Transcript: The ages of stars are derived from stellar models. The physics is complex so computers are used to simulate energy transport mechanisms. The details depend on heavy element abundance and on the mechanism for helium diffusion in the atmosphere of the stars. Thus there are uncertainties attached to the prediction of luminosity from stellar models. There are also uncertainties attached to the determination of luminosity from observation of stars. This can include the effect of interstellar extinction and uncertainties in the distance estimates. Since luminosity is proportional to distance squared, a ten percent error in the distance leads to a twenty percent error in luminosity. For all of these reasons it’s difficult to measure ages more accurately than ten or twenty percent.