What happens to a star much bigger than the Sun, once the star reaches the end of the main sequence' period of its life cycle?
The star will explode in a supernova.
Once a star reaches the end of it's lifespan it will increase in size and form a red giant. As the star fuses heavier and heavier elements in it's core it becomes more and more unstable. Provided the star is big enough it will explode in a supernova and the core will implode inward on itself and either form a black hole of a neutron star.