@goldensky - White dwarfs are not big enough to explode into a supernova. Normally they will expand similar to our sun and then shrink into a white dwarf and then go black.
They are dead stars and there isn't sufficient enough mass for a white dwarf to go supernova, however the giant stars do possess enough mass to create an explosion.
The only way it could harm the Earth is if it's pole's were directed right at us. The event of this happening is very unlikely though.
A supernova that occurs less than one hundred light years away would definitely destroy the Earths ozone layer exposing everything to cosmic radiation. Every species would be affected by extinction and horrific environmental changes.