Massive stars end their lives in spectacular explosions called supernovae. These cosmic fireworks shape the universe, forging heavy elements and leaving behind exotic remnants like neutron stars and black holes.

The journey to a supernova is complex, involving nuclear fusion, core collapse, and shock waves. Understanding this process reveals how stars create the building blocks of life and how they influence galactic evolution.

Evolution and Explosion of Massive Stars

Interior structure of pre-supernova stars

• Onion-like structure with layers of different elements
  • Iron core at the center forms the innermost layer
  • Layers of silicon, oxygen, neon, carbon, helium, and hydrogen surround the core in concentric shells
• High temperature and density in the core
  • Temperatures reach billions of degrees Kelvin (e.g., $10^9$ K)
  • Densities exceed $10^9 \text{ g/cm}^3$, comparable to the density of atomic nuclei
• Electron degeneracy pressure supports the core against gravity
  • Degenerate electrons resist further compression, providing a temporary stabilizing force
• Nuclear fusion no longer generates energy in the core
  • Iron cannot fuse to release energy under normal stellar conditions due to its high binding energy per nucleon
• Stellar nucleosynthesis occurs in different layers, producing heavy elements

Core collapse in supernova process

• Core collapse begins when iron core exceeds the Chandrasekhar limit ($\sim$1.4 solar masses)
  • Electron degeneracy pressure can no longer support the core against gravity, triggering collapse
• Inner core collapses and reaches nuclear densities ($\sim 10^{14} \text{ g/cm}^3$)
  1. Protons and electrons combine to form neutrons via electron capture
  2. Neutrinos are released and carry away energy from the core, further accelerating collapse
• Inner core becomes incompressible and rebounds, sending a shock wave outward
  • Shock wave stalls due to energy loss from dissociation of heavy nuclei (e.g., iron, silicon)
• Neutrinos deposited behind the shock wave revive it, leading to a successful explosion
  • Outer layers of the star are ejected at high velocities (thousands to tens of thousands of km/s)
• Explosion releases a tremendous amount of energy ($\sim 10^{44}$ Joules)
  • Luminosity can briefly outshine the entire host galaxy (e.g., Milky Way)
  • Supernova remnant forms from the ejected material and can persist for thousands of years (e.g., Crab Nebula)

Stellar Evolution and Mass Loss

• Hydrostatic equilibrium maintains the star's structure throughout its life
• Stellar evolution is influenced by the star's initial mass and composition
• Massive stars experience significant mass loss through stellar winds
• Stellar mass loss affects the star's final fate and supernova characteristics

Earth risks from nearby supernovae

• Gamma-ray burst (GRB) could pose a threat if directed towards Earth
  • GRBs are highly collimated beams of intense gamma radiation that can travel vast distances
  • Can cause damage to Earth's ozone layer and increase UV radiation at the surface, harming life
• High-energy particles (cosmic rays) from the supernova could reach Earth
  • Can cause increased radiation exposure and potentially harm living organisms through DNA damage
• Supernova shock wave could compress the solar system's Oort Cloud
  • May increase the rate of comets entering the inner solar system from the distant Oort Cloud reservoir
  • Higher likelihood of cometary impacts on Earth, potentially causing global catastrophes
• Supernova would need to be relatively close to pose significant risks
  • Within a few dozen light-years for GRBs and cosmic rays to be dangerous (e.g., 30-50 light-years)
  • No known supernova candidates within this distance at present, reducing immediate concerns