Last month I posted an observing report about the globular clusters of Spring. This weekend I observed the globs of Summer at the monthly "dark sky night" at our L.A. Astronomical Society's Lockwood Valley observing site. Initially, my agenda was to observe all of the globs of Ophiuchus (which is to globs what Virgo is to galaxies and what, for us northern hemisphere observers, Auriga is to galactic clusters). I listed 27 of these objects (including M62 on the border with Scorpius and NGC6539 on the border with Serpens), of which 7 are on the Messier list and no less than 13 are among the Herschel 400 (including NGC6171 which is also included among those on the Messier list as object 107). I had intended to view them in the order in which I had listed them, by declining magnitude, from bright M12 (magnitude 6.1) down to Palomar 15 (magnitude 14.2) and PWM78 2 (unknown magnitude); but as the night wore on and I was trying to see detail in 9th magnitude objects I ended this quest after observing 13 of the Ophiuchus objects and instead looked at numerous globs in other constellations.
When I later tried to produce coherent observing logs from the notes I had hastily scribbled in the field, I discovered that my visual impressions of these objects placed them into a few distinct categories. In addition to their variations in visual magnitude and diameter, which may be attributable to their distance as well as their intrinsic size, some of these globulars had distinct bright cores, others not; some had extensive halos of unresolved members, others not; for some the transition or borderline between core and halo was sharp, for others it was gradual; some were surrounded by outlying resolved stars, others not; some appeared to be round or spherical (i.e., globular), and others appeared oval or shapeless. I looked for some established system of classification, like the Trumpler classification system that is used for galactic clusters.
I found two kinds of classification systems for globulars. The kind used by serious scientists and astrophysicists is based on spectroscopic and statistical analysis and, though interesting, is of little use to amateur visual observers like me. The other system contains 12 classes based solely on the "concentration" of these objects, ranging from Class I (the most highly concentrated) to Class XII (the least so). This system is mentioned and applied to objects listed in Burnham's Celestial Handbook and in the Webb Society's Deep-Sky Observer's Handbooks (where it is attributed in one case to Harlow Shapley and in another to a 1965 publication by Halton Arp), but nothing in my library explains it further.
Can anyone guide me to a system by which globular clusters are classified on the basis of their several principal visual attributes, as these appear in our telescopes, or do I have to make up my own system?
When I later tried to produce coherent observing logs from the notes I had hastily scribbled in the field, I discovered that my visual impressions of these objects placed them into a few distinct categories. In addition to their variations in visual magnitude and diameter, which may be attributable to their distance as well as their intrinsic size, some of these globulars had distinct bright cores, others not; some had extensive halos of unresolved members, others not; for some the transition or borderline between core and halo was sharp, for others it was gradual; some were surrounded by outlying resolved stars, others not; some appeared to be round or spherical (i.e., globular), and others appeared oval or shapeless. I looked for some established system of classification, like the Trumpler classification system that is used for galactic clusters.
I found two kinds of classification systems for globulars. The kind used by serious scientists and astrophysicists is based on spectroscopic and statistical analysis and, though interesting, is of little use to amateur visual observers like me. The other system contains 12 classes based solely on the "concentration" of these objects, ranging from Class I (the most highly concentrated) to Class XII (the least so). This system is mentioned and applied to objects listed in Burnham's Celestial Handbook and in the Webb Society's Deep-Sky Observer's Handbooks (where it is attributed in one case to Harlow Shapley and in another to a 1965 publication by Halton Arp), but nothing in my library explains it further.
Can anyone guide me to a system by which globular clusters are classified on the basis of their several principal visual attributes, as these appear in our telescopes, or do I have to make up my own system?
NexStar 8i SCT, Orion 80ED APO refractor, Orion 120 mm EQ refractor, 3.5-inch Questar
