how long is the duration of a supernova?

Answer 1

The (Type II) supernova is the final stage of the death of a really massive star. The penultimate stage is where all the nuclear fuel of the star has been spent. All that remains in the earth sized core is iron which cannot yield anymore nuclear energy that could keep the core from collapsing. So gravity finally wins the tug-of-war and it collapses. At a quarter the speed of light, it goes. In 1/10 of a second it collapses from earthsize to a sphere 100 km across. While doing so the gravitational energy (not nuclear) released exceeds the energy output of all the stars in the universe combined... In short: boom.
But the event isn´t over. The red supergiant is so huge that the explosion cannot be seen from a distance just yet. The light from the explosion may travel at the speed of light but remember that the light from the sun takes 8 minutes to get to earth. A red supergiant, like Betelgeuse, is so big its radius is 7 times the earth-sun distance! So for an observer at a big biiig distance the actual event would see the doomed star brightening and brightening. And brightening. The event could last for days. Some supernovii has taken 100 days to reach its maximum after which the light decayed rapidly.

Answer 2

It takes a few days to reach maximum brightness and several months to fade away. You may think of a nuclear explosion as happening really fast, but that is because the bomb is only a few feet wide and the fireball is only a mile wide or so. A star hundreds of thousands of miles wide would take quite a while to explode. If the shock wave travels a million miles an hour, it would still take tens of minutes to travel from the core of the star to the surface, and even longer than that to blow the star apart. So it is as I said, a few days to brighten and some months to fade away.

Answer 3

about as long as a Nuclear Explosion