Massive stars in advanced evolutionary stages, and the progenitor of GW150914

Understand

The recent discovery of a gravitational wave from the merging of two black holes of about 30 solar masses each challenges our incomplete understanding of massive stars and their evolution.

• Critical ingredients comprise mass-loss, rotation, magnetic fields, internal mixing, and mass transfer in close binary systems.
• The imperfect knowledge of these factors implies large uncertainties for models of stellar populations and their feedback.
• In this contribution we summarize our empirical studies of Wolf-Rayet populations at different metallicities by means of modern non-LTE stellar atmosphere models, and confront these results with the predictions of stellar evolution models.