The presence of an extended blue horizontal branch ( HB ) in a stellar population is known to affect the age inferred from spectral fitting to stellar population synthesis models .
This is due to the hot blue component which increases the strength of the Balmer lines and can make an old population look spuriously young .
However , most population synthesis models still rely on theoretical isochrones which do not include realistic modelling of extended HBs .
In this work , we create detailed models for a range of old simple stellar populations ( SSPs ) , with metallicities ranging from [ Fe/H ] = -1.3 to solar , to create a variety of realistic HB morphologies , from extended red clumps , to extreme blue HBs .
We achieve this by utilising stellar tracks from the BaSTI database and implementing a different mass loss prescription for each SSP created .
This includes setting an average mass and a Gaussian spread in masses of individual stars coming on to the zero age HB for each model , and hence resulting in different HB morphologies .
We find that , for each metallicity , there is some HB morphology which maximises H \beta , making an underlying 14 Gyr population look \sim 5 - 6 Gyr old for the low and intermediate metallicity cases , and as young as 2 Gyr in the case of the solar metallicity SSP .
We explore whether there are any spectral indices capable of breaking the degeneracy between an old SSP with extended blue HB and a truly young or intermediate age SSP , and find that the Ca ii index of Rose ( 28 ) and the strength of the Mg ii doublet at 2800Å are promising candidates , in combination with H \beta and other metallicity indicators such as Mg b and Fe5406 .
We also run Monte Carlo simulations to investigate the level of statistical fluctuations in the spectra of typical stellar clusters .
We find that fluctuations in spectral indices are significant even for average to large globular clusters , and that various spectral indices are affected in different ways , which has implications for full-spectrum fitting methods .
Hence we urge caution if these types of stellar clusters are to be used as empirical calibrating objects for various aspects of SPS models .