Ch12_The_death_of_stars.pdf - Chapter 12 The deaths of...

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The deaths of starsChapter 12
The deaths of stars12.1The fate of low-mass stars12.2The fate of Sun-like stars 12.3White dwarf 12.4The fate of massive stars 12.5 The fate of binary stars2
Red dwarfSun-like star red giant; core Massive star supergiantsupernova 3Quick recap: a star’s fate depends on its mass
red dwarfs(紅矮星)mass < 0.4 Mconvection from their centers to their surfaces, sogas is well-mixed. Hydrogen, helium ashes distributed uniformly inside the star no hydrogen-burning shell; will notform giants 4Burn slowly, long life, 100’s billion years 12.1 The fate of low-mass stars
notmassive enough to burn heliumcontract slowly when fuels are used up, sogetting smaller, hotterMove to the left on the H-R diagrambecome 5Burn slowly, long life, 100’s billion years
0.4 M< mass < 4 Mdue to gravitydenseand coreradiation by hydrogen burning shellswelled, less denseand coolouter layerrapid expansionthe star is 612.2 The fate of Sun-like stars
The star becomes red giant, and moves to the right of the H-R diagramBefore helium burning starts at the core, core is compressed, becoming degenerated.7
The degenerated core produces huge pressure to resist further contraction8the resisting pressure is independent of temperatureAs core’s temperature reaches 108K, ever-increasing helium fusion rate at the core produces helium flashes, which terminates the degenerate state. The outer layer becomes smallerand , the star is less luminous
HeHCOProcess repeats until oxygen-rich core produced9oxygen-rich core is supported by electron degeneracy pressureStellar nucleosynthesis
Helium shell flash causes mass lossWe discussed helium flash in the previous chapter. It is also referred as “core helium flash”.A similar process, known as helium shell flash, also occurs in the star at some point.10HCHe
After helium fusion for a long time, helium-burning shellis created outside the core.Then, as helium in the shell is used up gradually, the shell contracts.It heats up peripheral hydrogen-rich shell just outside the helium-burning shell; causes hydrogen fusion, producing helium.11HCHeHelium shell flash causes mass loss
Newly produced helium rains downward onto the helium-burning shell. Hence, the helium-burning shell gains extra helium12HCHeHelium shell flash causes mass loss
HCHehelium-burning shell becomes more violent, producing helium shell flashes, provided that temperature reaches certain critical valueThe energy released pushes peripheral hydrogen-rich shell outward; hence decreases in temperature and ceases hydrogen burningProcess re-starts again13Helium shell flash causes mass loss

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Term
Spring
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Supernova, White dwarf

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