An Enigmatic Structure in Our Local Universe

Published in the December 2010 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 1

Our neighboring galaxy embraces, in its northern portion, a region not well known by amateur observers. It is an interesting subject with an enigmatic formation history. The structure I refer to is a large supergiant shell, a kpc-sized ring of HII regions dubbed LMC-4 by J. Meaburn (1980). You will see some not well known names when we discuss this region in the article but do not be concerned. Most of the components of LMC-4 are visible in a telescope as small as 8-inch. The region can even be glimsed using common binoculars (e.g. 10x50).

I felt really excited when I saw this region for first time from a magnificent observing site in Los Andes Mountains named "Paramillos", just a few kilometers northeast of Uspallata Valley, a site with very dark skies about 9,000 feet above see level.

A lot of papers about the origin of LMC-4 are found in some specific web sites like www.arxiv.org or “The Astronomical Journal”. We can get our “amateur point of view” using just our eyes and optical devices. As an amateur, I could observe one of the most enigmatic regions, unique in the nearby galaxies population. Backyard observers can also see objects that at first glance sound hard to be glimpsed with small and mid sizes telescopes.

Some years ago, I decided to go a step further and observe other than NGC and Messier deep-sky objects. For example, the observing project on RCW nebulae around the great Eta Carinae Nebula was a great experience. Now I can say that although that kind of objects are usually small and faint are sometimes within the reach of an 8-inch telescope for example, making also possible to test your instrument...and your patience in the observing field!

I needed three nights to observe a 1.12-degree field given by my telescope in this region, using different magnifications and filters. The last night I observed the galaxy from a new observing site in “La Carrera”, close to a small town named Tupungato, about 70 miles southwest of Mendoza City (latitude -33° 11´ 30”  longitude 69° 16´ W).

An Overal View of the Region

Image from Sky-map.org. I have labeled some of the most interesting 
objects to observe there. Upper picture by Steve Mandel and John Gleason.
(North is up).

So far, I have observed most of the LMC-4 as part of a big observing project on the Large Magellanic Cloud (LMC) I am currently carrying out. The viewed region includes the biggest stellar arc and a single HII region there, including also other objects visible in the field. In part 2 of this article I will include my reports about the observation of another stellar arc in LMC-4, the so-called “Sextant”.

LMC-4 is situated between the stars θ (Theta) and δ (Delta) Doradus (visual magnitudes 4.8 and 4.3) which are clearly visible with the unaided eye from a dark sky site and are labeled in any sky chart. Thus, these stars can be used as references to find the area in the galaxy where this zone lies. Moreover, if you recognize the 30 Doradus complex (Tarantula Nebula), you can find LMC-4 moving your telescope roughly 2.5° to the northnorthwest.

The region contains several structures noteworthy enough to be given names (read more in the paper “A Comprehensive Look at LH72 in the Context of Supergiant Shell LMC-4” by Knut A.G. Olsen et. al.). I personally enjoyed observing this part of our satellite galaxy because several structures and tiny open clusters can be found there.

At low magnification (42x) the field was very interesting showing stars with a range of brightness, with the brightest ones situated in the northeast part of the 1.12°field given by my eyepiece. In the region indicated by the blue ellipse in Figure 3 several fainter stars appear embedded in nebulosity. Moreover, some hazy patches are visible on the edge of the eyepiece field.

NGC 1978. North in the field (see Figure 2) a bright globular cluster in LMC lies, NGC 1978, that is, for a globular cluster, rather elliptical. This has been explained by a merger origin (read more about this object in the paper “NGC 1978 in the LMC: The Cluster and Surrounding Field” by D. J. Bomans et. al.). Discovered in 1826 by James Dunlop, NGC 1978 is clearly visible at 42x, appearing round at this power through an 8 inch telescope and with a small core seemingly looking a little brighter than the outer parts. It was more beautiful to observe at 78x, very obvious in the field of view of this eyepiece, also round and rather smooth in appearance (Concentration Class V). At 106x, you can see this cluster elongated and rather smooth in brightness with faint outer ends in the direction of its elongation.

