NGC 6334, 6357, & Pismis 24

In the well-known constellation Scorpius lie a lot of conspicuous deep-sky objects. It is not new if we talk about the open cluster NGC 6231, also known as the “Scorpius Jewel Box”, the globular cluster Messier 4, one of the nearest globular clusters to us, or the open clusters Messier 6 (Butterfly cluster) and Messier 7 (Ptolemy´s cluster), both visible to the naked eye. If you want to take a step ahead and observe other objects also situated in this constellation you can, for example, to try to find a couple of nebulae often nicknamed “Cat´s Paw” (NGC 6334) and “War and Peace” (NGC 6357). Within the last one, we found a peculiar open cluster cataloged as Pismis 24, containing interesting massive stars as members.

On July 4, 2009, I had a chance to drive toward the northwest and to reach the Andes mountains with the idea of observing those nebulae. I set up my telescope in a place named Paramillos (latitude –32° longitude 69° W), at 2700 meters above sea level and about 25 km (16 miles) from Uspallata Valley. 

Photo of the constellation Scorpius taken by the author. The white ellipses show the zones where IC 4628 and NGC 6334 & 6357 are situated.

The Southern Hemisphere winter is a good time to observe the region of the sky toward the Milky Way center, with constellations Scorpius and Sagittarius very high in the sky. I made the observations that night under a very clear and steady sky.  On the picture I took, I indicate with a big white ellipse the zone where these nebulae can be founded. You should include these objects in your own observing program if you have in mind to trip to the Southern Hemisphere to explore the southern constellations.

Before observing the two main nebulae in the observing program I aimed my 8-inch telescope to the south region of the constellation (smaller ellipse on the picture above), near the open clusters Collinder 316 and Trumpler 24, both of them part of the Scorpius OB1 association, with the idea of identifying and observe in detail a nebulosity known as “Prawn nebula” (IC 4628).

THE IC 4628 NEBULA

IC 4628 is situated about 1.5 degree North from the well-known cluster NGC 6231 and on the north side of the open cluster Trumpler 24. In fact, several stars in the field are members of this wide and sparse cluster. After identifying the starry field surrounding this nebula I observed it with low magnification (42x). At that moment the nebula was very high in the sky, roughly 79 degrees of altitude. It was visible like an elongated and faint nebulosity in a rich starry field with some stars forming short chains. You can improve the view using a UHC filter. With this kind of filter, the nebula looks wider, more contrasted, and more elongated east-west.

IC 4628 and surrounding deep-sky objects.

A few bright stars are visible surrounding the brighter part of the nebula, outlining it (the only part I can see with my telescope and filters). Fainter stars are visible embedded in the nebulosity.

A more contrasted, round, and small patch is also visible (number 1 on the picture to the left). A bright narrow filament connects that patch with the main part of the nebula. IC 4628 is a big nebula, for that reason it is not a good idea to use high magnifications if you want to observe the whole object, or at least the brighter and more detectable part.

I used an eyepiece that gave me a little higher magnification (53x). Using the UHC filter again the view was still very interesting. Some dark structures were visible in the nebula, especially in its west end. A narrow dark lane seems to cross through the middle of the nebula. Taking the UHC filter off, zone 1 shows a short line of faint stars very close to each other and embedded in a faint nebulosity.

Observing IC 4628 with the same low magnification but this time working with an OIII filter the contrast is maybe a little worse. In my opinion, the UHC filter definitively works better.

According with several deep-sky observers around the world, the H-beta filter works well in a very few objects only, like the California Nebula (NGC 1499) or the Cocoon Nebula (IC 5146). However, it was interesting to use it to observe this emission nebula. I was amazed because, in this case, this filter worked very well. Using low magnification (42x) and this kind of filter the image was very good, with good contrast. Some dark structures within the nebulae were visible also. Beyond the usefulness of the OIII and H-beta, the UHC was the best filter to use at the moment of observing this diffuse nebula.

