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Introduction

• Overview of stellar evolution, particularly the influence of mass on nuclear fusion.

Star Mass and Fusion

• Core Pressure and Temperature:

  • Main-sequence star mass determines core pressure and temperature.

  • Higher mass stars:

    • Higher core temperature.

    • Faster fusion rates leading to greater luminosity and shorter lifespans.

  • Lower mass stars:

    • Cooler cores and slower fusion rates.

    • Lower luminosity and longer lifespans.

Formation Times for Stars

• Lifespans vary significantly based on stellar mass:

  • Massive stars (e.g., 15M Sun): ~60,000 years.

  • Intermediate mass (e.g., 3M Sun): ~3 million years.

  • Low mass stars (e.g., 0.5M Sun): ~30 million years.

Star Clusters

• Observations from star clusters provide insights into stellar lifespans.

• Clusters contain stars of different masses formed around the same time.

Life Stages of Low-Mass Stars

• Main Sequence: Stars fuse hydrogen into helium.

• Post-Main Sequence: Stars expand and become larger and redder after hydrogen depletion.

• Red Giants:

  • Core contracts, initiates hydrogen fusion in a shell surrounding the core.

  • Increased luminosity but core contraction continues.

Helium Fusion

• Initiation: Requires higher temperature than hydrogen fusion; involves fusing three helium nuclei to produce carbon.

• Helium Flash: Sudden increase in temperature leads to a spike in helium fusion rates until thermal pressure stabilizes growth.

Life Track after Helium Flash

• Models predict red giants shrink and dim after helium fusion starts in the core.

• Observational data confirms this behavior in star clusters.

Double Shell Fusion

• Core helium fusion halts; further fusion occurs in shells around the core.

• Occasional thermal pulses cause carbon to be dredged to the surface.

• Ends with the expulsion of outer layers as a planetary nebula, leaving a white dwarf.

Life Track of a Sun-Like Star

• Transition from main-sequence to red giant, helium fusion, and finally to a white dwarf.

Sun's Future

• As the Sun ages, its luminosity will increase dramatically, potentially making Earth uninhabitable.

• Sun will expand near Earth's orbit.

High-Mass Stars

• High-mass stars (> 8M Sun):

  • Fuse hydrogen to helium at a higher rate using CNO cycle.

  • Life stages: similar to low-mass, but proceed quicker.

Origin of Elements

• Big Bang produced mainly hydrogen and helium; stars synthesize heavier elements through fusion processes.

• Heavier elements created from helium capture in high mass stars.

Death of High-Mass Stars

• Iron builds up until collapse occurs due to gravity, leading to a supernova.

• Energy from the collapse drives outer layers into space, contributing to element formation in the universe.

Supernova Remnants and Observations

• Notable supernova events include 1987A, which provided insights into star death and element creation.

Stellar Mass and Its Influence

• A star's mass dictates its life cycle and the types of elements it can produce.

• Low-Mass vs High-Mass: Low-mass stars end as white dwarfs; high-mass stars may explode as supernovae.

Key Takeaways

• Life Stages (Low-Mass Stars): Contains fusion stages from main sequence to planetary nebula.

• Life Stages (High-Mass Stars): Similar but culminate in supernova evolution.

• Understanding mass and fusion processes elucidates the life cycles and fates of stars.