Supernova is a universe parallel to Nova. Supposedly, there are infinite universes. The Cross-Multiverse Theory, as stated below, explains what Supernova's purpose is.
Since the beginning of time, there have been infinite universes. Each universe has its place in a perfect array -- except for two. The two universes, one being Nova, and the other being Supernova, are miraculous mistakes. Supernova is parallel to Nova. They are essentially the same, besides one difference. Supernova contains everything Nova has -- except for living creatures. It does, however contain a strange gas, that is believed to be the fumes of magic itself. Due to, perhaps, a coincidence, the two universes collided at some point, even though they were completely static. This caused several ripples in reality, time, and space itself. Chaos consumed all of the universes that ever existed and ever will exist, until the day came that Nova and Supernova formed some sort of a bond. Both universes, stuck to each other, were the only imperfection in the endless infinity of a perfect array of universes.
And with the bond of the universes, Nova became dynamic, and life began. The eternal seal between the two universes allowed some power to pass into Nova. And with the dawn of humanity, some have been able to borrow and harness the strange power from Supernova.
Besides magic, Supernova has still interacted with Nova. An example of this would be The Great Tree myth.