An Enigmatic Structure in Our Local Universe - Part 2

Published in the April 2011 issue of the Rosette Gazette newsletter (Rose City Astronomers, Portland Oregon USA)

A Fascinating Region in the Large Magellanic Cloud to Explore in Detail Through Amateur Telescopes

Part 2

In Part 1 of the article “An Enigmatic Structure in Our Local Universe” I have described a peculiar zone of the Large Magellanic Cloud (LMC) situated to the northwest of the well-known object 30 Doradus (also known as “Tarantula Nebula”), a starburst region. That zone is the Quadrant, a big circular arc of stars in the supergiant shell called LMC-4.

Part 2 is concerned about the other conspicuous structure there, an arc of stars called “Sextant”, also including some clusters and OB associations1 present in the field of view.

The Large Magellanic Cloud. Photo courtesy of Pat Hanrahan (Rose City Astronomers, Portland Oregon USA)

This Part 2 is focused in a field centered at R.A. 5hs 28m 42s  Dec. -67° 43' 37” (epoch J2000.0), about 45 arcmin to the southwest of the biggest stellar arc “Quadrant” and about 1° 30' to the east-southeast of the 4.8 magnitude star θ (Theta) Doradus, easily visible to the naked eye under a dark sky. Thus, it can be used as a guide to easily find the field.

A Brief Description of the Field

Canota (Mendoza, Argentina). A view toward the northwest.

I spent two nights (around the first days of March) to observe the 1° field under analysis. The observations were made from Canota, Mendoza Argentina (Latitude -32° 34' Longitude 68° 56' W). Although a dome of light is present to the South this nearby observing site, about 25 miles to the North of Mendoza city, usually offers a 6.1/6.2 limiting magnitude sky. The sky was totally clear both nights after several days of cloudy and rainy days. However, the seeing was not the best but the sky was steady enough to observe the faint and small details of some of the objects in the zone. The second night had worse transparency and a higher level of humidity.

At low magnification (42x) the field shows stars with visual magnitudes around 8.5 and 9 (the brightest ones). To the East, some hazy objects containing several stars were visible. To the North you will see maybe the most interesting and conspicuous structure in the whole field, the “Sextant”, which harbors an interesting history (read more below). It appears, at a first glance, as a fine curved formation of nebulosity and stellar aggrupation.

The Stelar Arc “Sextant” (Constellation III?)

Figure 1. DSS image of the stellar arc known as Sextant. I have plotted the different 
zones easily discerned through a common amateur telescope.

To the southwest of the structure known as “Quadrant”, described in Part 1 of the article, published in the December 2010 issue of the Rosette Gazette (Rose City Astronomers, OR USA), lies a smaller but more noteworthy structure referred to as “Sextant”. At first glance, through an 8-inch telescope at low power, this structure looked like a remarkable arc of nebulosity.

The Sextant is a large arc of young stars and clusters shaped like one-sixth part of a ring. A plausible model for the formation of Sextant is that it was triggered by the collapse of a small swept-up shell around a cluster that formed inside a giant expanding HI ring.

The question arises again, “Is the Sextant arc the structure called “Shapley´s Constellation III”? McKibben Nail & Shapley (1953) designated NGC 1974 (a component of the stellar arc) as the identifier of Constellation III, including an area of 28’ x 28’ around it. They also noted that Constellation III is a triple cluster, so in fact, they were probably referring to Sextant (paper “Triggered Star Formation in the LMC4/Constellation III Region of the Large Magellanic Cloud I have observed” by Efremov and Elmegreen, May 1998).

The only thing resembling the Quadrant and Sextant stellar arcs is a large region in the Sc galaxy NGC 1620 (also UGC 3103) studied by Vader & Chaboyer (1995), a galaxy situated in constellation Eridanus, with a Declination of -00° 09', thus being visible from both Hemispheres and accessible to most of the amateur observers because of its brightness (magnitude 12.4  Surface brightness 13.4). Due to its inclination, it is difficult to know if this is a stellar arc or a spiral arm.

