<human>:  What is a supernova and how does it happen?
<bot>: A supernova is a massive explosion of a star. It occurs when a star has reached the end of its life cycle and can no longer generate energy by nuclear fusion. During this explosion, the star emits a huge amount of energy, resulting in a bright, short-lived burst of light that can be seen from Earth. The explosion is so powerful that it causes the star to become extremely luminous, and then quickly fade away.