AstronomyLesson 4 of 5 in the course Stars from Adolescence to Old Age

Late Stellar Evolution, Cosmic Recycling, and the Sun’s Future

5 flashcards · 5 quiz questions · 5 match pairs · 5 fill-in sentences · English

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Summary

This lesson explains what happens after a Sun-like star exhausts the hydrogen in its center and becomes a red giant. The increasing heat and pressure in its contracting core eventually allow helium fusion to begin.

A red giant has an inert helium core surrounded by a shell in which hydrogen continues to fuse. Because the core no longer generates energy through fusion, it cannot provide enough pressure to counter gravity. The core contracts, and compression heats it. The surrounding hydrogen-fusion shell and the star’s expanded outer layers help maintain the red-giant structure.

All flashcards

What is the triple-alpha process?
Fusion of three helium nuclei into a carbon nucleus under the high-temperature conditions in a star’s core.
Why does a red-giant helium core heat up after central hydrogen is exhausted?
Without central fusion to provide enough pressure against gravity, the core contracts and compression raises its temperature.
What is a helium flash?
A rapid increase in helium fusion in a low-mass star’s core, driven by feedback between heating and faster reactions.
What is cosmic recycling?
The return of stellar material to space and its later reuse in new generations of stars and planetary systems.
What is a planetary nebula?
A glowing shell of gas expelled by an aging low-mass star and illuminated by its hot central star.

Quiz questions

  1. What surrounds the inert helium core of a red giant after central hydrogen is exhausted?Answer: A shell where hydrogen continues to fuse
  2. Why does a helium core contract after central hydrogen fusion ends?Answer: It no longer generates enough fusion energy to oppose gravity
  3. At approximately what core temperature can helium fusion begin in the low-mass stars described?Answer: 100 million K
  4. What does the triple-alpha process produce?Answer: A carbon nucleus from three helium nuclei
  5. Why can helium ignition become a flash in a low-mass star?Answer: Heating speeds up fusion, which releases more energy and causes further heating

Based on Astronomy 2e by Andrew Fraknoi, David Morrison, Sidney Wolff (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.

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