About 19 arc minutes to the West, and situated at the edge of LMC-4, we find a region labeled 1 in Figure 2, NGC 1948, a cluster with nebulosity according to the Wolfgang Steinicke's Revised NGC and IC Catalog. It looks like an interesting nebular structure easily visible at 42x without a filter, appearing round in shape and with a number of stars present there. A careful observation and the use of averted vision made possible to detect a faint arc-shaped nebulosity spreading in the zone enclosed by the blue ellipse in Figure 2, with some stars also visible there. 106x is a good magnification to get a more clear view of this area. Also in this figure, the stars within the red ellipse seem to be surrounded by faint nebulosity. 

At higher magnification (78x) the stars in NGC 1948 look embeeded in nebulosity, round in shape, and being the most prominent part of the mentioned wider structure that reaches the asterisms labeled 3 (see Figure 2). The nebulosity is even fainter in the middle part of the structure. An even fainter nebulosity seems to be situated toward the northwest side of the area (upper right corner on lower panel in Sky-map.org picture). At 106x, a small zone of nebulosity, which is more conspicuous and bright, is visible coincident with the position of NGC 1948 in Figure 2, on the western edge of the stellar swarm. An UHC filter improved the view of this area.

NGC 1951 is an open cluster that looks like a small patch of smooth bright nebulosity (maybe like a small globular or a slightly defocused star). You can see it easily at low magnification. I got the same view at a little higher power (78x).

Using averted vision I could see for moments, close to this stellar cluster, a very faint and irregular nebulosity (2 in Figure 2) that I also observed at 78x, viewing some stars embedded there. I had a similar view at 106x, with very faint stars embedded in nebulosity weak in brightness. The use of a UHC nebular filter did not help to improve the view, at least at that last power. Looking at the DSS image some stars seem to be there. At 78x, on the northeartern edge of NGC 1951, some faint stars that certainly were very hard to see even with averted vision seems to lie (the arrow in Figure 2 indicates the position). A group of stars lies very close to NGC 1951 (to the west). At 106x, NGC 1951 looks interesting appearing bright and small (we are observing extragalactic clusters!).

NGC 2003. On the other hand, this 11.3 magnitude open cluster, discovered in 1834 by John Herschel, appears even smaller than NGC 1951 at low magnification, so it was necessary to know its position in advance among the surrounding stars to find it. The pattern of stars marked with red lines in Figure 3 helped me to recognize the cluster. At 42x it looks like a small star surrounded by nebulosity, being small but bright enough to be seen through an 8-inch telescope. At 78x the cluster was clearly seen, looking small and round, with a brighter core and with a fainter periphery. Observing with averted vision, a very small star was visible very close to the cluster. At 106x this cluster is obvious at a first glance as a small hazy and little elongated spot. The faint star is again visible very close to it.

NGC 1941. This object is labeled as a dark nebula in the old version of Skymap Pro 6.0 (upper right in Figures 2 and 3). However, in the interactive charts in Sky-map.org a bright nebula appears there. The NGC/IC Project web page classifies it as a cluster with nebulosity. Using 78x, I saw a small group of stars associated with a faint and irregular nebulosity, achieving the best view using averted vision. It was discovered in 1835 by John Frederick William Herschel.

NGC 1946. It is a 12.6 magnitude open cluster according to the NGC/IC Project. It is a very challenging object for an 8-inch telescope. It was hard to see at 78x and even at 106x, appearing very small and faint. For very brief moments this object looked sharper and similar to the cluster in B (Figure 2), which looked like a small hazy spot at 78x, with a brighter core visible using averted vision when I observed it at 106x. Two asterisms (3 in Figure 2) were useful to identify NGC 1946. The asterism to the right looked beautiful and reminded me a drop`s shape.

Also in that figure, I have indicated with number 4 a very small and elongated nebulosity, appearing in DSS images (left here) similar to a bipolar planetary nebula. I am talking about KMHK 915, a cluster of stars according to the Simbad astronomical database. I tried to observe it using my telescope. At 78x I just could see a star that I identified using two faint stars situated to the north of this target. Using averted vision, there seems to be a pair of faint stars but I could not see the nebula and the shape showed in the image here at all.

Approaching to the Stellar Arcs...

