The Aristotelian paradigm of unchanging supra-lunar regions has been shown wrong on two fundamental observable aspects: stars change position, and they may change their brightness. The subject of variable stars is an "old" subject of astronomy which started to develop in the 19th century, and still is a modern cutting-edge science (dark energy has been established thanks to supernovae). Today, there are hundreds of thousands variable stars known. The General Catalogue of Variable Stars (GCVS) lists more than 100 types and subtypes of variable stars. In our figure, we made an attempt to cover the different types of variable sources according to the physical phenomena at the origin of their variability (see Eyer and Mowlavi 2008 for details).
The Gaia satellite will give a very substantial contribution to variability studies. First, it will provide a complete survey of the whole sky down to magnitude 20, probably detecting many tens of millions of variable objects. Second, the measurement of the parallax will allow the determination of their luminosity.
Gaia will have quasi-simultaneous G photometry, BP and RP spectrophometric, and RVS measurements (in half of the cases for this latter instrument). As the shortest integration time is 4.4 seconds, time scales of seconds to years can be detected. The photometric precision should reach the millimagnitude level at the bright end, and about 20 millimag at a magnitude of 20.
Most of the known variability types shown on the figure will benefit from the Gaia mission, thanks to its multi-epoch observations. Though the Gaia time sampling is not dense, and hence not very suitable for the study of some non-periodic variabilities, it will be efficient in the analysis of strictly periodic signals, for which the predicted period recovery rate is high.
For some events that occur uniquely and on a short time scale, a flux-based alert will be issued by the Coordination Unit 5 (CU5) of the Gaia ground segment. The analysis of all types of variables outside the solar system is done by the Coordination Unit 7 (CU7) based on the calibrated data received from CU5. This includes the characterisation of the light curve variability of all variable sources, their classification into the different types of variabilities, and specific analysis of several variability types. CU7 will also do variability announcements that are less time critical, on specific groups of variable stars such as producing a list of RR Lyrae stars.
A larger version of the picture is also available.