The resulting database will contain information on a billion stars that together cover a significant fraction of the Galaxy's volume: the accuracy and sensitivity of Gaia allows stars to be detected and their position and velocity to be measured from the solar neighbourhood, all the way through the disc of the Milky Way to the bulge at the Galactic centre (~8.5 kpc from the Sun). In addition globular star clusters in the halo that surrounds the Galactic disc and bulge will be observed.
The three-dimensional velocity of a star will be determined by combining its observed motion across the sky during the 5-year mission with the star's radial velocity that is derived from its measured spectrum. The distributions of stars in the Galaxy over position and velocities are linked through gravitational forces, and through the star formation rate as a function of position and time.
The star formation history can be derived from the currently observed population of stars by determining their distribution over stellar type (colour and luminosity). A star's colour is dependant on the surface temperature of the star. Both quantities, luminosity and colour, change in the course of a star's lifetime as it passes through different evolutionary stages. When plotted in a colour-luminosity diagram, known as the Hertzsprung-Russel diagram, the position of a star in the diagram reveals its age.
The observed distribution over the Hertzsprung-Russel diagram from the Galaxy's population of stars can be compared with those of models containing collections of stars of different ages and colours. This method, however, is limited in accuracy, due to ambiguities in the effects of age and chemical composition on the star's observed colour and luminosity.
The Gaia astrometric data (position and distance), photometric data (luminosity), and spectroscopic data (metallicity, distance and extinction), combined with specifically-developed tools, will resolve this ambiguity. Direct-inversion tools will make the full evolutionary history of the Galaxy accessible.
The complete survey of the sky will provide the details necessary to study the structure and dynamics of our Galaxy, including the build up of its different stellar components. Accurate knowledge of stellar velocities and positions will lead to identification of structures and will give insight into the Galactic dynamics, driven by gravitational interactions (including mergers with smaller satellite galaxies). The Galaxy's star formation history will be derived from the observed current distribution of stars over stellar type.
The detailed knowledge of our Galaxy obtained from the study of the Gaia data, will provide a firm base for the analysis of other galaxies for which this level of accuracy cannot be achieved through direct observations like in our own Galaxy, where we have a close-up and inside view.
Questions that the Gaia results will be able to help answer are:
- Do large galaxies form from accumulation of many smaller systems which have already initiated star formation?
- Does star formation begin in a gravitational potential well in which much of the gas is already accumulated?
- Does the bulge pre-date, post-date, or is it contemporaneous with the halo and inner disc?
- Is the thick disc a mix of the early disc and a later major merger?
- Is there a radial age gradient in the older stars?
- Is the history of star formation relatively smooth or highly episodic?