Star clusters used to be consid­ered to consist of stars that all formed simul­ta­ne­ously and with the same elemen­tal abundances. The surpris­ing discov­ery that these clusters contain multi­ple popula­tions with charac­ter­is­tic abundance inhomo­geneities remains an enigma. I will inves­ti­gate whether rotational mixing is a plausi­ble culprit, using massive emission-line stars as tracers of rapid rotation. Also, I will assess the valid­ity of certain light elements as signa­tures of multi­ple populations.

Star forma­tion in galax­ies occurs through the collapse of giant gas clouds and typically leads to star clusters. For decades, these clusters were believed to consist of stars that all formed simul­ta­ne­ously and with the same elemen­tal abundances. The discov­ery that such clusters contain multi­ple stellar popula­tions with charac­ter­is­tic light element abundance anticor­re­la­tions and possi­bly age varia­tions came as a surprise. Despite intense efforts and a multi­tude of theories, the origin of multi­ple popula­tions remains a major unsolved problem in astronomy.

One of the proposed culprits is rotation­ally induced mixing in stars. I will explore this possi­bil­ity by conduct­ing a survey of young star clusters using massive emission-line stars, so-called Be stars, as tracers of rapid stellar rotation and of nitro­gen (N) enhance­ment. This first part of my thesis will be mainly based on a propri­etary multi-wavelength survey of a compan­ion galaxy of the Milky Way, the Small Magel­lanic Cloud.

In fact, N enrich­ment is one of the widely used indica­tors of multi­ple popula­tions since it is easily measur­able spectro­scop­i­cally, but it might also have other origins. In the second part of my thesis, I will there­fore assess the valid­ity of this tracer in compar­i­son with other light elements that exhibit varia­tions using novel propri­etary spectro­scopic surveys.