A Tale of Three Globulars: Messier 2, Messier 13 and Messier 15
Back in early October, I spun around and picked out three of the most fantastic globulars: Messier 13, (“The Great Cluster in Hercules”), Messier 2, and Messier 15. All three of these look spectacular even with a big and bright moon out and they are all great both for visual viewing as well as imaging. And here I am now, with clouds in Woodinville — beside a big bright moon. And I’d be imaging comet 103p/Hartley 2 anyway, so what else to do!? Play with processing old data!
The most brilliant of them all, Messier 13 covers nearly 16 arc-minutes of sky and shines at a whopping magnitude 5.9. Locating M13 is quite easy once you have found the body of Hercules since it lies along the connecting edges of the box making Hercules ‘body’.
Imaging Messier 13 is generally easy since it is so big and bright, although visually it is simply captivating. The longer you stare off into the void, the more stars seem to pop into the cluster. In that rare moment of pristine seeing, it’s as if your eye is a billion pixel camera, with each star hitting a single pixel. If you have never looked at M13 through a telescope, you are missing out on the best globular cluster out there. In the Celestron CPC 1100, any eyepiece in the 10-20mm (13mm Ethos or 21mm Ethos are definite winners) will give you a spectacular sight!
The next two, M2 and M15 are very similar: both around 12 arc minutes in size and right about magnitude 6.5. M2 is slightly larger and brighter but they are both similar and easy to find. M2 lays between Sadalsuud (Aquarius) and Enif (Pegasus); about 1/3 of the way. M15 lays past the end of Pegasus’ Enif; about half as far as the distance from Enif to the previous star of Pegasus.
Both of these globulars are also spectacular to view, although nothing as majestic as M13. At a distance of about 37.5k light years and 150k stars, M2 has about half as many stars as M15, which is almost the same distance away. (For comparison, M13 is only about 25k light years away, and most estimates put it around 500k stars, but the upper end of estimates are in the million range). Obviously it’s pretty hard to count the number of stars here — instead the smart astrophysicists look at parameters such as total luminosity, distance, and magnitude to make their guesses. Sadly, counting pixels doesn’t quite work
All these images were from the 11 inch Celestron CPC 1100 mounted AltAz with f/6.3 focal reducer with the Orion Starshoot Pro v2 CCD camera. M2 and M15 were processed in MaximDL and DeepSkyStacker, although I used my Mac and Nebulosity for M13. Since the exposure times here are short (30 seconds), rotation is not much of an issue and the need for the Pyxis derotator is unncessary since all these programs have built in derotation routines! Take a bunch of them, drop the junk ones, sigma stack them, and you’re off to the races!
So, what do you think? Which is your favorite? How do you like to view them? Which other globulars do you like?