Globular cluster
A Globular cluster is a special group of star clusters, which have a symmetrical structure. 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 glases one can see just a blurred nebula. Single stars can only be observed with medium sized telescopes.
The mass of glubular clusters is between 10,000 and 1,000,000 of 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.