Is it just nuclear fusion?
A nova is a sudden increase in the brightness of a star, caused by a thermonuclear explosion on the surface of a white dwarf star. A supernova is a much larger explosion that occurs at the end of a massive star's life cycle, resulting in the release of massive amounts of energy.
The energy in a nova and supernova is created through nuclear fusion. In a nova, hydrogen atoms fuse together to form helium, releasing a large amount of energy. In a supernova, heavier elements such as carbon, oxygen, and iron fuse together, creating even more energy.
The energy released in a supernova is much greater than that of a nova. A supernova can release up to 100 billion times the energy of a nova.
In a nova, the trigger is the buildup of hydrogen on the surface of a white dwarf star. When the hydrogen reaches a critical mass, it ignites and causes a thermonuclear explosion. In a supernova, the trigger is the collapse of a massive star's core, leading to the fusion of heavier elements and the subsequent explosion.
Yes, both novae and supernovae can occur in our galaxy. In fact, supernovae occur about once every 50 years in the Milky Way, while novae occur more frequently.