Globular star clusters are a symmetrical collection of ancient stars (up to a million such stars) which are bound together gravitationally. Recent estimates indicate that about 150-200 globulars exist throughout our galaxy with only three being readily visible to the naked eye (the Andromeda Galaxy has been estimated to contain approximately 500 globular clusters). Since most of the globular clusters are more common in the southern hemisphere, scientists have deduced that our sun must lie away from the galactic core of the Milky Way. One of the most beautiful such globular clusters is M13 in Hercules.

Note: A survey of the POSS (Palomar Observatory Sky Survey) plates during the 1950's by various astronomers including Edwin Hubble, Halton Arp and George Abell revealed fifteeen new globular clusters which are diverse in both apparent diameter (1.8' to 10.9'x8.8') and magnitude (9.2 to 15.1). Some of the Palomar globulars (ex. PAL 6-7, 9-11) are typical in both size and distance but dim due to intervening galactic dust; other clusters, such as PAL 3-4 and 14, are significantly larger but lie at the outer limits of our galaxy. Similar to the Abell catalog of planetary nebulae, this particular list of globular clusters is a popular target of observers with large-aperture instruments such as Dobsonians and including an annual "Palomar marathon".

Note: PAL 15 in Ophiuchus is a globular cluster with a very weakly concentrated core and as indicated by the image below and its Trumpler classification of "XII". It is characterized with a surface brightness of 18.9 mag/arc-min² with its brightest star being only of magnitude 17.1, thus making it the dimmest globular clusters within the PAL catalog. Its apparent diameter of approximately eleven arc-minutes makes PAL 15 the largest member of the PAL catalog but this large apparent diameter leads to a very prohibitive and low surface brightness. PAL 15 lies at a distance of 145,500 light-years away, thus making it one of most distant globular clusters within the Milky Way. PAL 15 was discovered by Zwicky in 1959 and who suggested it was comprised of approximately 400 member stars ranging between magnitude 19 and 22.