= Solution
A close <binary star> can produce two <compact stellar objects> if both initial stellar <masses> are sufficiently large. The initially more massive star evolves first, transfers material or loses its envelope, and leaves a <neutron star> or <black hole> after core collapse. The system must remain bound despite <mass loss> and any natal kick. Subsequent evolution of the companion can produce a <common envelope>: orbital <energy> transferred to the envelope can expel it and greatly reduce the separation. A second collapse must again leave a bound pair. A sufficiently close surviving <binary star> then loses orbital <energy> and <angular momentum> through <gravitational waves> and may merge. Stable transfer, winds and kicks make this a possible channel rather than an inevitable outcome; dynamical encounters in dense stellar systems provide another channel.
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