What chemical signature indicates the primordial composition of Globular Cluster stars?
Answer
Metal-poor (few elements heavier than helium/hydrogen)
Globular Clusters (GCs) formed very early in the universe, before numerous generations of massive stars had lived, died, and enriched the interstellar medium through supernova explosions. Consequently, the stars within GCs are metal-poor, meaning they contain very few elements heavier than the primordial hydrogen and helium. This low metallicity acts as an intrinsic chemical clock. For instance, clusters with iron content significantly below $Z=0.001$ (one-thousandth of the sun's abundance) are reliably identified as ancient GCs. Open Clusters, forming later in the disk, have incorporated elements from more past stellar cycles and thus exhibit generally higher metallicity.
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