NGC 6334, THE “CAT´S PAW” NEBULA 

This nebula, also known as “Bear Claw Nebula”, is one of the most prominent sites of massive star formation. The central region consists of a long filament with seven sites of massive star formation. At a distance of about 5500 light-years, it is visible in a rich starry field when you observe it through a telescope. In the same eyepiece field, the dark nebula Barnard 257 (B257) was barely visible at low magnification. The identification of a stellar configuration situated to the West of the dark cloud (to the right on the picture below) helped to find it.

Focusing the attention on NGC 6334, three hazy patches come to the view at low magnification and without any filter. These nebulosities surround stars which are visible within them. Some nebulosity is visible in the area indicated by number 1 in the picture below. To the West (right on the image) there are some very faint stars very close to each other and embedded in faint nebulosity.

The Cat`s Paw Nebula. North is up.

The most conspicuous and bigger nebulosity is that surrounding the star of magnitude 9.4 HD 156738 (number 2 on the image here). The nebula is also visible to the East of the star, always showing a smooth appearance.

The patch indicated by number 3 is also visible without a filter. Between nebulosities 1 and 3, some of nebulosity is detected, embedding the stars situated there. The east end is more contrasted and easier to see visually.

The Observation with UHC filter

This kind of filter allows you to see more contrasted nebulosity in zones 1,2, and 3. The zone in 2 is again the brighter one visible in the field. More structures are visible in this nebula when you use nebular filters. For example, with the UHC filter, you can see the elongated nebulosity marked by the white ellipse in the picture above. I also observed this nebula with an OIII filter but the view was not so sharped and contrasted. Even if the nebulosity 2 is clearly visible through this filter, some other structures begin to fade and they become more difficult to see.

The H-beta filter also works to see this nebula, at least to see its three brighter patches, but it is not the best option when you want to observe NGC 6334. In my opinion, the best filter was the UHC, followed by the OIII filter. H-beta was the last option to choose.

THE NGC 6357 NEBULA & THE OPEN CLUSTER PISMIS 24

DSS image of NGC 6357 and Pismis 24. North is up

About 3 degrees from the bright star Shaula, one of the bright stars in the tail of Scorpius, we found the diffuse emission nebula NGC6357, usually known as “War and Peace nebula”, discovered by John Herschel on June 8, 1837.

The first attempt to see this nebula was using my 8-inch telescope without a nebular filter. After aiming the telescope to that region of the sky, the identification of the surrounding field was easy because there exist four bright stars with magnitudes of 6 and 7 forming a straight line. Actually, they are the brighter stars in the eyepiece field. An open cluster can be found within the nebula, Pismis 24. After having the accurate position of this star cluster among the stars (see comments below) I began with the observation of it and the associated nebula NGC 6357.

There is an interesting story about this open cluster,

NGC 6357 is classified as an emission nebula. The whole nebula was not visible at all. However, the brighter part of it is easily visible even without using a nebular filter. That brighter part, indicated by the red circle in the DSS image above, is better viewed as a small patch of nebulosity, using averted vision. Very close to it, the small open cluster Pismis 24 is clearly visible. According to the software Skymap, this stellar group has a magnitude of 9.6 and its Trumpler classification is IV,2,p,n, thus indicating associated nebulosity. If you use low magnification, you will see a sort of defocused star (the brighter in the cluster) and two fainter stars forming a line with that one. Some very faint stars are visible between that stars and the nebulosity. Using low magnification but this time with a UHC filter, you can get a more detailed view of the nebulosity. This looks better, showing an elongated shape becoming wider toward the direction where the cluster lies. The elongated shape of the nebula is more obvious using a little higher magnification and the same filter.

I used more power (156x) to see the cluster in detail. At this magnification, I could see four stars that remember me a very small version of the constellation Sagitta (The arrow) in the northern sky.

At this magnification I could clearly see the bright star Pismis 24-1 (indicated on the DSS image at the top of this section) and also the star Pismis 24-17 (immediately to the left of Pismis 24-1 on the image). Pismis 24-17 is also a member of the cluster. 

As you can read, awesome nebulosities inhabit the marvelous constellation Scorpius, in the southern skies.