M5 is one of the oldest globular clusters with an estimated age
of 13 billion years. Its photographic diameter is about 165 light
years with a tidal diameter (beyond which member stars would be
torn away) of over 200 light years. The cluster shows an intermediate
level of metallicity. Metallicity is a term used that refers to
elements heavier than helium, although some heavier elements like
carbon for instance, are not really metals. These heavier elements
were not present when the universe formed and could only be created
through nucleosynthesis within the cores of stars. Although globulars
as a group are metal deficient they are divided into metal poor
and metal rich groups. The metal rich groups still have much lower
metallicity than a younger star like our sun. This is called the
Oosteroff dichotomy. Metallicity is estimated by analyzing the
spectra of a star and computing the Iron to hydrogen ratio (Fe/H).
An FE/H <2.0 places a globular in the metal poor group. Metallicity
seems to decrease with distance from the galactic center suggesting
a possible separate origin or even a captured origin of metal
poor globulars from a dwarf satellite galaxy in the remote past.