After observing some of the clusters and nebulae in the field of view I aimed the telescope to the south part of the zone, reaching the western edge ot the biggest arc in LMC-4.

James Dunlop. He observed the Southern

Skies from Australia in the XIX century.

NGC 2002. The identification of this cluster was not easy. Using some stars in the area as reference I could see it, looking almost stellar at 42x. Higher power is undoubtly useful for a better view and analysis of this LMC object. 

106x worked very well to see this open cluster. It is a compact, small, and rather bright (10.1 magnitude, 2arcmin in size according to the Wolfgang Steinicke's Revised NGC and IC Catalog) cluster, appearing in my eyepiece like a small defocused star or a star with nebulosity. A small and faint nebulosity is visible with averted vision close to this cluster (A in Figure 2) and also a small hazy object in C. It was discovered in 1826 by James Dunlop using a 9-inch telescope. It appears as Dun 214 in a list of 244 objects compiled by Hartmut Frommert.

Stellar Arcs, Shapley´s Constellation III

The arc of stars known as "Quadrant". North is up.

About 53 arc minutes southeast of NGC 1978 and about 1.7° southwest from the star (δ) Delta Doradus lies the biggest stellar arc of clusters and young stars in LMC-4, the “Quadrant” (Shapley´s Constellation III?), receiving this name because it is a quarter segment of a ring. It consists of OB associations LH 65, 77, and 84 (Lucke & Hodge 1970) and is often referred to as LH 77.

Through an 8-inch telescope working at 42x it looked like a large arc of stars and nebulosity with some brighter stars situated in the eastern part (left in Figure 2). The groups indicated with α in Figure 4 looked like short chains of very faint stars. The easternmost part of the Quadrant (left in Figure 4) shows faint nebulosity, reaching the edge indicated with a white line. There, several faint stars are visible in the field. There is also an elongated zone which looks brighter just in the region where the objects NGC 2027 and NGC 2034 are situated. Southwest of this arc there is another one, smaller and brighter, known as “Sextant” (I include my observing reports about it in the second part of this article). The Quadrant and Sextant arcs are indeed unique features; there is nothing similar in the LMC and similar arcs of star clusters have not been reported elsewhere either.

What is wrong with Constellation III?

There are some mixed opinions about which part in this region is really Shapley´s Constellation III. The brightest of these arcs was first noted by Westerlund and Mathewson (1966), who wrongly identified as "Shapley’s Constellation III”; nowadays it is known as association LH77 or ”Quadrant”. Van den Bergh (1981) called the whole LMC-4 region Constellation III. On the other hand, McKibben Nail & Shapley (1953) designated NGC 1974 as the identifier of Constellation III, including an area of 28’ x 28’ around NGC 1974, a cluster with nebulosity according to the Wolfgang Steinicke's Revised NGC and IC Catalog. According to these researchers, Constellation III is a triple cluster, so they were probably referring to Sextant, in which eartern part NGC 1974 lies. You can read more about this in section 2 “Stellar Cluster Arcs” in the paper “Triggered Star Formation in the LMC4/Constellation III Region of the Large Magellanic Cloud” by Yuri & Bruce Elmegreen.

A Binary Open Cluster

Located northwest of the OB association LH77, and within the arc “Quadrant”, a pair of open clusters that actually looked globular through an 8-inch telescope are visible at low magnification (42x) if you carefully observe the area about 7 arc minutes southeast of the cluster NGC 2002. I am talking about NGC 2006 (also SL 537) and SL 538, that have a projected angular separation of about 55 arc seconds. They looked like two very small round formations very close to each other. What surprised me is the fact that just one of these clusters has a number in the NGC catalogue when both of them are clearly visible at low magnification through an 8-inch telescope and look very similar in size and brightness. I checked an image of the pair (Figure 1 in the paper “The Cluster Pair SL 538/NGC 2006 (SL 537)” by Andrea Dieball and Eva K. Grebel, 1998). There you can see that the NGC cluster is the southernmost one and “is the fainter member of the pair”. The northernmost cluster (SL 538, also ESO 56-7) looks a little brighter and more compact. The other one (NGC 2006) is a little bigger, fainter, and more dispersed. The difference is not so clear in the DSS image above. As far as I know, the brighter cluster does not have an NGC number. A possible scenario is that the designation NGC 2006 be for both clusters, but the original summary description for this object, Cl, eL, vRi, vBvSNM (Cluster, extremely large, Very Rich, Very Bright and Very Small Nucleus in the Middle) does not mention a double nature. For understanding the “Summary Description” visit http://www.ngcicproject.org/abbrev.htm

