How do supernova explode?

Answer 1

There are several mechanisms proposed for supernova explosions. Since we have never seen one close up (which is probably just as well!) this is mostly theoretical.

As large stars (bigger than about 3 solar masses) grow old, they run low on hydrogen, the nuclear fuel that they convert into helium. It isn't really so much that the hydrogen is "used up", it's more as if it were a wood fire in a fireplace; at some point, the ashes prevent the wood from burning properly. In a star, the helium "ash" of the nuclear fire builds up to the point where it's interfering with the hydrogen fusion, and the star begins to collapse under gravity.

When the core pressure and temperature increases enough, the star begins fusing helium into carbon and oxygen, and the "ash" becomes new "fuel", and the star expands, into a red giant phase. Eventually, they cycle repeats, and the carbon and oxygen begin to interfere, and the star begins to collapse again. When the core becomes hot enough, it begins fusing carbon and oxygen into much heavier elements. As each fusion reaction occurs, it contributes less and less energy, until the fusion process begins to produce elements heavier than iron; at that point, fusion SUCKS ENERGY OUT of the star, and the star collapses abruptly. Between the gravitational collapse and the nuclear fusion, the star creates elements all the way to the transuranics, including everything lighter. We know this because our Earth, formed out of the debris from old supernova stars, contains heavy elements like gold, lead, and uranium.

The collapse of the star compresses the core of the star into either a black hole or a neutron star, depending on the mechanics of the explosion and the initial mass, and the remainder of the star's mass is blasted back into space, to become new nebulas and perhaps new solar systems.

Answer 2

Supernovae can explode through two main processes: core-collapse supernovae occur when a massive star runs out of nuclear fuel, causing its core to collapse and trigger the explosion. Type Ia supernovae happen in binary star systems where a white dwarf accretes matter from its companion, surpassing a critical mass limit, leading to a runaway nuclear fusion reaction that causes the explosion.