At low magnification (42x), four zones were visible in the Sextant, three of them prominent and one fainter and more elusive. The easternmost one is called NGC 1974, discovered by James Dunlop in 1826, a cluster with nebulosity according to both the NGC/IC Project and the Wolfgang Steinicke's Revised NGC and IC Catalog, appearing round in shape and nebular. Observing carefully, and using averted vision, some very faint stars are visible embedded there.

Immediately to the West of NGC 1974, another nebular patch is visible, NGC 1968. There, an elongated stellar formation, composed by stars brighter than those in NGC 1974, was clearly visible. Moving the telescope more to the West, the structure in Sextant is less conspicuous but a continuum of faint nebulosity seems to connect the whole complex. Two faint stars are the most prominent feature there (gray ellipse in Figure 1). Moving farther West, the westernmost of the prominent “patches” is very similar to NGC 1968, clearly visible without a filter at this magnification showing a very similar stellar configuration. We are talking about NGC 1955. Harder to see is a patch of nebulosity, LH251 (Figure 1). With averted vision, I could see a small area of smooth nebulosity and some stars. The view of the whole Sextant at this magnification but using a UHC filter was marvelous, with NGC 1974, 1968, and 1955 being the most conspicuous patches and with LH51 looking fainter, round, and smooth.

The Sextant was also clearly visible at 78x. The nebulosity appeared rather continuous and the stellar configurations in NGC 1968 and 1955 looked similar, both in appearance and orientation. However, the configuration of NGC 1955 looked like a rather arched chain of stars. Using averted vision, the zones appeared very well detached in the entire structure. LH 51 was also visible, improving the view using this technique.

Figure 2 (DSS image).

At the same magnification and using a UHC filter, the Sextant showed four zones of nebulosity, being NGC 1968 and 1955 the most conspicuous ones. At the center of the complex, the two stars mentioned above were again visible.

At 104x, the Sextant was visible across the field, filling it and offering a wonderful view of the entire structure. NGC 1974 looked like a swarm of faint stars of similar brightness in a round configuration and embedded in faint nebulosity. NGC 1968 looked more obvious containing brighter stars in an elongated configuration NE-SW and, like NGC 1974, embedded in faint nebulosity. Immediately to the West, the “central” zone showed no nebulosity, with a “dark” zone dividing the two patches from those situated to the West. The two stars mentioned have visual magnitudes 11.2 and 12.8.

Figure 3

Displaying a rather symmetric configuration, NGC 1955 and LH 51 are seen very similar to NGC 1968 and 1974 respectively (see Figure 1). NGC 1955 showed stars with similar brightness to that in NGC 1968. Finally, LH 51 was seen similar to NGC 1974 but a little fainter and showing less amount of stars. Its appearance, similar in shape, looks more nebular. Through a UHC filter, the Sextant looked very impressive. A wider nebulosity was visible in NGC 1955 in the direction N-S (visible in Figure 1). LH 51 appeared bigger than NGC 1974 and 1968 through this filter, but fainter and smoother.

To the East of the Sextant, an object appears labeled in sky charts as NGC 1991 (see Figure 3). However, I could not see anything there. According to the NGC/IC Project, NGC 1991 is another denomination for NGC 1974 so surely there is a mistake in the original eyepiece field chart.

A little bigger and round nebulosity was detected in zone 1 in Figure 3 being barely visible at 42x using a nebular filter (UHC). Observing it at 78x without a filter , and using averted vision, it appeared like a faint and rather round nebulosity not very well detached from the background sky. Through a UHC filter, it looked round in shape and bigger in size compared with earlier observations. On the other hand, an extremely faint nebulosity seems to cover the zone between the Sextant and zone 1. At this power, and with this filter, the nebulosity seems to have a triangular shape narrowing northward. 

A final observation at 104x showed this not very well detached nebula very barely, visible with averted vision.

Figure 4 (DSS image).

