How far from earth is the Cassiopeia A supernova remnant?
Approximately 11,000 light years. [See Link]
That depends entirely on how far the supernova is from Earth. If it is our Sun going supernova, a little over 8 minutes. And a few Milli-seconds later all life on Earth would be gone. Any other star, it would just appear slightly brighter for a while and then disappear. It could take centuries for the light to reach us.
Scientifically, the end of the world is possible when the sun uses up all of it's hydrogen and creates a supernova. The Earth might get caught in the supernova, and the Earth will be too hot for any organisms to survive. +++ Almost. The Sun won't end as a supernova because it is far too small. Instead it will probably become a red giant, swelling possibly enough to engulf the Earth in its atmosphere. Everything…