`How did they form?’ is a question many asked when LIGO announced the first direct detection of gravitational waves originating from two surprisingly heavy stellar-mass black holes. With masses of about 30 solar masses each, they outweighed all of the known black holes known from X-ray binaries. Now, four years after the first detection, alerts of new triggers come in at a rate of almost one per week and we have learned that the first system was not exceptional: the majority of detected events involve heavy black holes.

In parallel, conventional telescopes have been revolutionizing our understanding of the properties of young massive stars. One of the most remarkable findings is that the majority of massive stars have one or more companions so close that the exchange of mass between them in inevitable during their lifetime. What does it take for a such stellar couples to end their lives as GW events?  I hope to address some of the exciting new insights as well as the challenges and open questions that we are still facing.