What is a supernova? Why can a supernova explosions in space be seen but not heard on Earth?

B/c there isn't enough oxygen to produce the sound. The universe is "empty".

A supernova is the last stage of a massive star's life; when its nuclear fuel has run out and its core collapses. Supernova explosions can be seen because the light it emits travels through space, making it visible to us. A supernova--or anything else in space can't be heard because sound doesn't travel in space; there is no molecules or particles for it to travel through.