A globular cluster is a special group of star clusters, which has a symmetrical structure shaped like a ball. Only sometimes one can observe some kind of flattening. With naked eye only a few globular clusters can be observed. In the southern sky ω Centauri (NGC 5128) looks like a star of apparent magnitude 4m.0. Already with small telescopes one can see millions of stars in this gigantic cluster. The brightest globular cluster in the northern sky is M 13 located in the constellation Hercules. With opera glasses one can see just a blurred nebula. Single stars can only be observed with medium sized telescopes.
The mass of globular clusters is between 10,000 and 1,000,000 times of the solar mass. Most of the members of the cluster are old stars of population II Therefore the age of these clusters might reach 10 billion years. Spectroscopical analysis showed that the abundance of heavier elements is similar to that in old objects.
In the center of a globular cluster, the number of stars is very dense and the median distance between two stars is 10000 times less than in the neighbourhood of the sun.
The distance of globular clusters can be measured by the variability period and the absolute magnitude of RR Lyrae stars. This type of (old) variable stars is typical for globular clusters. Most of them lie within a distance of 10,000 up to 100,000 ly or even above. They form some kind of framework around our own galaxy. More than 100 globular clusters seem to belong to our galaxy and even in distant galaxies like M 31, which is the Andromeda Nebula, globular clusters can be found. In rare cases, globular clusters do not belong to any galaxy.