Sharpless Sh 2-308, tau CMa Cluster, and NGC 2467 home

 

Deep South in Canis Major

NGC 2467, the t CMa cluster, Sharpless 301, and Sharpless 308

 

March 2007

NGC 2467, the t CMa Cluster and Sharpless 301 and 308 are four very interesting Deep Sky objects of the winter sky, which are quite unpopular with observers of Northern latitudes as they are very far south in the Canis Major/Puppis region and for which there are only few observing reports. Except for the the two Sharpless objects, they are relatively bright and conspicuous objects. But even Sharpless 301 and 308 are not at all extreme challenges and are accessible with much detail also in medium sized telescopes.

NGC 2467 / Sharpless 311

NGC 2467 or Sh2-311 in Puppis (07h53, -2623') is a HII region embedding a small Open Cluster. The emission nebula is not very large, but nevertheless quite bright. With my 14" Dobson, I could discern two darker region that extend from the eastern border to the center of the HII region (around the little cluster Haffner 18) and a marked dark lane that separates a northern bright patch from the main mass of the nebula. Due to these dark clouds and filaments NGC 2467 resembles a smaller version of M8, the Laguna Nebula. To the North is a little Stroemgren Sphere around the little cluster Haffner 19.

The HII region was visible without problem even without filter or with a smaller telescope, such as my 8-Inch Dobson. It responds very well to UHC narrow band or OIII filtering. I found the view with the broader UHC filter more pleasing, as the stars were not as suppressed as with OIII, which gives this HII region a special touch.

With the OIII filter, another large part of the nebula stands out to the NW of the main mass. This part forms a 20-30 arcminutes long crescent, facing away from the main nebula. This remarkably large part appears to emit light mainly in the OIII bands and is hence not very pronounced in the DSS image to the left.

t CMa Cluster

NGC 2362 (07h 19', -2457') is a small Open Cluster of approximately 8 arc minutes diameter around mag 4.4 star t CMa. It is still debated, whether t CMa is a true cluster member or not. If it belongs to the cluster and hence at the same distance, t CMa would be one of the brightest super giants of our galaxy. The other members of the quite young cluster have all quite similar magnitude and form a nice contrast to the shining blue O8 star t CMa. This contrast makes this little cluster a particularly pleasing object.

Image by Adam Block

Sharpless 301

Sh2-301 (07h10, -1830') is not that different from NGC 2467 and responds as well to both UHC and OIII filtering. With my 14-Inch Dob and OIII filter, the HII region appeared richly structured. Superimposed stars and a dark lane crossing the nebula diagonally were quite distinct.

A narrow band image of Sharpless 301 by Dean Salman is here. By the way, Dean Salman has an excellent collection of images of the Sharpless catalog, which is very helpful in determining good observing targets.

Sharpless 308

Sh2-308 (06h54, -2355') is by far the weakest of the four objects, but also one of the most interesting ones. Sh 308 is the blown away outer atmosphere of the Wolf-Rayet star EZ CMa (that little star in the center of the image), similar to the Crescent Nebula, Thor's Helmet, or the WR137 shell (which is a complete list of the brighter WR nebula. The DSS image measures 1 and is a composite of red and blue plates of the southern SERC survey. The WR shell highly excited by EZ CMa and distinctly blue. Hence it responds extremely well to the OIII filter.

The brightest parts are the western filaments, that curve from bright  o1 CMa (in the lower part of the image) to the North and form a huge crescent shaped shell around EZ CMa. These filaments resemble both in shape and brightness those of IC443, the SNR in Gemini. Under very good conditions and low magnification, the shell can be seen as a complete bubble, including the eastern part bending back to o1 CMa. With 7mm exit pupil, a very dim OIII glow can be seen to fill the interior of the entire shell.

Despite of being a spectacular WR shell, there are only few observing reports of Sharpless 308. I could find only three reports by Andreas Domenico, Ronald Stoyan and Steve Gottlieb.

A spectacular image of Sharpless 308 was taken by Don Goldman and is shown here.

Directly south of o1 CMa is the loose Open Cluster Collinder 121.

click for larger map

All four objects are deep at the horizon for observers at northern latitudes. They are rewarding targets for winter nights with good transparency and the Big Dog well visible down to its feet.

Image composites of the SERC Survey on http://archive.stsci.edu/cgi-bin/dss_plate_finder (DSS copyright), Data from www.seds.org

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