Stellar models describing the evolution of a star's composition and internal processes with stellar age, give predictions on the observed luminosity and colour (or equivalently surface temperature) of the star.  During a star's lifetime, its surface temperature and luminosity evolve as the star's energy source, the fusion of light elements in its core and shells, progresses through a series of elements.

Although the fundamental principles of stellar evolution are well understood, there are still several aspects of the evolution and interior of stars for which the current theories require further improvement.

As a star evolves it will follow a track in the Hertzsprung-Russel diagram, which is a plot of the star's luminosity versus its colour. The high-precision positions in the Hertzsprung-Russell diagram of stars of known surface abundances, provided by Hipparcos and by high-resolution spectroscopy, have revealed discrepancies between the observations and the predictions of standard stellar models.

Gaia will return accurate luminosities, surface temperatures, chemical abundances, masses, and determinations of the extinction of stellar light by the interstellar medium for all types of stars and hence for the full range covered in the Hertzsprung-Russel diagram. 

Science Goals

The large sample of stars over different stellar type gathered by Gaia, will greatly extend our understanding of stellar structure and evolution and will allow further improvement of theoretical models of stellar interiors, for example in the areas of:

• The size of the convective cores
  Here the generated heat is dissipated by convection of the gas. The size of the core defines the amount of available nuclear material for sustaining the star's energy output
• The internal diffusion of chemical elements
  Diffusion of elements can result in helium being transported to the core, increasing the amount of helium that can be converted into carbon and extending the time spent at this stage by the star
• The outer convective zones
  Gaia will greatly enhance our capabilities of dealing with non-local convective models for stellar interiors, as opposed to most of the current stellar models that are still built by treating convection based on a classical theory that works best only for stars in the Main Sequence phase (when they are in the process of burning hydrogen into helium at their cores)