Mjöllnir (literally, "that which smashes") is the hammer wielded by Thor, the Norse god of thunder.
According to Marvel Universe the blacksmiths used the core of a star as a forge to construct Mjöllnir.
[at Thor's coronation]
Odin: Thor Odinson, my heir, my first born. So long entrusted with the mighty hammer, Mjöllnir, forged in the heart of a dying star. It's power has no equal! It's a weapon to destroy or as a tool to build. It is a fit companion for a king. I have defended Asgard, and the lives of the innocent across my realms in the time of the great beginning.
Asgardians are presented in the Marvel Universe as 'just' an advanced, physically durable and long lived race with strange technology. So has there actually been any attempt to show or explain how blacksmiths could actually survive inside the heart of a dying star?