Photometric Variability in the HR Diagram

The Hipparcos satellite has detected systematically variable stars when the amplitudes exceed a magnitude dependent threshold, as small as a few millimag for bright stars. The published data about variable stars result from the work of two groups at the Geneva Observatory and at the Royal Greenwich Observatory. It contains information about periodic variables (extrema, periods and epochs) and about `unsolved' variables (amplitudes). Here, a more general description of the HR-diagram is made in terms of stability, microvariability, variability, and variation time scales. A particular emphasis is put on the limits of stability areas and their signification.