Question

what is the reason behind supernova?

Answer 1

Supernova is basically a very huge explosion and occurs in the latter stages of star's life. The outer layers of the star are thrown out and the central core remains.

This only happens in case of massive stars which are much bigger than our own Sun. A supernova basically occurs as the outer layers of the star rebound after bouncing back from its dense solid core. The outer layers are ejected outwards in the form of an immense explosion while the remaining core is either a pulsar or a neutron depending upon its spin rate.