Astrophysics of Life - The Lives and Deaths of Stars - The...

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The Lives and Deaths of StarsProtostars. These objects radiate energy away in the form of light. That energy comesfrom gravity – released by contraction.A star’s mass determines its luminosityOn the main sequence, the contraction stops because gravity and internal pressureexactly balance each other.Nuclear reactions occur at exactly the right rate to balance gravity.Nuclear reactions slowly convert H to He in the coreNewly formed stars are typically: ~91% H and ~9%HeIn the Sun’s core, the conversion of H to He will take ~10 billion years (its mainsequence lifetime)What happens when the core Hydrogen is used up? Nuclear reactions stop. Corepressure decreases. Core contracts and gets hotter – heating overlaying layers. 4 H1He burning moves to a hot “shell.”4 H1 He reactions occur faster than beforeThe star gets brighter (more luminous)!The hot shell causes the outer layers to expand and cool!The star moves off the Main Sequence, . . . up the “Red Giant” branch.Ascension up the red giant branch takes ~100 million years

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Term
Spring
Professor
Eikenberry
Tags
Astrology, White dwarf

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