After some time off due to other requirements on my time, I am bringing back the TSS
Challenge. However, I am changing the format to a bi-monthly effort. Because the English language is a bit screwy at times, the term “bi-monthly” can mean either twice monthly or once every other month. In this case, my intent is the latter rather than the former. This will give me more time to prepare each article and you more time to complete the challenge as you have to deal with the variables of your local conditions and individual demands on free time.
Those of you who know me even modestly know well that I am a hardcore galaxy hunter. However, what some may not know is that I really, really like globular clusters almost as much. Therefore this challenge will present a triple-play globular event, which will highlight only one constellation in each celestial hemisphere. Each of those constellations contain three primary globular clusters, which all appear in the New General Catalogue, and in the case of the northern entry, two also appear in the Messier
catalogue. For the northern folks we will feature the constellation of Hercules, and for our southern friends the constellation Ara. The data for each object is sourced from Telescopius.com, and this includes visual magnitude, angular size (in arc minutes), surface brightness (in mag per arcmin2) and the core concentration class from the Shapley-Sawyer scale (1-densest thru 12-loosest). Without further ado, let’s reveal what objects I have for you to pursue over the July/August timeframe, and I wish you all good luck with your observing, sketching and imaging pursuits. As always I encourage you to post your results here so that all, including you, may learn from your experience.
Northern Celestial Hemisphere – Hercules, the mythological hero and strongman:
Messier 13 / NGC 6205 (globular cluster, mag=5.8, size=20.0’, SBr=12.0, class=5):
Known famously as Great Globular Cluster in Hercules or the Hercules Globular Cluster, the 13th entry in Charles Messier
’s Catalogue of Nebulae and Star Clusters published in 1784 is well known to the vast majority of amateur astronomers with even a modicum of experience. It was actually discovered by Edmond Halley 1714 and independently discovered by Charles Messier
in 1764. Often the very first globular observed by beginning observers, it can be glimpsed naked eye under excellent and dark conditions, and is readily apparent as a non-stellar object with minimal optical aid. Sporting a very bright and compact core, it blazes into glory in even smaller scopes, where it is possible to resolve some stars in its outer fringes. As aperture
increases, one can resolve more stars across its face all with its intensely bright and compact core as a backdrop. Not only is it a prominent visual target, but it is also very easy to locate. It is situated along the western side of the most prominent portion of Hercules, the famous “keystone” asterism
about one-third of the way along a line from mag 3.5 Eta to 2.8 Zeta Herculis. With increasing aperture
more and more stars become resolved, and one can glimpse strings of stars emanating outward from the core into the outer halo.
A real challenge with this object is the visual detection of a structural detail known as the “propeller.” This is a small and very dim three-pronged feature emanating from a common center point that gives the impression of a propeller. It is caused by intervening dust within the cluster. This elusive detail is easier to discern using imaging than via visual observation, but it can be seen with great care using larger apertures under a steady dark sky. If you don’t feel quite up to the “propeller” challenge, but still feel sporty, there is a nearby galaxy you can try. NGC
6207 is a spiral galaxy glowing at magnitude 11.6 that is only 27.5’ northeast of the center of M13. Spend some time with M13 and you will see just why so many observers return to it year after year.
Messier 92 / NGC 6341 (globular cluster, mag=6.3 size=14.0’, SBr=12.0, class=4):
This globular cluster is often thought of as M13’s little brother. Though it does appear similar to the more famous globular, it is smaller and about half a magnitude dimmer with a subtly more condensed core. It is located in northern Hercules about 6° 20’ north of the northeastern corner star of the keystone asterism
, mag 3.2 Pi
Herculis. Discovered in 1777 by Johann Bode, it was independently discovered by Charles Messier
Visually in it can be located with binoculars and small apertures as a small glowing orb. Even in moderate aperture
, it begins to reveal some resolution at its fringes and across its face. In larger aperture
the resolution becomes deeper into its body with some of the outer portions of the core starting to resolve. It is a beautiful object on its own, and while it is not as often observed as is M13, it truly does not play second fiddle. It reveals several strands of stars flowing outward from the core and displays noticeable resolution of its members, so while it is smaller and slightly dimmer, it truly does hold it own visually against its more famous neighbor. If one does return to M13 year after year, they would depriving themselves of something equally as beautiful if they did not revisit M92 at the same time.
