1 Introduction

Following the effort undertaken by Hubble Space Telescope for improving the determination of the Hubble constant and by taking advantage of several improvements of the input physics a number of authors have recently revisited the prediction of stellar evolutionary theory to re-evaluate the age of galactic globular clusters (see, e.g., VandenBerg et al. 1996; D'Antona et al. 1997; Salaris et al. 1997; Brocato et al. 1997; Chaboyer et al. 1998; Salaris & Weiss 1998a,b). In a previous paper (Cassisi et al. 1998: hereinafter Paper I) we discussed on purely theoretical grounds the effect of the improved physical inputs on stellar evolutionary models, evaluating the additional influence of element diffusion on the evolutionary history of such stars. In this paper we take advantage of such an homogeneous set of stellar models to present theoretical predictions concerning observational magnitudes and colors, and to discuss the theoretical scenario leading to the evaluation of cluster ages.

Such age evaluations, as primarily based on the calibration of the Turn-Off luminosity in terms of the cluster age, requires the adoption of suitable standard candles belonging to the cluster, as given either by MS or by HB stars. In the last case one is dealing with the so called $\Delta V$ method, which is only formally independent of the cluster distance modulus. In all cases, connecting theory to ages requires several steps worth being discussed in some details. Thus, presenting the evolutionary results, we will also investigate the degree of reliability concerning theoretical predictions about the magnitude of MS, TO and HB stars. The main aim of this investigation is to point out theoretical uncertainties but also to show how reliable the new models are, testing the results with available observational constraints and, in particular, with the absolute magnitudes of metal poor stars recently provided by Hipparcos.