Origin of multiple stellar populations in globular clusters
In plain words
Most old star clusters contain two or more groups of stars with different chemical makeup, as if part of the cluster formed from gas polluted by earlier stars. The proposed polluters cannot make enough polluted gas unless the cluster was once ten or more times heavier.
Precise statement
Second-population stars (enhanced He, N, Na, Al; depleted C, O) form approximately 30 to 90 percent of stars in Galactic globular clusters. Identify the polluter (AGB stars, fast-rotating massive stars, supermassive stars, massive binaries) and a formation scenario that supplies the needed polluted mass without a first population 10 to 20 times more massive than the present cluster, and that explains why the phenomenon appears in clusters older than about 2 Gyr but not in younger massive clusters.
What would settle it
A formation model that reproduces the observed abundance patterns, population ratios versus cluster mass, and the age threshold without exceeding mass limits set by the Galactic halo field population.
Status in the literature
A 2018 review concluded that no proposed scenario accounts for the main observations (Bastian and Lardo, ARA&A, 2018).