  Globular star clusters are compact and massive stellar systems old enough to
have witnessed the entire history of our Galaxy, the Milky Way. Although recent
results suggest that their formation may have been more complex than previously
thought, they still are the best approximation to a stellar population formed
over a relatively short time scale (less than 1 Gyr) and with virtually no
dispersion in the iron content. Indeed, only one cluster-like system (omega
Centauri) in the Galactic halo is known to have multiple stellar populations
with a significant spread in iron abundance and age4,5. Similar findings in the
Galactic bulge have been hampered by the obscuration arising from thick and
varying layers of interstellar dust. Here we report that Terzan 5, a
globular-cluster-like system in the Galactic bulge, has two stellar populations
with different iron content and ages. Terzan 5 could be the surviving remnant
of one of the primordial building blocks that are thought to merge and form
galaxy bulges.
