A supernova is a powerful and catastrophic event that occurs at the end of a massive star's life cycle. When a massive star exhausts its nuclear fuel, it can no longer sustain the outward pressure generated by nuclear fusion in its core. Without this pressure to counteract gravity, the core collapses rapidly under its own weight.

The collapse of the core triggers a series of dramatic events. The outer layers of the star are suddenly compressed and then violently expelled outward in an incredibly energetic explosion. This explosion releases an immense amount of energy, temporarily outshining an entire galaxy of stars. A supernova can briefly outshine its entire host galaxy and may even be visible from Earth across vast distances.

There are two main types of supernovae:

1. **Type II Supernovae**: These occur when massive stars, typically more than eight times the mass of the Sun, reach the end of their lives and undergo core collapse. Type II supernovae are characterized by the presence of hydrogen in their spectra.

2. **Type Ia Supernovae**: These occur in binary star systems where one star is a white dwarf and the other is a companion star. When the white dwarf accretes enough mass from its companion, it can reach a critical mass known as the Chandrasekhar limit, triggering a runaway nuclear fusion reaction that completely destroys the white dwarf. Type Ia supernovae are known for their consistent peak luminosity, making them valuable tools for measuring cosmic distances and studying the expansion of the universe.

Supernovae play a crucial role in the universe's evolution, enriching interstellar space with heavy elements forged in their intense nuclear reactions. They also release vast amounts of energy and can influence the formation of new stars and planetary systems. Additionally, the remnants of supernovae, such as supernova remnants and neutron stars, provide valuable insights into stellar physics and high-energy astrophysics.