Pre-main sequence evolutionary theory is not well-calibrated to observations .
With care , the observed quantities can be converted into effective temperature and luminosity ( i.e .
the Hertzsprung-Russell diagram ) which the theoretical calculations also predict as a function of stellar mass and age .
For a sample of nearby young stellar clusters and associations ranging in age from < 1 Myr to > 100 Myr , we have tested the loci of luminosity as a function of effective temperature against various sets of predicted pre-main sequence isochrones .
As we found in Hillenbrand & White ( 2004 ) which tested stellar masses , here for the stellar ages there are two conclusions : some evolutionary calculations fare better than others in reproducing the empirical sequences , and systematic differences between all pre-main sequence evolutionary calculations and the data are apparent .
We also simulate hypothetical clusters of varying star formation history and compare the resulting HR diagram predictions to observed clusters .
Our efforts are directed towards quantitative assessment of apparent luminosity spreads in star forming regions and young clusters , which are often erroneously interpreted as true luminosity spreads indicative of true age spreads .