axes, 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.

The combination of Gaia astrometry and photometry will also contribute significantly to star formation studies.
2.4.

Stellar variability and distance scale On average, each star is measured astrometrically ∼70 times during the five-year nominal operations phase (Sect. 5.2).

At each epoch, photometric measurements are also made: ten in the Gaia G broadband filter and one each with the red and blue photometer (Sect. 8.2).

For the variable sky, this provides a systematic survey with the sampling and cadence of the scanning law of Gaia (Sect. 5.2).

This full-sky survey will provide a census of variable stars with tens of millions of new variables, including rare objects.

Sudden photometric changes in transient objects can be captured and the community can be alerted for follow-up observations.

Pulsating stars, especially RR Lyrae and Cepheids, can easily be discovered from the Gaia data stream allowing, in combination with the parallaxes, calibration of the periodluminosity relations to better accuracies, thereby improving the quality of the cosmic-distance ladder and scale.
2.5.

Binaries and multiple stars Gaia is a powerful mission to improve our understanding of multiple stars.

The instantaneous spatial resolution, in the scanning direction, is comparable to that of the Hubble Space Telescope and Gaia is surveying the whole sky.

In addition to resolving many binaries, all instruments in Gaia can complement our understanding of multiple systems.

The astrometric wobbles of unresolved binaries, seen superimposed on parallactic and proper motions, can be used to identify multiple systems.

Periodic changes in photometry can be used to find (eclipsing) binaries and an improved census of double-lined systems based on spectroscopy will follow from the Gaia data.

It is again the large number of objects that Gaia will provide that will help address the fundamental questions of mass distributions and orbital eccentricities among binaries.
2.6.

Exoplanets From the whole spectrum of scientific topics that Gaia can address, the exoplanet research area has been the most dynamic in the past two decades.

The field has expanded from hot, giant planets to smaller planets, to planets further away from their host star, and to multiple planetary systems.

These advancements have been achieved both with space- and ground-based facilities.

Nevertheless, the Gaia astrometric capabilities remain unique, probing a poorly explored area in the parameter space of exoplanetary systems and providing astrophysical parameters not obtainable by other means.

A strong point of Gaia in the exoplanet research field is the provision of an unbiased, volume-limited sample of Jupiter-mass planets in multiyear orbits around their host stars.