The presence of blue straggler stars ( BSs ) in star clusters has proven a challenge to conventional simple stellar population ( SSP ) models .
Conventional SSP models are based on the evolution theory of single stars .
Meanwhile , the typical locations of BSs in the colour-magnitude diagram of a cluster are brighter and bluer than the main sequence turn-off point .
Such loci can not be predicted by single-star evolution theory .
However , stars with such properties contribute significantly to the integrated light of the cluster .
In this paper , we reconstruct the integrated properties of the Large Magellanic Cloud cluster ESO 121–SC03 , the only cluster populating the well-known age gap in the cluster age distribution , based on a detailed exploration of the individual cluster stars , and with particular emphasis on the cluster ’ s BSs .
We find that the integrated light properties of ESO 121–SC03 are dramatically modified by its BS component .
The integrated spectral energy distribution ( ISED ) flux level is significantly enhanced toward shorter wavelengths , and all broad-band colours become bluer .
When fitting the fully integrated ISED of this cluster based on conventional SSP models , the best-fitting values of age and metallicity are significantly underestimated compared to the true cluster parameters .
The age underestimate is \sim 40 per cent if we only include the BSs within the cluster ’ s half-light radius and \sim 60 per cent if all BSs are included .
The corresponding underestimates of the cluster ’ s metallicity are \sim 30 and \sim 60 per cent , respectively .
The populous star clusters in the Magellanic Clouds are ideal objects to explore the potential importance of BSs for the integrated light properties of more distant unresolved star clusters in a statistically robust manner , since they cover a large range in age and metallicity .