Terzan 5 is a stellar system in the Galactic bulge commonly catalogued as a globular cluster .
Through dedicated NIR photometry and spectroscopy we have discovered that it harbors two main stellar populations defining two distinct red clumps ( RCs ) in the colour-magnitude diagram , and displaying different iron content : [ Fe/H ] \simeq - 0.2 and [ Fe/H ] = +0.3 for the faint and the bright red clumps , respectively .
In addition , a third minor population with significantly lower metallicity ( [ Fe/H ] = -0.79 ) has been recently detected , thus enlarging the metallicity range covered by Terzan 5 to \Delta [ Fe/H ] \sim 1 dex .
This evidence demonstrates that , similarly to \omega Centauri in the Galactic halo , Terzan 5 is not a genuine globular cluster , but a stellar system that experienced a much more complex star formation and chemical enrichment history .
Moreover the striking chemical similarity with the bulge stars suggests that Terzan 5 could be the relic of one of the massive clumps that contributed ( through strong dynamical interactions with other similar sub-structures ) to the formation of the Galactic bulge .