Published in the June & July 2008 issues of the Rosette Gazette Newsletter (Rose City Astronomers, Portland Oregon USA)
For those southern observers who like to identify and study bright emission and reflection nebulae, late March and April is a good time to observe in detail an interesting region in the sky where different nebulae, ranging from easy-to-see to challenging, are visible. Some of them are too faint to be observed through an 8-inch telescope, however.
In the Vela-Puppis-Canis Major Region a famous complex, the Gum Nebula, is found. It is a huge HII region, or maybe an old supernova remnant according to some researchers.
In this article, I have included reports and comments about my experience observing this region from Mendoza (Latitude -33°, Longitude 69° W ) using an 8-inch reflector telescope.
The Vela – Puppis – Canis Major region in the Southern Sky
I have chosen a 50-degree wide region in these constellations, centered on Right Ascension 8 hs and Declination –35°. From this part of the planet, this zone is at the zenith at 10 pm local time (UT-3hs.) in middle March, so the Vela-Puppis-Canis Major area is very high in the sky. That is very important to try to observe nebulae in the best conditions.
The following map shows the area of the several nebulae included in the observing program.
The Observing Site
Four nights were needed to complete this observing program. The first night to observe this part of the sky was Sunday, March 30th (2008). At this time of the year, we are at the beginning of autumn in the Southern Hemisphere but temperatures are still good to observe from a site not too far from the city.
The site chosen was Jocoli, situated about 43 miles north of Mendoza city (Argentina). The limiting visual magnitude that night was 6.2, in my opinion not bad for an observing site relatively close to a city.
The Canis Major Region
I began observing a group of nebulae and star clusters to the east of the constellation Canis Major. The Seagull Nebula ( IC 2177) was my first target.
I first observed the north end of this long nebulae, in my opinion, the more interesting region of this huge object because some open clusters and an interesting starry field are visible there.
Using low magnification (42x) you can see a rich starry field with bright and faint stars. Also, three open clusters are clearly visible in the same field: NGC 2335, NGC 2343, and Collinder 465 (Cr 465). NGC 2343 is a 6.7 magnitude open cluster. It looks very compact and is the brighter cluster in the field. An interesting S-shaped chain of faint stars (visual magnitude 11) crosses the cluster, whose Trumpler classification is III,3,p,n.
NGC 2335 looks fainter and a little bigger than NGC 2343. However, it is clearly visible and seemingly with nebulosity ( its Trumpler classification is III,3,m,n, where n means that some nebulosity is present). Its magnitude is 7.2.
Between NGC 2343 and the star V569 Monocerotis, the brighter star in the field with visual magnitude 6.5, two other open clusters lie, Collinder 465 and Collinder 466. Both are situated in a very rich area with several faint stars that inhabit the border of IC 2177.
A conspicuous dark lane is also visible, situated between Collinder 465 and the star V569 Monocerotis, toward the open cluster NGC 2335.
To the East, at approximately 24 arc minutes, there exists an emission and reflection nebula cataloged as Gum 1. This object is not visible when you try to observe it with an 8” telescope without a nebular filter (perhaps it would be interesting to try to observe it under a darker sky!). Three bright stars are visible in the middle of the eyepiece field, but nothing obvious is visible around the star HD 53367 (magnitude 7) where this nebula lies, according to sky charts and catalogs.
Observing again Gum 1 with the same magnification but this time using a UHC filter, the nebula is barely visible using averted vision. It looks like a very faint hazy patch. I had a similar view using 78x and UHC filter.
The elusive nebula NGC 2327
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| Figure 1 |
Moving south, we found a very small reflection and also an HII region situated in a rich starry field. Using low magnification NGC 2327 is not obvious at all. A big stream of stars is visible following the nebula IC 2177.
Observing the field carefully and using the star configuration as a guide ( the stars within a red circle in Figure 1), to find the position where NGC 2327 is situated, an object with stellar appearance is visible. At 106x with UHC filter this object looks almost stellar with a small nebulosity surrounding it. The question remains, is it NGC 2327?
Finally, I moved my telescope to the south end of the Seagull Nebula. This is also a very impressive field, several stars and interesting stellar chains are visible here. To the right side of the eyepiece field, a beautiful crown of 5 stars is clearly noted. Cederblad 90 (Ced 90, also Gum 3), an emission and reflection nebula, is situated in the south end of the Seagull Nebula. In that region, and with low magnification, a small region embedded in nebulosity comes to view. Also, some dark structures were visible to the south in the same eyepiece field. Faint nebulosity surrounds the star HD 53623 and it is necessary to use averted vision. Using a nebular filter (e.g. UHC) Ced 90 looks a little bigger, embedding the faint stars situated to the left. I had a better view of these characteristics using a little higher magnification (53x).
The Thor´s Helmet Nebula (NGC 2359)
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| NGC 2359, the Thor`s Helmet nebula |
A very beautiful diffuse nebula lies about 4 degrees northeast of the star γ (Gamma) Canis Majoris (visual magnitude 4.1). It is NGC 2359, a striking nebula. It is clearly visible even without a nebular filter and you can identify some of the shapes showed in the picture to the right.
