AstronomyLesson 2 of 2 in the course Stars from Adolescence to Old Age

Star-Cluster H–R Diagrams and the Main-Sequence Turnoff

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

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Summary

A star cluster’s H–R diagram shows how its stars are distributed by temperature and luminosity. Because stars of different masses evolve at different rates, the diagram also provides clues to the cluster’s age.

Stars form from clouds of interstellar gas and dust. Once a star has condensed, it contracts and heats until it reaches the main sequence, the region of the H–R diagram where stars spend most of their lives fusing hydrogen in their cores.

All flashcards

Main sequence
The H–R diagram region where stars spend most of their lives fusing hydrogen in their cores.
Main-sequence turnoff
The H–R diagram location where cluster stars begin leaving the main sequence.
Why do massive stars leave the main sequence before lower-mass stars?
They evolve more quickly and use up hydrogen in their cores sooner.
What does a sparse region between the main sequence and red giants indicate?
It may represent a brief evolutionary stage that few stars occupy at one time.
How can a cluster’s main-sequence turnoff help estimate its age?
Its observed position can be compared with theoretical H–R diagrams for different ages.

Quiz questions

  1. Why might an H–R diagram show few stars between the main sequence and the red-giant region?Answer: Stars may pass through that evolutionary stage quickly.
  2. What does the main-sequence turnoff mark on a cluster’s H–R diagram?Answer: Where stars begin moving away from the main sequence.
  3. How do astronomers use a main-sequence turnoff to estimate a cluster’s age?Answer: They compare its observed position with calculated H–R diagrams for different ages.
  4. As a star cluster ages, how does its main-sequence turnoff generally change?Answer: It shifts to lower points on the main sequence, corresponding to lower-mass stars.
  5. What do the ages of the oldest globular clusters imply about the universe?Answer: The universe must be at least about 11 billion years old.

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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