In the Andrea Dieball et. al. paper, the authors state that they saw an enhanced star density between SL 538 and NGC 2006. This may indicate a stellar bridge and thus gravitational interaction between them. Thus, we can be talking about a real binary open cluster. I think maybe it could be a much smaller, compact, and extragalactic version of the “Double Cluster of Perseus”.

At 106x, both clusters looked very similar with some difference in brightness. Maybe the best magnification to see this pair was 148x. At this power you can see better the little difference in size between the members.

About 4 arc minutes to the east-southeast of the pair, and visible in an 8-inch dobsonian , another small cluster, known as KMHK2 1019, has a brightness similar to that of the fainter cluster of the pair.

The OB Association LH72 and the “Eighth-Note” Nebula

Images of LH72 and DEM 228 showing their relationship to SGS-143
and LMC 4. Picture courtesy of Knut Olsen (NOAO*).

Reproduced by permission of the ASS.

In the northeastern corner of the field of view (Figure 3) and about 1.4° southwest of the star δ (Delta) Doradus I found maybe the most interesting structure in the entire region, the OB association1 LH72 (RA 05hs 32m 12.00s Dec. -66° 27' 00.0) and its associated nebula DEM 228 (Davies, Elliot & Meaburn, 1976). This nebula, also known with the more friendly name “Eighth-Note”, was visible without a filter at 42x. I indicate with A in Figure 5 the brightest and most prominent part visible in my telescope, looking round in shape and with a central core even brither and containing star-like dots. A view at higher magnification was necessary to see these details better. These dots are maybe stars member of the association related with the nebula (LH 72). Using averted vision I could see the nebula more clearly, extending to the north-northwest reaching the 11.6 magnitude star TYC 8891-3384-1 and suggesting me the characteristic shape you can see in the more deep images of this object (see Figure 5).

At 78x, the view was similar. The round prominent zone shows some stars within, maybe members of LH72, and the extention of nebulosity follows the shape and distribution indicated in Figure 5. Through a UHC filter an extremely faint nebulosity seems to reach the bright stars in the upper left corner of Figure 5. However, no nebulosity is visible at all in the DSS image. In C a little brighter patch was visible in my telescope, matching well with the photos of this object.

Finally, I observed DEM 228 at 106x, with the galaxy about 31° of altitude under a not exceptional but good enough dark sky. At this power the nebula was also visible without a filter. Matching with the description at lower magnification, the most prominent nebulosity surrounds the association LH72, appearing a little elongated at this magnification in the direction NW-SE. Using, at this magnification, an UHC filter the zone in LH72 looked rather round with an inner part a little brighter.

This HII region does not appear in well known planetarium software and sky charts, like Skymap Pro 6, SkyAtlas 2000, Skycharts, etc. The book “Uranometría 2000.0 Volume 2 by Tirion, Rappaport, and Remaklus does not indicate the nebula too. It only shows the position of LMC-4 in LMC with letter “X” in a detailed map of the galaxy, in page 212. I definitively think that DEM 228 or “Eighth-note” nebula (call it as you want) is an LMC object to include in your list when you observe this galaxy.

The description of all these objects were made when the galaxy had an average altitude of 35° in the sky. The objects could be seen even sharper with the galaxy at its highest (about 53° during the transit) and of course using bigger telescopes. Moreover, a well dark adaptation is always necessary.

Whether you are observing at a star party or just alone in a field, remember that when you observe these arcs of stars you are looking at a very unique structure, not found in any other region of our local universe!

* Special thanks to Knut Olsen, the author of a paper about LMC4, and Jill Membrey, the American Astronomical Society Managing Editor, for authorizing me to include a picture of that paper in this article.


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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_ 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).

3_ SGS 14 is an HI (neutral hydrogen) Shell.