2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/4370The large galaxies in the Local Group, while all disk galaxies, have diverse stellar populations. A better understanding of these differences, and a physical understanding of the causes, requires more detailed study of the older populations. This presents a significant challenge to GAIA but the scientific returns are also significant.Nine pages, two figures. Review for proceedings of Les Houches School May 2001, `GAIA: A European Space Project', eds O. Bienayme and C. Turon. To appear, 2002, as European Astronomical Society Pub Ser Vol 2 (Published by Edition de Physique). Style file includedAstrophysicstext