1◦ degree field of view. I used HD 34017 and the patterns of stars
encircled with blue ellipses as guides to find the exact place where NGC 1841
lies (Figure 1).
At 78x, NGC 1841 appears in the field of view relatively big, round, faint, and smooth in brightness. As with lower magnification, averted vision helps for a better detection of this elusive object for an 8-inch telescope. It was not possible to resolve the cluster in some of its members.
106x is
a good power to observe this cluster. It looks pretty big, faint, and smooth
without resolved members. Even higher magnification (148x) makes the view of
NGC 1841 not very good, appearing very faint and being difficult to see because
of the low contrast. Optimum Detection Methods give, if we consider a visual magnitude of 11.4 and angular dimension of 0.9x0.9 arc min for this cluster, an optimum magnification of 80x under a 6.3 or 6.4 limiting magnitude sky, and 75x under a 6.5 sky.
Remember, beyond all the theory and methods for improving the detection of any deep sky object, the best you can do is to go to the mountains, or a place with dark skies, and have your own experience observing the targets in your observing programs. It is observing how you can draw your own conclusions about how difficult is to detect an object through the telescope that you have. But do not stop there, try to observe the same object in different nights, so you can compare the results. Every observing night is a unique experience that enriches your passion for stargazing.
At 78x, NGC 1841 appears in the field of view relatively big, round, faint, and smooth in brightness. As with lower magnification, averted vision helps for a better detection of this elusive object for an 8-inch telescope. It was not possible to resolve the cluster in some of its members.
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| John Frederick William Herschel |
Remember, beyond all the theory and methods for improving the detection of any deep sky object, the best you can do is to go to the mountains, or a place with dark skies, and have your own experience observing the targets in your observing programs. It is observing how you can draw your own conclusions about how difficult is to detect an object through the telescope that you have. But do not stop there, try to observe the same object in different nights, so you can compare the results. Every observing night is a unique experience that enriches your passion for stargazing.
R.A. 04 36 09.0 Dec. -58 51 30.0 (J2000.0)
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| Figure 2. Position of the Reticulum Cluster in the sky |
In 1974, Sèrsic discovered an object in constellation Reticulum on plates taken with the
0.7m Maksutov telescope at Cerro El Roble Observatory. At first, he catalogued it as a probable dwarf galaxy
and member of the Local Group. More recently, Demers &
Kunkel (1976), and Gratton y Ortolani (1987) suggest
that "Reticulum system" is a globular cluster of the Large Magellanic
Cloud. Reticulum is an old and sparsely populated globular cluster that is
located ≈ 11◦ from the center of the LMC (Demers & Kunkel 1976).
Paper "Variable Stars in Large Magellanic Cloud Globular Clusters III: Reticulum" by Charles A. Kuehn et. al. 2013.
Reticulum is classified as an OoI type1 cluster (Oosterhoff
classification). It is located at approximately 11◦ from the LMC bar
(l =269◦ , b=-40◦).
This cluster resides in constellation Reticulum, very close to the border with Dorado (see Figure 2. Enlarge
for a more detailed view). A way to find it is to use the stars Alpha (α) Reticuli, Alpha (α) Doradus, and Zeta Doradus that form a triangulum
in the sky. The cluster lies roughly in the spot where bisectors of that
triangle cross each other.
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| Figure 3 |
At higher magnification (63x), the view is similar. The same stars are visible (labelled with blue circles in Figure 3). Even higher power (118x) did not show more than that faint stars scattered in the area. The faint star labelled with a red circle could be very barely glimpsed using averted vision at this last magnification.
As a final remark, this globular cluster is a faint object for 8-inch telescope. Owners of bigger mirrors should be glimpse this member of the Large Magellanic Cloud when observing from a dark sky site.
ESO 121-SC03
R.A. 06 02 02.50 Dec. -60 31 25.5 (J2000.0)
R.A. 06 02 02.50 Dec. -60 31 25.5 (J2000.0)
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| ESO 121-SC03. Picture from AladinLite view |
At 42x, the field where this object, also known as KMHK2 1591, is situated looks interesting with some stars forming fine shapes. The brightest star in the 1◦ field of view is the 6.4 magnitude SW Pictoris. The chain of four stars (indicated with blue lines in Figure 4), which contains the 6.9 magnitude star HD 41451 at its eastern end, and the stars forming a sort of crown (center of the field in Figure 4) helped to find the zone where this faint and obscure member of the Large Magellanic Cloud lies. This 14 magnitude cluster with angular dimension 2.0 x 2.0 arc minutes is, without a doubt, a faint object for an 8-inch telescope. Obviously at this low magnification nothing was visible in the field.
