50 light years is the generally accepted distance that would cause problems. There are no candidates this close that could cause a type II supernova (this is when a massive old star, like Betelgeuse, collapses), but there could be potential type I supernovae lurking closer. These are binary pairs of a white dwarf and another star where the white dwarf is accreting mass from the companion and reaches a limit (the Chandrasekhar mass) where runaway nuclear fusion spontaneously occurs.
