What is a globular cluster? An observational perspective

@article{Gratton2019WhatIA,
  title={What is a globular cluster? An observational perspective},
  author={Raffaele Gratton and Angela Bragaglia and Eugenio Carretta and Valentina D’Orazi and Sara Lucatello and Antonio Sollima},
  journal={The Astronomy and Astrophysics Review},
  year={2019},
  volume={27},
  pages={1-136}
}
Globular clusters are large and dense agglomerate of stars. At variance with smaller clusters of stars, they exhibit signs of some chemical evolution. At least for this reason, they are intermediate between open clusters and massive objects such as nuclear clusters or compact galaxies. While some facts are well established, the increasing amount of observational data are revealing a complexity that has so far defied the attempts to interpret the whole data set in a simple scenario. We review… 
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