Indicated by an arrow in Figure 3 there is a small nebula that was hard to observe at low magnification (42x). Using averted vision, a small smooth nebulosity seems to be situated very close to the star GSC-9162-0489. The small nebula (0.7 x 0.7 arcmin in apparent dimension) has the not colorful denomination LHA 120-N51B. It is the small and round nebulosity visible to the west (to the right) of the star GSC-9162-0489 in Figure 2. Very close to this nebula lies the OB association LH 55.

At higher magnification (78x), the small nebula is still barely visible. For moments, using averted vision, a very small nebulosity was visible and an extended faint nebulosity became visible surrounding it, reaching and engulfing the star GSC-9162-0489. The extended nebulosity (actually an HII region) is known as LHA 120-N 51E and it is associated with the OB association LH 55.

104x was an appropriate magnification to observe this region, with LH120-51B appearing like a small nebulosity with a bright central spot. A more detailed inspection of the small object LH120-N51B at 213x made possible to see the faint small nebula with a bright stellar-like core.

The DSS image (Figure 2) shows a very small object situated to the East of the region LH 55 and indicated by a red arrow. I used the asterism indicated with a red circle to try to find it but at 78x and 104x it was not clearly visible. That object is named BCDSP 4 and it is classified as a stellar cluster by the SIMBAD Astronomical Database.

Moving the telescope to the northeast of the Sextant, at the north edge of the eyepiece field, a sort of small nebula with smooth appearance was visible at 78x. Observing carefully with averted vision there seems to be a group of faint stars. This object is the cluster KMHK3 943, situated in an HII region named DEML212 (see Figure 3). Observed at 104x, KMHK 943 appeared like a small and little elongated hazy patch with a few stars embedded there. Always averted vision improved the details.

NGC 2004 was a noticeable 9.6 magnitude open cluster through my 8-inch telescope at 42x, situated in a region rich in faint stars. A bright and condensed core was visible, surrounded by fainter stars.
Even without a nebula filter, some zones of nebulosity were visible there (toward the northwest and the southeast from this cluster). A faint lane of nebulosity seems to connect NGC 2004 with the Sextant, situated to the southwest. At the same magnification but using a UHC filter, the cluster was visible very detached, suggesting the existence of nebulosity there. At 78x, this cluster appeared interesting with a small and bright core showing a “grainy” appearance when I saw it using averted vision. The outer parts of the cluster showed faint stars, some of them discerned and visible. The core of NGC 2004 seems to be a little offset.

At even higher power (104x), this cluster showed a “bright granular core”, where I could start to discern some stars observing carefully with averted vision. The outer zones, much fainter, appeared with faint stars, some of them being identified at this magnification.

NGC 2004 is mentioned together with NGC 2100, another LMC cluster, in the paper “A Hypervelocity Star from the Large Magellanic Cloud” (Alessia Gualandris and Simon P. Zwart, 2008) as possible parent clusters of HE 0437–5439, an 8M⊙ hypervelocity star with a possible origin in our satellite galaxy, now located at a distance of 61 kpc from the Sun.

Also, a round object which is classified as a galaxy by the SIMBAD Astronomical Database, situated in a region populated by a group of faint stars (see the upper panel in Figure 3) was not visible. At 78x, the identification of this object was unfruitful. The stars immediately surrounding it were barely visible that night through the 8-inch telescope, making the accurate identification of the zone more difficult. Seemingly, a faint star seems to be there but even higher magnification and perhaps a nebula filter would be more useful to try to see this challenging object.

