In-depth Notes on the Lives and Deaths of Stars

Introduction to Stellar Evolution

• Duration of Stars' Lives:
  • A star like our sun has a lifespan of about 10 billion years.
  • Some stars live much shorter lives, only about 4 million years.
  • Knowledge of stellar life cycles is derived from observing stars at various points in their evolution, similar to observing different ages of a human.

Stages of Stellar Evolution (for Sun-like stars)

• I. Collapse of a Gas Cloud:
  • Stars form from the gravitational collapse of gas clouds.
• II. Main Sequence:
  • Stage characterized by hydrogen fusion; stars burn hydrogen into helium in their cores.
• III. Red Giant:
  • Core helium fusion and outer hydrogen shell burning cause expansion.
• IV. Helium Flash:
  • Initiates helium fusion into carbon under extreme temperature conditions.
• V. Second Red Giant (Asymptotic Giant Branch):
  • Further helium burning and additional expansion occur.
• VI. Planetary Nebula and Formation of White Dwarf:
  • Outer layers of the star are expelled; core becomes a white dwarf.

Main Sequence Stars

• Energy Production:
  • Fusion of hydrogen (H) into helium (He) provides energy.
• Hydrostatic Balance:
  • Stars maintain balance between gravitational pull inward and fusion energy pushing outward.

Red Giant Phase

• Phase Characteristics:
  • The core contracts, increasing temperature and density, leading to rapid hydrogen fusion in the outer shells.
• Temperature Impact:
  • As the photosphere expands, it cools and appears redder; this process continues as the core becomes hotter from ongoing fusion.
• Luminosity and Size Changes:
  • The star's outer envelope expands significantly, potentially engulfing inner planets like Earth.

Helium Flash

• Initiation of Helium Fusion:
  • Occurs when the core temperature reaches 100 million K, enabling carbon production from helium fusion.
• Explosion Dynamics:
  • The helium flash is an explosive fusion event that leads to a temporary expansion of the core and cessation of outer hydrogen shell fusion.

Asymptotic Giant Branch (AGB) Stage

• Helium and Hydrogen Shells:
  • Following the helium flash, stars undergo rapid helium fusion in a shell around the core, contributing to further expansion.
• Energy Sources:
  • Energy is derived from core contraction, helium fusion, and hydrogen fusion in shells.
• Planetary Nebula Creation:
  • Eventually, stars lose outer layers, resulting in a planetary nebula.

Stellar Death and White Dwarf Formation

• Final Stages:
  • After fusion ceases, gravity causes core contraction, leading to white dwarf formation supported by degeneracy pressure, preventing further collapse.
• Characteristics of a White Dwarf:
  • Typically around the size of Earth, extremely dense (about 5 tons per teaspoon).

Supernova Events and Element Formation

• Type Ia Supernova:
  • Occurs when a white dwarf approaches the Chandrasekhar Limit (1.4 solar masses), leading to catastrophic fusion reactions, creating heavier elements up to iron and beyond in the supernova explosion.
• Nuclear Reactions in Supernovae:
  • Iron is the most stable element; during collapse, elements heavier than iron are formed and expelled into space, enriching the universe with these elements.

Gamma-Ray Bursts and Hypernovae

• Exceptional Explosions:
  • Massive stars (> 50 solar masses) may end their lives in hypernovae, producing gamma-ray bursts, which release vast amounts of energy.

Age Determination of Star Clusters

• Using Stellar Evolution:
  • Main sequence turn-off points reflect the age of star clusters; older stars take longer to evolve off the main sequence, providing insights into the cluster's age.
• HR Diagram Analysis:
  • By constructing H-R diagrams of star clusters, astronomers can determine various stellar ages.