As huge stars grow old, they become red super giants. They become red super giants because their core fuses all its hydrogen into helium. Their core shrinks and becomes hotter and denser.
The core of the star collapses in an instant. The iron atoms are crushed together during the gravitational collapse. The temperature of the core reaches 100 billion degrees.
The electrical forces between the atoms' nuclei become greater than the gravitational forces, which cause a bright, short explosion called a supernova. The shock wave of the explosion blows away the outer layers of the star.
The star will become a neutron star or a black hole depending on its remaining mass. If its mass is 1.5 to 3 times the Sun's mass, (1.99 x 1030 kg) it will collapse and become a neutron star. Its mass is 1.4 times the Sun's, it has a strong magnetic field, and it spins rapidly.
If the mass of the star is more than 3 times the Sun's mass, it will become a black hole. A black hole has a gravitational field strong enough to keep light from escaping.