Overview
This lecture covers the life cycle of low-mass stars, such as our Sun, outlining their evolutionary stages from main sequence to white dwarf.
Stages of Low-Mass Star Life Cycle
- Starts as a protostar, then progresses to the main sequence where core fuses hydrogen.
- Main sequence ends when core hydrogen is exhausted, causing star instability.
- Gravity causes the core to shrink and heat up as fusion in the core stops.
- Hydrogen in a shell around the core starts fusing, expanding the surface and cooling it.
- This expanded, cooled star is called a red giant.
- The core gets hot enough to fuse helium, entering the horizontal branch stage.
- During horizontal branch, the core grows and cools, while the surface shrinks and heats up.
- Helium in the core eventually runs out, leaving a core of carbon and oxygen that cannot fuse.
- Core shrinks and heats up again, creating two fusion shells (helium and hydrogen), leading to the AGB (Asymptotic Giant Branch) giant stage, an extreme version of the red giant stage.
- During the AGB stage, fusion shells alternate turning on and off, making the star unstable.
Death of a Low-Mass Star
- Unstable AGB star ejects its outer layers, forming a planetary nebula.
- The leftover core becomes a white dwarf, which is small, hot, and dense.
Patterns and Relationships
- Core and surface always do opposite (when core shrinks/heats, surface expands/cools and vice versa).
- Luminosity follows core temperature (hotter core = higher luminosity).
- Stage transitions marked by shifts in fusion type or shell activity.
Key Terms & Definitions
- Main Sequence — star stage where hydrogen in the core is fusing.
- Red Giant — large, cool, red star with a helium core and hydrogen shell fusion.
- Horizontal Branch — stage with helium core fusion and hydrogen shell fusion.
- AGB Giant — very large, cool star with carbon-oxygen core and two fusion shells.
- Planetary Nebula — ejected outer layers forming a glowing shell of gas.
- White Dwarf — dense, hot, Earth-sized stellar remnant of a low-mass star.
Action Items / Next Steps
- Draw and memorize the arrows chart for each stage (core size/temp, surface size/temp, luminosity).
- Remember the order and characteristics of each life cycle stage.
- Review the definitions and link them to each life stage.
- Prepare for discussion on white dwarfs in the next class.