NGC 2011, according to B. Kumar, R. Sagar and J. Melnick in their paper “CCD Photometric and Mass Function Study of 9 Young Large Magellanic Cloud Star Clusters” (February 2008) is located in the OB association region LH 75, with its photographic image indicating that it is an elongated, fairly condensed, and partly resolved cluster. Clearly visible through an 8-inch telescope like a single stellar swarm, this open cluster is actually a double cluster with its fainter component being not visible (read more in the paper “HST WFPC2 Observations of the Peculiar Main Sequence of the Double Star Cluster NGC 2011 in the Large Magellanic Cloud”, by D. A. Gouliermis et. al.). At low magnification (42x), it looked bright but much smaller than NGC 2004. The view at this magnification reminded me that of a small and bright planetary nebula. Near this cluster, faint stars and nebulosity suggest to me the S-Shaped type distribution I have indicated with gray color in Figure 3, with the end near NGC 2021, that appears like an oval-shaped group of faint stars (indicated by the arrow in Figure 4). Actually, the S-shaped structure I saw does not match very well with the DSS image of that zone.


A higher magnification (78x) made possible to show NGC 2011 like a defocused star immersed in the way of the S-shaped structure of stars and nebulosity present there. The oval aggrupation showed several stars of rather similar brightness. Through a UHC filter, NGC 2011 appeared like a star surrounded by small nebulosity. At 104x NGC 2011 was bright, looking like a star with a very small surrounding nebulosity. On the other hand, the zone near NGC 2021, a small 12.1 magnitude open cluster according to the NGC/IC Project, is a notable zone of stars and nebulosity, a structure that seems to be present in other regions of the Large Magellanic Cloud. The S-shaped nebulosity was visible too.

NGC 2014 (LH 76; Lucke & Hodge 1970) is an OB association that lies at the southern edge of LMC-4, embedded in the H II region DEM 229. At 42x, it was a beautiful stellar aggrupation easily visible. In its southern edge the brightest star of the swarm was detected (about 9.8 visual magnitude). The cluster looked elongated very closely east-west. Using a nebular filter (UHC) the view showed better the nebulosity present there and the stars looked dimmer, but they were visible. At 78x with a UHC filter, some stars are visible in a hazy environment.

High magnification (104x) shows a stellar swarm, with its members easily discerned and some of nebulosity engulfing the region.

DSS image of NGC 2020.

NGC 2020, discovered by John Frederick William Herschel in 1836, was barely visible at low magnification (42x), needing averted vision for brief moments of better detection. It looked round in shape and with smooth brightness. Cataloged as a diffuse nebula, some sources like the Wolfgang Steinicke's Revised NGC and IC Catalog suggest that this nebula is maybe a supernova remnant.
The UHC filter improved very well the view of this object, looking again round in shape and smooth in appearance but much more detached from the background sky.

At 78x, it looked round and mostly smooth in appearance. Averted vision was necessary for a better view. At least two stars are superimposed. Using a nebular filter (UHC) I got a good view, being easily detected and also appearing almost round in shape and smooth in brightness, seemingly showing an inner dark feature when it was visible in detail using averted vision, suggesting the inner “hole” visible in the DSS image here.

At 104x I got the same view. With averted vision and using a UHC filter, the inner part appeared like a dark lane or “bar” across the object.

Number 2 in Figure 3 indicates a very small cluster with nebulosity that was visible through my 8-inch scope.

Figure 5

Obscure Stellar Clusters

Before beginning with the visual observation of the field I was studying the DSS image. This made possible the identification of some faint and small stellar clusters that you will not find in common amateur sky charts and planetarium software. Most of these clusters are located in the lower half part of the eyepiece field. In Figure 5, I have plotted the clusters with red dots and numbers. The goal was to try to see these objects visually through my 8-inch telescope under the usual dark skies we have in Canota.

Numbers 1 and 2 indicate the positions where two small stellar clusters lie. In the case of cluster 2 I used the stars encircled by the blue ellipse as a guide because it is situated about the same distance that exists between those stars, in the same imaginary line that connect them. At low magnification (42x), and with higher power (78x), they were not visible at all needing even higher magnification to know if they are visible in a small telescope. However, even at 104x the clusters were out of reach of my 8-inch telescope.

Moving across the field and reaching the eastern edge, four more small objects lie there. In 3 the DSS image shows a small hazy object and a small stellar cluster in 4. Both were not visible at low magnification (42x). I observed this zone again using 78x and 104x but both were not visible.


