HR Diagram

Introduction to Stellar Classification

  • Stars can be categorized based on mass and their life cycles.

Stellar Mass Classification

  • High Mass Stars:

    • Short lifespan

    • High luminosity

    • Typically end as neutron stars or black holes

  • Low Mass Stars:

    • Longer lifespan

    • Lower luminosity

    • End as white dwarfs

Groups Based on Solar Mass

  • Low Mass Stars: Less than 2 solar masses

  • Intermediate Mass Stars: Between 2 and 8 solar masses

  • High Mass Stars: Greater than 8 solar masses

Stellar Lifecycle and Energy Processes

  • All stars undergo nuclear fusion beginning with hydrogen.

  • The lifecycle is dependent on mass and its gradual evolution through various phases.

Fusion Processes

  • Stars receive hydrogen fusion at temperatures required for this process to occur.

  • Main Sequence Phase:

    • Stars fuse hydrogen into helium, maintaining a balance between energy and gravity.

    • Core fusion begins when a star reaches the necessary temperature.

Differences in Stellar Evolution

Low Mass Stars

  • Main sequence stars may have stable luminosity for extensive periods.

  • As hydrogen depletion occurs:

    • Core shrinks due to lower particle counts.

    • Hydrogen fusion continues at a slower rate.

  • Transition to Red Giants: Stars expand, cool, and begin helium fusion.

  • Helium fusion occurs under specific conditions:

    • Three helium nuclei must combine for the process to begin.

    • Possible product: Carbon.

Transition to Planetary Nebula

  • Stars eject their outer layers after significant fusion processes.

  • Remaining core becomes a white dwarf.

High Mass Stars

  • CNO Cycle: Catalyzing process using carbon, nitrogen, and oxygen.

  • High mass stars rapidly transition through various fusion stages.

  • Produce heavier elements until the core reaches iron:

    • At iron, fusion ceases as it doesn’t produce energy.

    • Core collapses leading to a supernova explosion.

Final Stage Outcomes

  • For Low Mass Stars:

    • Ends as a White Dwarf.

  • For High Mass Stars:

    • Leads to a Neutron Star.

    • Potential phase described includes supernova remnants with neutron stars cooling over time.

Stellar Fusion Processes and The HR Diagram

Stellar Movement on the HR Diagram

  • Stars progress from the main sequence to red giant phase, changing position on the HR graph.

Core Processes in Low Mass Stars

  • Double Shell Fusion:

    • Occurs when both hydrogen and helium fusion happen in the same star.

    • Leads to thermal pulses observed in the star’s evolution.

Binary Systems

  • Involve two stars, where mass transfer can occur.

  • Galactic interactions between stars may alter their lifecycle and longevity.

Summary of Stellar Life Cycles Based on Mass

  • Low Mass Stars:

    • Longer lifetimes

    • Commonly produce carbon through fusion, end as white dwarfs.

  • High Mass Stars:

    • Shorter lifetimes

    • Able to fuse heavier elements, ending in supernova explosions and neutron stars.

  • The finality of the stellar lifecycle is determined by the star's mass and temperature conditions in their cores.