NGC 6229 (globular cluster, mag=9.4, size=4.5’, SBr=12.4, class=4):
The third globular cluster in Hercules that we find in the New General Catalogue was discovered by William Herschel in 1787. This small and fairly dim cluster is most definitely lesser known to many observers and imagers. Its weak visual appearance makes it much more of a challenge for smaller apertures, though it may be glimpsed as a very small fuzzy star-like object. Even it moderate to larger apertures it does not yield much in terms of resolution.
Located nearly 5° ENE of mag 3.9 Tau Herculis, it forms a triangle with a widely spaced pair of 8th magnitude field stars (HD 151651 and HD 151689) to its west. It’s very small and compact core is noticeably bright within a small and dim halo. Depending upon aperture
it may take on a mottled appearance, hinting that one is near some level of stellar resolution, but it takes larger aperture
to pick out a very modest number of member stars at best. It is not showy in any form of the word, and certainly gets lost in the rush to observe M13 and then M92. But, it certainly is worth hunting down and taking a peek so that one can claim to have observed the Hercules trio of globular clusters.
Southern Celestial Hemisphere – Ara, the mythological altar of Zeus:
NGC 6397 (globular cluster, mag=5.3, size=31.0’, SBr=12.5, class=9):
Moving south of Scorpius we come to the constellation of Ara. The brightest globular of the three within the celestial altar is this large and showy cluster. It was discovered in the 1751-1752 timeframe by Nicolas-Louis de Lacaille using his ½ inch 8x telescope while surveying the southern skies from South Africa. This cluster lies at about 7.8 KLY distance from us, making it the second closest globular to the Earth.
Depending upon one’s sky quality it can be easily glimpsed with the naked eye at best, or at a minimum with binoculars and small aperture
scopes. In medium to large scopes it is a sight to behold, revealing many curving streamers of stars emanating outward from it with many stars resolved at its fringes and across the face. It contains a smaller bright core and one may see some some resolution at the edges of this core as well as some mottling in the core itself, all forming a backdrop for the resolved stars across the cluster’s face. This is a beautiful globular that if located farther north in the sky would rival the best we have in the northern hemisphere without a doubt. Look for this delightful cluster about 55’ NNE of mag 5.2 Pi
Arae, it is difficult to miss!
NGC 6352 (globular cluster, mag=7.8, size=9.3;, SBr=12.3, class=11):
Our next cluster in Ara is located in the north central part of the constellation about 1° 47’ northwest of mag 2.9 Alpha Arae.. The mag 7.0 star HD 157555 is just 14.5’ southeast of the cluster in the direction of Alpha Arae. This cluster was discovered by James Dunlop in 1826 during his survey of the southern skies from Paramatta, New South Wales, Australia.
Visually it should not be difficult in binoculars or small apertures. Medium aperture
will begin to show some resolution and display its loose central area with a very small modestly concentrated core. Larger aperture
will reveal more and deeper resolution of both its halo and central region with its small core as a backdrop. Not as large or showy as NGC
6392, it still holds its own visually and is a beautiful cluster in its own right.
NGC 6362 (globular cluster, mag=8.1, size=15.0’, SBr=13.7, class=10):
Our final globular in Ara and for this month’s challenge is found in deep southern Ara close to the border with Apus. It can be found about 1° 12’ northeast of mag 4.8 Zeta Apodis, situated within a box of four field stars (mags 6th to 9th). This cluster was also discovered in 1826 by James Dunlop during his time at Paramatta.
Another target accessible to binoculars and small apertures, it starts to reveal its true nature more with moderately sized scopes. It contains a loosely structured broadly concentrated central region with a bit of an uneven halo structure. Resolution of many stars is possible with moderate and particularly larger aperture
, and one may glimpse a rambling line of stars crossing the face of the cluster. Visually it is a bit more uneven than are the two previous globulars, but it is no less beautiful and will not disappoint.
I now leave the above objects in your hands to pursue over the next two months. Obviously given the declinations involved, not everyone would be able to observe all of these objects. But should you be lucky enough to live at a more southerly latitude in the northern hemisphere or a more northern latitude in the southern hemisphere and have access to all of them, or at least a portion of the opposite hemisphere’s objects, please feel free to observe any that you have access to. Again, I encourage you to post your results here. Whether your game be visual observation, sketching or imaging, we would like to see just what you can come up with for any of the above objects that grace your night sky. So good luck and enjoy the fun of the challenge.