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| Figure 4 |
At 78x, ESO 121-SC03 is not visible after observing carefully under a dark and steady sky. I observed it again from another place, higher in Los Andes mountains, but I had the same results after using 118x. In both nights and from both sites, something seems to be there when observing with averted vision, but it is hard to assure that the cluster can be glimpsed. As I always say, if you are not sure you saw any object, you must say "I did not see it".
The brightness of this extragalactic object reserves the view only for bigger instruments. Higher magnification (e.g. 160x, 222x) gave the same results, no object visible in the field.
NGC 1466
R.A. 03 44 32.4 Dec. -71 40 16 (J2000.0)
R.A. 03 44 32.4 Dec. -71 40 16 (J2000.0)
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| Figure 5 |
With roughly 8◦ of separation with the Large Magellanic Cloud in the sky, this member of our satellite galaxy is visible at 42x in a field that offers a beautiful image. The 6.3 magnitude HD 24188 is, by far, the brightest star in the 1◦ field, visible close to the cluster. This makes easy to find NGC 1466. At this low power, NGC 1466 looks like a slightly defocused star situated between the stars HD 24115 (visual magnitude 9) and the fainter GSC-9156-0534 (visual magnitude 12). At this magnification, the globular cluster reminds me a round planetary nebula. With averted vision, the core of NGC 1466 appears somewhat brighter.
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NGC 1466. ESO Online DSS |
106x seems to be a good magnification to observe NGC 1466, appearing very clear in the field of view as a round hazy patch. Some members of this cluster are easily discerned.
Even higher magnification, like 148x, is a good power to see this cluster under a very dark sky. You can see a nebulous patch and some stars within when using averted vision..
NGC 2257
R.A. 06 30 13.86 Dec. -64 19 32.2 (J2000.0)
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| Figure 6 |
I observed this LMC cluster when it was about 59◦ in the sky, just a few minutes after its transit by the local meridian, so that the altitude was the best to try to catch this old member of our satellite galaxy. At 48x, I could identify an asterism which I indicate with lines in Figure 6. This asterism was very userful to focus the atention on the spot where NGC 2257 should appear. This cluster can be glimpsed at this magnification, appearing as a very faint, round, and smooth patch of light immediately to the West of the stars TYC 8902-1763-1 and GSC-8902-0650 (visual magnitudes 10.9 and 12.4 respectively). Averted vision sligthly improves the view of it.
At 63x the view of the cluster is similar to that at lower power. Without a doubt, a "nebulosity" with smooth brightness and without any discerned star can be seen in the field. It looks round in shape. It is a faint object (magnitude 12.6) so averted vision helps for a little sharper image of the cluster against the starry field.
118x shows this object faint, a ghostly image of a globular cluster. Even with averted vision the view is hard and elusive. No stars are discerned in the cluster at this power. The angular dimension of NGC 2257 looks similar to the distance between the aforementioned stars TYC 8902-1763-1 and GSC 8902-0650 (see Figure 7).
There were not better results applying higher magnification (namely 222x). Again, a faint and featureless image of the cluster showed up in the eyepiece field. Even if NGC 2257 is visible through an 8-inch telescope, the view through bigger scopes should be more interesting and fruitful.
At 63x the view of the cluster is similar to that at lower power. Without a doubt, a "nebulosity" with smooth brightness and without any discerned star can be seen in the field. It looks round in shape. It is a faint object (magnitude 12.6) so averted vision helps for a little sharper image of the cluster against the starry field.
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| Figure 7 |
There were not better results applying higher magnification (namely 222x). Again, a faint and featureless image of the cluster showed up in the eyepiece field. Even if NGC 2257 is visible through an 8-inch telescope, the view through bigger scopes should be more interesting and fruitful.
“Astronomy, as nothing else can do, teaches men humility.”
Arthur C. Clarke (1917-2008)
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1_ The Dutch astronomer Pieter Oosterhoff noticed that there appear to be two populations of globular clusters, which became known as Oosterhoff groups. The second group has a slightly longer period of RR Lyrae variable stars.[39] Both groups have weak lines of metallic elements. But the lines in the stars of Oosterhoff type I (OoI) cluster are not quite as weak as those in type II (OoII).[39] Hence type I are referred to as "metal-rich" (e.g. Terzan 7[40]) while type II are "metal-poor" (e.g.ESO 280-SC06[41]).
2_ KMHK refers to a list of LMC clusters published in 1990 by M. Kontizas, D.H. Morgan, D. Hatzidimitriou and E. Kontizas (Astronomy and Astrophysics Suppl. Series, Vol. 84, p. 257).
2_ KMHK refers to a list of LMC clusters published in 1990 by M. Kontizas, D.H. Morgan, D. Hatzidimitriou and E. Kontizas (Astronomy and Astrophysics Suppl. Series, Vol. 84, p. 257).
