Object Classification and the Determination of Stellar Parameters

Gaia will observe more than one billion objects brighter than G = 20, including stars, asteroids, galaxies and quasars. As Gaia performs real time detection (i.e., without an input catalogue) the intrinsic properties of most of these objects will not be known a priori. An integral part of the Gaia data processing is therefore to classify everything observed. This will be based primarily on multi-band photometry provided by Gaia, but should also make optimal use of the high resolution spectroscopy (for brighter stars) and the parallaxes. In addition to a broad classification, we can also determine fundamental stellar parameters, in particular effective temperature, metallicity and the line-of-sight interstellar extinction. Such information will be essential for fully exploiting the astrometric part of the Gaia catalogue for stellar population studies. However, extracting this information is a significant challenge, and will need to make use of appropriate multi-dimensional data analysis techniques. I outline some of the problems and the strategies being developed to tackle them.