The mystery of chemical anomalies in clusters.

Globular clusters are among the first objects formed in the early Universe and host some of the oldest stars known. We can access the memory of the star formation and assembly mechanism of galaxies by measuring ages, chemical abundances and kinematic properties of globulars. The discovery that stars in globular clusters do not fit within the traditional picture of hosting stars with the same age and chemical composition, has led to a renewed interest in globular cluster studies. Indeed, it has been discovered that these stellar systems host stars with variations in many elements like helium, carbon, nitrogen, oxygen, and sodium. Severals scenarios have suggested to explain the origin of the chemical variations observed in clusters, but none of them is able to entirely reproduce observations, especially when a global view is taken. After more than 30 years from the discovery of such chemical anomalies, their origin is still a mystery and it is unlikely that most of the questions on globular clusters formation and evolution will be answered without a new insight on this baffling phenomenon. 

In this review we provide an outline of observations of the chemical variations along with a critical comparison against theoretical models. While many of the previous reviews have concentrated on the chemistry, we explore that as only one line of evidence, and also consider global properties and correlations, relation to field stars and the physical properties of both young and old massive clusters. We also attempt to highlight topics that are particularly uncertain and which new theoretical and observational studies are likely to lead to important advances. Finally, we suggest some of the directions that we feel are likely to be the most successful in the coming years.