AstronomyLesson 1 of 1 in the course Stars from Adolescence to Old Age

Main-Sequence Evolution, Stellar Lifetimes, and Red-Giant Formation

5 flashcards · 5 quiz questions · 5 match pairs · 4 fill-in sentences · English

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Summary

This lesson introduces the zero-age main sequence and explains how hydrogen fusion powers a star during its long, stable main-sequence phase. It also explores how gradual changes in a star’s core affect its position on the H–R diagram.

The H–R diagram compares stars by luminosity and surface temperature. A star’s position on this diagram changes as its properties change.

All flashcards

Zero-age main sequence (ZAMS)
The H–R diagram locus where stars begin stable hydrogen fusion in their cores.
Main-sequence energy source
Core hydrogen fusion releases energy that helps balance the inward pull of gravity.
Why do massive stars have shorter main-sequence lifetimes?
Their hotter cores fuse hydrogen much faster, using up their larger fuel supply sooner.
Hydrogen-shell fusion
Hydrogen fusion in a layer surrounding a contracting helium-rich core.
Evolutionary track
A model star’s changing luminosity and temperature plotted on an H–R diagram.

Quiz questions

  1. Why can a massive star have a shorter main-sequence lifetime than a low-mass star despite having more hydrogen fuel?Answer: Its hotter core consumes hydrogen at a much faster rate.
  2. After core hydrogen fusion ends, what initially happens to the helium-rich core?Answer: It contracts under gravity and heats its surroundings.
  3. Why can a star become cooler at its surface while its luminosity increases as it develops into a giant?Answer: Its expanding outer layers have a larger surface area and a lower surface temperature.
  4. On an H–R diagram, which direction does a star move as it becomes more luminous and its surface temperature falls?Answer: Upward and to the right.
  5. What does an evolutionary track on an H–R diagram represent?Answer: A model star’s changing luminosity and temperature over time.

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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