What are Supernovas??

 A Supernova is a powerful and luminous stellar explosion. This event occur during the last evolutionary stages of a massive star or when a white dwarf is triggered into runway nuclear fusion.

Having too much matter and iron atom causes a repulsive force which explodes the star causing a supernova. It releases enormous amount of energy. The total energy output maybe as much as the total output of the sun during it's 10 billion years lifetime.

It way shine with the brightness of 10 billion suns. 

A Supernova may outshine the entire universe. They are also the primary source of heavy elements in the universe. According to NASA, they are the largest explosion that takes place in the space.

If a Supernova explodes, then it could take years for it to fade away.

If a Supernova hits the Earth, then the X-rays and Gamma Ray's from it would destroy the ozone heavily. It could cause  a mass extinction and perhaps could perish all the life forms.

For a Supernova to hit Earth it should be within the range of 50 light years. As of now we are out of danger in the near future.