Star clusters as simple stellar populations
  In this paper, I review to what extent we can understand the photometric
properties of star clusters, and of low-mass, unresolved galaxies, in terms of
population synthesis models designed to describe `simple stellar populations'
(SSPs), i.e., groups of stars born at the same time, in the same volume of
space, and from a gas cloud of homogeneous chemical composition. The
photometric properties predicted by these models do not readily match the
observations of most star clusters, unless we properly take into account the
expected variation in the number of stars occupying sparsely populated
evolutionary stages, due to stochastic fluctuations in the stellar initial mass
function. In this case, population synthesis models reproduce remarkably well
the full ranges of observed integrated colours and absolute magnitudes of star
clusters of various ages and metallicities. The disagreement between the model
predictions and observations of cluster colours and magnitudes may indicate
problems with or deficiencies in the modelling, and dioes not necessarily tell
us that star clusters do not behave like SSPs. Matching the photometric
properties of star clusters using SSP models is a necessary (but not
sufficient) condition for clusters to be considered simple stellar populations.
Composite models, characterized by complex star-formation histories, also match
the observed cluster colours.
