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 4 in Ursa Major 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 15.6 mag/arc-min2 with its brightest star being only of magnitude 18, thus making it one of the dimmest globular clusters within the PAL catalog along with PAL 3 in Sextans. Its apparent diameter of approximately two arc-minutes makes PAL 4 also one of the smallest members of the PAL catalog. PAL 4 lies at a distance of 356,000 light-years away, thus making it the second most distant globular clusters within the Milky Way, and is very slightly red-shifted (z=0.023).
The image below is characterized with a number of small and dim galaxies including PGC 1856881 at mag 18.6 ("A"), PGC 1854936 at mag 17.9 ("B"), PGC 1846558 at mag 17.9 ("C"), PGC 1855809 at mag 18.4 ("D"), PGC 1853973 at mag 18.1 ("E"), PGC 1846201 at mag 18.2 ("F"), PGC 1846998 at mag 16.4 ("G"), PGC 1845035 at mag 17.3 ("H") and PGC 1847049 at mag 17.6 ("I").
Please click on the image below to display in higher resolution (1200 x 900)
RA / Dec:
11h 29m 16s /
28° 58' 23"
Feb 18, 2010
01:40 - 04:20 UT+2
AP 160 f/7.5 StarFire EDF
AP 1200GTO GEM
SBIG LRGB filters
1.17" per pixel