Stellar evolution

Figure: Open cluster Colour-Magnitude diagrams from the revised Hipparcos reduction (van Leeuwen 2009, A&A, 497, 209). The clusters are colour-coded by age; precision membership, kinematics and abundance measurements will be provided here for a much larger cluster sample.

Each star cluster provides a (near-)coeval snapshot of the stellar mass function. This survey contributes to testing stellar evolution models from pre-main sequence phases right through to advanced evolutionary stages. Much of the input physics in stellar models can be tested by its effects on stellar luminosities, radii and the lifetimes of different evolutionary phases. Homogeneous spectroscopy will provide estimates of stellar parameters and reddening for large samples of stars over a wide range of masses, in clusters with a wide range of ages and mean chemical compositions. Such data are essential in testing, calibrating, and refining both evolutionary tracks and stellar parameters derived from spectra (Soderblom 2010). While of enormous standalone value, when later combined with Gaia astrometry, and supplemented by asteroseismology, these data will isolate and probe all the theoretical uncertainties, whilst simultaneously identifying and quantifying important perturbing factors such as binarity, rotation, accretion and magnetic activity.