ASTR 1020 1st Edition Lecture 13 Outline of Last Lecture I. Star BirthA. Trapping of Thermal EnergyB. ProtostarC. Brown DwarfsD. Largest Stars: Radiation PressureOutline of Current Lecture I. Star LivesA. Helium FlashB. Last Stages of Nuclear BurningC. Planetary NebulaeCurrent LectureI. Star Lives- Low mass stars: post Main Sequence - Observations of star clusters show that a star becomes larger, redder, more luminous after its time on the main sequence is over- Mirror Principle: Core contracts, envelope expands- After H burning, He ash remains in core - As the core contracts, H begins fusing to He in a shell around core - Luminosity increases: increasing fusion rate in shell does not stop the core from contracting- Helium fusion does not begin right away because it requires higher temperatures (larger charge leads to greater repulsion); Combine three He nuclei to make one carbon.A. Helium Flash- Thermostat is broken in low-mass red giant because degeneracy pressure supports core- Core temperature rises rapidly when helium fusion begins- Helium fusion rate skyrockets until thermal pressure takes over and expands core again to reach a balance- Life After Helium Flash- Models show that a red giant should shrink and fade after He fusion begins in the core - Mirror principle: core expands, envelope shrinks- Observations of star clusters agree with models - Helium-burning stars are found in a horizontal branch on the H-R diagramB. Last Stages of Nuclear Burning• After core He fusion runs out, He fuses into C in a shell around the C core, H fuses into He in a shell around the He layer • Double-shell burning stage never reaches equilibrium—fusion rate periodically spikes upward in a series of thermal pulses • Star large, luminous, and unstable (Sun will grow out nearly to Earth’s radius)C. Planetary Nebulae- Ends with a pulse that ejects the H and He envelope into space as a planetary nebula - The core left behind becomes a white dwarf (supported by degeneracy