  The Gaia satellite will survey the entire celestial sphere down to 20th
magnitude, obtaining astrometry, photometry, and low resolution
spectrophotometry on one billion astronomical sources, plus radial velocities
for over one hundred million stars. Its main objective is to take a census of
the stellar content of our Galaxy, with the goal of revealing its formation and
evolution. Gaia's unique feature is the measurement of parallaxes and proper
motions with hitherto unparalleled accuracy for many objects. As a survey, the
physical properties of most of these objects are unknown. Here we describe the
data analysis system put together by the Gaia consortium to classify these
objects and to infer their astrophysical properties using the satellite's data.
This system covers single stars, (unresolved) binary stars, quasars, and
galaxies, all covering a wide parameter space. Multiple methods are used for
many types of stars, producing multiple results for the end user according to
different models and assumptions. Prior to its application to real Gaia data
the accuracy of these methods cannot be assessed definitively. But as an
example of the current performance, we can attain internal accuracies (RMS
residuals) on F,G,K,M dwarfs and giants at G=15 (V=15-17) for a wide range of
metallicites and interstellar extinctions of around 100K in effective
temperature (Teff), 0.1mag in extinction (A0), 0.2dex in metallicity ([Fe/H]),
and 0.25dex in surface gravity (logg). The accuracy is a strong function of the
parameters themselves, varying by a factor of more than two up or down over
this parameter range. After its launch in November 2013, Gaia will nominally
observe for five years, during which the system we describe will continue to
evolve in light of experience with the real data.