With 42x NGC 2359, also known as the “Duck Nebula”, is easily visible as smooth nebulosity in a rich starry field. An awesome view is reached when you put on a UHC filter. A fainter filament (indicated with number 2 in the picture) is visible toward a place where another nebula (IC 468) is situated. In this zone, there is an extremely faint nebulosity, barely visible (maybe part of IC 468?).
With 42x NGC 2359, also known as the “Duck Nebula”, is easily visible as smooth nebulosity in a rich starry field. An awesome view is reached when you put on a UHC filter. A fainter filament (indicated with number 2 in the picture) is visible toward a place where another nebula (IC 468) is situated. In this zone, there is an extremely faint nebulosity, barely visible (maybe part of IC 468?).
With 53x and a UHC filter, the filament is again observed and the brighter zone (number 1 in the picture above) is visible very well. Moreover, a narrow dark lane seems to cross the nebula in its central part.
Using higher magnification (106x) some stars are visible in the brighter part.
After the observation of NGC 2359, I moved the telescope near the star τ (Tau) Canis Majoris where a small nebula (15´x 15´ in size) lies, vdB 96. When the observation of this nebula was made, its altitude was 45° above the west horizon. The nebula was not visible at all even using a UHC filter, I think maybe a bigger telescope or darker skies are necessary (including a higher altitude of the object) to try to see something.
The second observing night was on Saturday, April 5 (2008) from a different observing site, a nearby one called Canota, at 1,400 meters above sea level. That night the sky was very clear so I decided to go ahead and continue the observing program of some nebulae in Canis Major and Puppis.
An interesting nebula named Sh2-301 (also Gum 5) is found about 3.2° from the star γ (Gamma) Canis Majoris. I had a chance to observe this nebula through a 16-inch telescope. It was clearly visible using an OIII filter. To the East of this nebula, a line of three bright stars is visible.
Observing this nebula with 75x, it looks like a round and smooth nebulosity with some faint stars within. Averted vision helps a lot. A prominent bright lane of nebulosity is visible emerging from the main part, reaching and surrounding the star HD 54957 (visual magnitude 7.4). Nebulosity is also visible around the star HD 54977 (visual magnitude 8.1)
The Puppis Region
In the observing program, there are four (4) bright nebulae and two (2) clusters with nebulosity that lie in this constellation. I began with a close pair of nebulae: Sh2-302 & vdB 97, at almost 3° to the south-southwest of the very well known open cluster Messier 47. I tried to observe it at 42x and 78x with the UHC filter but no nebulosity was visible in the eyepiece field.
On May 1 I had another chance to continue with this observing program. The observing site was again Canota.
NGC 2467
Situated in the northern part of Puppis, close to the star ο (Omicron) Puppis (visual magnitude 4.5) you will find an excellent object. Actually, it is an open cluster with nebulosity. At first, I observed it using 42x and even without a filter, the nebula was clearly visible. The nebula looks round, with a relatively bright star visible near its center. The surrounding field is very rich, with a lot of faint stars forming interesting groups and shapes, and also some brighter stars. The UHC filter works very well to see this nebula appearing more contrasted. Moreover, other faint nebulae structures are visible to the left where the brighter stars in the field are situated. Besides this, some dark structures are visible in the eyepiece field.
With 106x, the nebula looks wider and some faint stars are visible within it. Using a nebular filter NGC 2467 looks smooth. It reminds me a little bit about the planetary nebula NGC 1514 in Taurus.
The observation of this small nebula, or cluster with nebulosity according to some sources (see DSS image below), was one of the most challenging objects of this observing program. When I pointed my 8-inch telescope to that area using low magnification I thought, “Well, nothing obvious there”. I checked my charts and eyepiece fields and at first, I could identify a sort of spiral-shaped asterism (see right panel of the picture below). Then I focused my attention on the star TYC 7134-2023-1 (visual magnitude 10.2) because, according to the DSS image and detailed charts, the nebula should be there.
Using this time higher magnification (106x), NGC 2579 came to view even without a nebular filter. The nebula is visible, just great!. It looks like a very small, round, and smooth hazy patch very close to the star TYC 7134-2023-1.
With the help of a UHC filter, the nebula looks a little brighter and bigger. With this filter, you can see the nebula surrounding the bright star TYC 7134-2023-1. With averted vision, a small bright spot (a star?) is visible within the nebula. Observing again with 213x the bright point in the nebula is more obvious (especially with averted vision).
As soon as I came back home I looked for further information on NGC 2579. I found an interesting paper recently published (July 2007) in the Cornell University e-library (www.arXiv.org). The scientific team points out that this object is an HII region with a small companion, a ringed nebula (40”x50” in size) named ESO 370-9 (visible lower left in the DSS image). According to this paper, there have been identification problems with NGC 2579. Despite its description as a “double star in a pretty small nebula” and sufficiently precise coordinates in the NGC, it has been misclassified with other nearby objects. It has been misclassified as a reflection nebula in the SIMBAD database and it is identified as a planetary nebula in the Strasbourg-ESO Catalogue of Galactic Planetary Nebulae.
