Massive stars live fast and die young, burning through their fuel at breakneck speeds. Unlike our slow-burning Sun, these cosmic giants evolve in mere millions of years, creating heavy elements through intense nuclear fusion.

These stellar powerhouses end with a bang, exploding as supernovas and leaving behind exotic remnants like neutron stars or black holes. Their fiery deaths seed the cosmos with new elements, shaping future generations of stars and planets.

Evolution of Massive Stars

Evolution rates of stars

• Massive stars (>8 solar masses) evolve much faster than lower-mass stars like our Sun due to higher core temperatures and pressures enabling faster nuclear fusion reactions, causing them to consume their fuel more rapidly (few million years vs. billions of years)
• Lower-mass stars (<8 solar masses) evolve more slowly because of lower core temperatures and pressures leading to slower nuclear fusion reactions, allowing them to conserve their fuel and have longer lifetimes (main sequence lifetime of Sun is ~10 billion years)
• Massive stars experience significant stellar mass loss through powerful stellar winds, which affects their evolution and final fate

Nucleosynthesis in massive stars

• Massive stars undergo nucleosynthesis to create elements heavier than carbon through a process of creating new atomic nuclei from pre-existing nucleons (protons and neutrons)
• Nucleosynthesis in massive stars occurs in stages, each requiring higher temperatures and pressures than the previous:
  1. Hydrogen fusion: Hydrogen fuses into helium ($4 \times 10^7$ K)
  2. Helium fusion: Helium fuses into carbon and oxygen ($2 \times 10^8$ K)
  3. Carbon fusion: Carbon fuses into neon and magnesium ($8 \times 10^8$ K)
  4. Neon fusion: Neon fuses into oxygen and magnesium ($1.6 \times 10^9$ K)
  5. Oxygen fusion: Oxygen fuses into silicon and sulfur ($2 \times 10^9$ K)
  6. Silicon fusion: Silicon fuses into iron and nickel ($3 \times 10^9$ K)
• Fusion process continues until the core is composed primarily of iron and nickel, at which point fusion in the core stops because fusing elements heavier than iron and nickel requires energy input rather than releasing energy, leading to the star's core collapse and a supernova explosion
• The formation of an iron core marks the final stage of nuclear fusion in massive stars

Stellar Remnants

• After a supernova explosion, the fate of a massive star depends on its initial mass:
  • Stars between 8-20 solar masses typically form neutron stars
  • Stars above 20 solar masses may collapse into black holes
• The type of stellar remnant left behind influences the surrounding interstellar medium and future star formation

Stellar Populations and Chemical Composition

Cluster composition and stellar age

• Globular clusters and open clusters have different chemical compositions due to their age and the evolution of their stars
• Globular clusters are older (>10 billion years) and contain stars with lower metal content (elements heavier than helium) because they formed early in the universe's history when fewer heavy elements were available
  • First generation of stars in globular clusters consisted mainly of hydrogen and helium
  • As these first-generation stars evolved and died, they enriched the interstellar medium with heavier elements through supernovae (e.g., Crab Nebula) and stellar winds
• Open clusters are younger (<1 billion years) and contain stars with higher metal content because they formed later in the universe's history, after multiple generations of stars had enriched the interstellar medium with heavier elements
  • Stars in open clusters (e.g., Pleiades) formed from gas clouds already enriched with heavier elements from previous stellar generations
• Difference in chemical composition between globular and open clusters provides evidence for stellar evolution and gradual enrichment of the universe with heavier elements over time