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