A “Hidden” Pair of Very Faint Clusters. A Case for Applying High Magnification

I had a very interesting experience trying to find out if two small clusters in the field, indicated with a red ellipse in Figure 5, were cataloged or not, and in which catalog they would appear. Two extremely faint stellar clusters are located in the southeast part of the field. They were in the threshold of visibility in my 8-inch telescope under a dark sky and with our neighbor galaxy about 38° high in the southwest late summer sky.

Figure 6. A composite image of ordinary clusters in LMC, with a 
close up RGB image of the clusters (upper panel).

I could not find these clusters labeled in planetarium software or sky charts because they are very small, faint, and unknown objects. I included them in the observing list after analyzing the DSS image of the field proportioned by the STSCI DSS web page. I did not have the equatorial coordinates (epoch J2000.0), so I made a first approximation of their positions working with that image and obtaining the coordinates proportioned by this web page. After achieving a first approximation (within 1 arc minute in accuracy) I checked the list of the “Catalogue of extended objects in Magellanic Clouds (Bica+, 2008)” available at CDS “The Strasbourg astronomical Data Center”. There I selected the Open Clusters and Associations table options, with 3740 and 3326 rows (objects) respectively (you can get it at http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=J/MNRAS/389/678)
I found them!!! The clusters are cataloged and they are named KMHK 1027 (also BRHT38a) and KMHK 1032 (BRHT38b). Figure 6 shows both clusters in an RGB composition image (upper right) obtained using the Aladin4 previewer at the SIMBAD query results page. 

At 42x the small stellar clusters, situated close to the 11 magnitude star GSC-9162-0091 (indicated with the gray ellipse in Figure 6) were not detected.

At 78x, the clusters were hardly glimpsed. With averted vision, two very small hazy spots were visible for moments. Using an eyepiece that gave me 104x the two clusters were detected like small and faint spots, needing averted vision for an improved view.

At 213x I got the best view of the cluster pair. With averted vision, they looked like very small hazy patches. KMHK 1027, the southernmost one (lower in Figure 6) and with an apparent dimension of 1 x 0.9 arc min., is a little bigger and brighter than KMHK 1032 (0.7 x 0.7 arcmin.) and it was easier to see for me. The very faint star on the south edge of KMHK1027 could be glimpsed two. Definitively they are good targets to test your visual acuity!!!

As a final note, I can say that the Large Magellanic Cloud hosts a zoo of deep-sky objects to test your telescope, visual capability, and know how far you can go in visual observation.

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1_ OB Association: The concept of a stellar association was originally introduced in 1949 by V. A. Ambartsumian, who later separated them into OB and T associations (Ambartsumian 1968). Morgan, Sharpless, & Osterbrock (1952) considered as a stellar association any loose group of stars within an area where bright OB stars exist and with evidence of a common origin.
A recent definition of a stellar association (Kontizas et al. 1999) refers to it as a single, unbound concentration of early-type luminous stars, embedded in a very young star-forming region.

2_ LH Luke, P. B. and Hodge, P. W. “A Catalogue of Stellar Associations in the Large Magellanic Cloud”. Astronomical Journal 75, 2 (1970): 171 – 175.

3_ 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. 527).

4_ Aladin is an interactive software sky atlas allowing the user to visualize digitized astronomical images, superimpose entries from astronomical catalogs or databases, and interactively access related data and information from the Simbad database, the VizieR service and other archives for all known sources in the field(see available data). Created in 1999, Aladin has become a widely-used VO tool capable of addressing challenges such as locating data of interest, accessing and exploring distributed datasets, visualizing multi-wavelength data. Compliance with existing or emerging VO standards, interconnection with other visualization or analysis tools, ability to easily compare heterogeneous data are key topics allowing Aladin to be a powerful data exploration and integration tool as well as a science enabler. Copyright © UDS/CNRS - distributed under GPL v3 license