The white dwarf HR 8210 may be set to go supernova, as its mass approaches the Chandrasekhar Limit. The star is 150 light-years away, which puts our planet well into the danger zone. Even more sobering is the fact that, if the star has already gone nova, we won’t know till 150 years after the event. So if HR8210 had decided to start violently blowing off its excess mass in 1853, we would only start seeing the first wave of light and radiation around next year.

Update: The danger is mostly just media-induced sensationalism. More here.