AST- 105 1st EditionLecture 16Outline of Last LectureI. Reflection NebulaeII. Interstellar ReddeningIII. The Evolution of a 1 M ʘ StarIV. The Process of Main Sequence Evolution Outline of Current Lecture I. Main Sequence EvolutionII. Red Giant PhaseIII. Helium Flash PhaseIV. Horizontal Branch PhaseV. Second Red Giant PhaseVI. Planetary Nebula PhaseVII. White Dwarf PhaseCurrent Lecture I. Main Sequence Evolutiona. Nuclear fusion in core converts H to Heb. Star leaves main sequence when H is used up in the core.c. Sun spends 10^10 years in the main sequence.d. More massive stars burn through H faster. e. Track of a star next moves to higher luminosity (L) and lower temperature (T).II. Red Giant Phasea. H fusion ends in core, but continues in shell around the core.b. Gravity causes core to contract and heatc. Energy output from shell increasesd. Outer region of star expands and coolsi. Expansion caused by increased pressuree. Star is now a Red Giant.III. Helium Flash Phasea. When central T = 10^8 K, He fusion begins in the coreIV. Horizontal Branch Phasea. He fuses to C in coreb. Some He fuses with C to form oxygen (O)c. Core expands and coolsd. Envelope contracts and becomes bluere. “L” higher than for Sun.f. HB stars form horizontal grouping in H-R diagram.V. Second Red Giant Phasea. No He left in coreb. Core is now carbon (C) and oxygen (O)c. No nuclear fusion occurs in cored. Core contracts and heats; envelope expands and coolse. Star becomes a Red SupergiantVI. Planetary Nebula Phasea. Red Supergiant envelope pulsates unstablyb. Envelope is ejectedc. Core is exposedd. Structure:i. Expanding spherical gas shell around small, hot starii. Appears as bright ring on sky, since path length is longer near edges of shell than toward the core.e. NOT related to planets, but may look like a planet with a small telescope.f. Other Planetary Nebulaei. Structure of ejected gas con be nonspherical due to:1. Star orbiting a binary companion2. OR disk around star produces an “hourglass” shape.VII. White Dwarf (WD) Phasea. Hot, dense core becomes white dwarf = collapsed star about size of Earthb. NO nuclear burningc. Structure:i. Density: 10^9 kg/m^3 = 10^6 density of waterii. Electrons are packed as closely as possibleiii. Does NOT contract furtheriv. T and L decrease in
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