The science case for the Gaia mission was compiled in the year 2000 (Perryman et al. 2001). The scientific goals of the design reference mission were relying heavily on astrometry, combined with its photometric and spectroscopic surveys. The astrometric part of the science case remains unique, and so do the photometric and spectroscopic data, despite various, large ground-based surveys having materialised in the last decade(s). The space environment and design of Gaia enable a combination of accuracy, sensitivity, dynamic range, and sky coverage, which is practically impossible to obtain with ground-based facilities targeting photometric or spectroscopic surveys of a similar scientific scope. The spectra collected by the radial-velocity spectrometer (Sect. 3.3.7) have sufficient signal to noise for bright stars to Article number, page 3 of 36 make the Gaia spectroscopic survey the biggest of its kind.

The astrometric part of Gaia is unique simply because global, microarcsecond astrometry is possible only from space.

Therefore, the science case outlined more than a decade ago remains largely valid and the Gaia data releases are still needed to address the scientific questions (for a recent overview of the expected yield from Gaia, see Walton et al. 2014).

A non-exhaustive list of scientific topics is provided in this section with an outline of the most important Gaia contributions.
2.1.

Structure, dynamics, and evolution of the Galaxy The fundamental scientific-performance requirements for Gaia stem, to a large extent, from the main scientific target of the mission: the Milky Way galaxy.

Gaia is built to address the question of the formation and evolution of the Galaxy through the analysis of the distribution and kinematics of the luminous and dark mass in the Galaxy.

By also providing measurements to deduce the physical properties of the constituent stars, it is possible to study the structure and dynamics of the Galaxy.

Although the Gaia sample will only cover about 1% of the stars in the Milky Way, it will consist of more than 1000 million stars covering a large volume (out to many kpc, depending on spectral type), allowing thorough statistical analysis work to be conducted.

The dynamical range of the Gaia measurements facilitates reaching stars and clusters in the Galactic disk out to the Galactic centre as well as far out in the halo, while providing extremely high accuracies in the solar neighbourhood.

In addition to using stars as probes of Galactic structure and the local, Galactic potential in which they move, stars can also be used to map the interstellar matter.

By combining extinction deduced from stars, it is possible to construct the three-dimensional distribution of dust in our Galaxy.

In this way, Gaia will address not only the stellar contents, but also the interstellar matter in the Milky Way.
2.2.

Star formation history of the Galaxy The current understanding of galaxy formation is based on a combination of theories and observations, both of (high-redshift) extragalactic objects and of individual stars in our Milky Way.

The Milky Way galaxy provides the single possibility to study details of the processes, but the observational challenges are different in comparison with measuring other galaxies.

From our perspective, the Galaxy covers the full sky, with some components far away in the halo requiring sensitivity, while stars in the crowded Galactic centre region require spatial resolving power.

Both these topics can be addressed with the Gaia data.

Gaia distances will allow the derivation of absolute luminosities for stars which, combined with metallicities, allow the derivation of accurate individual ages, in particular for old subgiants, which are evolving from the main-sequence turn-off to the bottom of the red giant branch.

By combining the structure and dynamics of the Galaxy with the information of the physical properties of the individual stars and, in particular, ages, it is possible to deduce the star formation histories of the stellar populations in the Milky Way.
2.3.

Stellar physics and evolution Distances are one of the most fundamental quantities needed to understand and interpret various astronomical observations of stars.

Yet direct distance measurement using trigonometric parallax of any object outside the immediate solar neighbourhood Article number, page 4 of 36
or not emitting in radio wavelengths is challenging from the ground.

The Gaia revolution will be in the parallaxes, with hundreds of millions being accurate enough to derive high-quality colour-magnitude diagrams and to make significant progress in stellar astrophysics.

The strength of Gaia is also in the number of objects that are surveyed as many phases of stellar evolution are fast.

With 1000 million parallaxes, Gaia will cover most phases of evolution across the stellar-mass range, including pre-mainsequence stars and (chemically) peculiar objects.

In addition to parallaxes, the homogeneous, high-accuracy photometry will allow fine tuning of stellar models to match not only individual objects, but also star clusters and populations as a whole.