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The red spot in the picture indicates the position of NGC 2579. With red lines, I indicate the spiral-shaped or question mark-shaped asterism I used as a guide to finding that nebula.
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The Vela Region
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| Figure 2 |
Some bright nebulae and a famous supernova remnant are found in the west part of this southern constellation.
For this latitude, this constellation reaches about 75° of altitude early in the night during late March, so that season is the best moment to explore that region. Part of the famous Gum nebula lies in this constellation.
Discovered by Collin S. Gum, this huge and almost circular object, with an angular size of 36°, is possibly an HII region, but its nature remains unclear.
The first target I observed was NGC 2626, part of the Gum Nebula complex. Through an 8-inch telescope at low magnification, this nebula looks faint, in an interesting starry field. At the center of the nebula, a star is clearly visible. I think maybe that nebula looks faint because it is situated in the Milky Way path (almost 0° of galactic latitude) and the background sky does not look very dark, so the contrast is not the best. I tried to see it with higher magnification and UHC filter but I had a bad view. I read a report by the observer Steve Coe (Saguaro Astronomy Club) where he also claims that UHC does not help much on this nebula. With even higher magnifications, but without filter, the nebula looks a little better, with a slightly elongated shape and smooth appearance.
Gum 15 is situated almost 2° east of NGC 2626. The red circles in Figure 2 indicate zones where I could see very faint nebulosity, getting a better view using a UHC filter. Lower on the eyepiece field the open cluster Collinder 197 is visible, embedded in a very faint nebulosity.
Then I moved my telescope 1.5° from Gum 15 to the southeast, to try to observe the wide nebula Gum 17. It is a very big nebula (about 1.3°) so it is necessary to use wide fields (low magnifications) to observe it. Some nebulosity is visible around the brightest star in the field (HD 75759, visual magnitude 6). Maybe the most prominent feature in this area is the dark nebula SL4 (see picture below) situated in the southeast part of the bright nebula. Close to this nebula the open cluster (OB Association?) Trumpler 10 is visible. It looks like a sparse swarm of bright stars.
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Trumpler 10 comments
Trumpler 10, a stellar swarm visible with binoculars even from urban skies, is cataloged as an open cluster, with a Trumpler classification of
II,2,p.
However, some researchers suggest that Trumpler 10 is an OB association situated near the
morphological center of the Gum Nebula, and it can account for most of the photoionization of the Gum Nebula with another association, Vela OB2. Vela OB2, first identified and discussed by Kapteyn (1914) as the “Vela Group”, spans a large part of the Gum Nebula.
Trumpler 10 lies in the Northwest part of constellation Vela.
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| Figure 3 |
Saturday, May 3 I completed this observing program. The observing site was again Canota. I worked in constellation Vela observing some nebulae that are part of the big Gum complex. To scan this part of the sky was a very exciting experience.
Near the 3.8 magnitude star HD 78004 there are two Gum nebulae. That star can be used as a guide to finding them (see map at the top of this article).
50' southwest of that star we find Gum 23. With low magnification and without a filter I could see a little and very faint nebulosity. In the eyepiece field, the brightest star is FZ Velorum (visual magnitude 5).
The nebulosity is better viewed using a UHC filter. It was visible in the zone encircled by the small ellipse (lower left in Figure 3), where a line of three stars was also visible. The other faint patch of nebulosity is situated between the stars FZ Velorum and CD—47 64560 (middle right on the image) appearing bigger than the other one. Close to the star FZ Velorum and between both patches a dark region is visible.
Observing this object with higher magnification (53x) and the UHC filter the nebula still looks faint but a small hazy and brighter spot is visible in the region indicated by the white arrow. Averted vision is necessary to see this small feature.
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| Figure 4 |
The Vela Supernova Remnant and the “ Pencil Nebula” (NGC 2736)
The Vela Supernova Remnant, produced by a star that exploded approximately 11,000 years ago, is roughly spherical in shape and with an angular diameter of about 7°.
Last year (2007) I could see a small filament situated in the south part of this big Supernova Remnant using an OIII filter and my own telescope. I had some advantages, a very dark and steady sky and a site at good altitude (2,000 meters above sea level). This time I tried again but under a little brighter background sky and from a site situated not so high. The experience was not the same. This supernova remnant is a very challenging object, at least for an 8-inch telescope, and personally, I think some conditions are really necessary to see it: To know very well where and what to observe, to stay as alone as you can to get concentrated and to spend all time you need observing through the eyepiece to try to identify and see those faint filaments.
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The Pencil Nebula (NGC 2736). Its brighter part
is visible through an 8-inch telescope. |
An easier part to see, and maybe the brighter part of the Vela Supernova remnant is its eastern filament, cataloged as NGC 2736, situated about 1.4° north-northwest of the star HD 78004.
Also known as RCW 37, it was discovered in the 1840s by Sir John Herschel, It is usually nicknamed “Pencil Nebula” and also referred to as “Herschel´s Ray Nebula”. This filament of nebulosity is almost the only sign of the eastern part of the vast bubble of expanding shock wave from a stellar explosion thousands of years ago, the Vela Supernova Remnant.