Universe 8e Lecture Chapter

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Roger A. Freedman William J. Kaufmann III
Universe Eighth Edition
CHAPTER 19 On and After the Main Sequence
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By reading this chapter, you will learn

• 19-1 How a main-sequence star changes as it
  converts hydrogen to helium
• 19-2 What happens to a star when it runs out of
  hydrogen fuel
• 19-3 How aging stars can initiate a second stage
  of thermonuclear fusion
• 19-4 How H-R diagrams for star clusters reveal
  the later stages in the evolution of stars
• 19-5 The two kinds of stellar populations and
  their significance
• 19-6 Why some aging stars pulsate and vary in
  luminosity
• 19-7 How stars in a binary system can evolve very
  differently from single, isolated stars

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Key Ideas

• The Main-Sequence Lifetime The duration of a
  stars main-sequence lifetime depends on the
  amount of hydrogen available to be consumed in
  the stars core and the rate at which this
  hydrogen is consumed.
• The more massive a star, the shorter its
  main-sequence lifetime. The Sun has been a
  main-sequence star for about 4.56 billion years
  and should remain one for about another 7 billion
  years.

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Key Ideas

• During a stars main-sequence lifetime, the star
  expands somewhat and undergoes a modest increase
  in luminosity.
• If a stars mass is greater than about 0.4 M?,
  only the hydrogen present in the core can undergo
  thermonuclear fusion during the stars
  main-sequence lifetime. If the star is a red
  dwarf with a mass less than about 0.4 M?, over
  time convection brings all of the stars hydrogen
  to the core where it can undergo fusion.

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Key Ideas

• Becoming a Red Giant Core hydrogen fusion ceases
  when the hydrogen has been exhausted in the core
  of a main-sequence star with mass greater than
  about 0.4 M?. This leaves a core of nearly pure
  helium surrounded by a shell through which
  hydrogen fusion works its way outward in the
  star. The core shrinks and becomes hotter, while
  the stars outer layers expand and cool. The
  result is a red giant star.
• As a star becomes a red giant, its evolutionary
  track moves rapidly from the main sequence to the
  red-giant region of the H-R diagram. The more
  massive the star, the more rapidly this evolution
  takes place.

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Key Ideas

• Helium Fusion When the central temperature of a
  red giant reaches about 100 million K, helium
  fusion begins in the core. This process, also
  called the triple alpha process, converts helium
  to carbon and oxygen.
• In a more massive red giant, helium fusion begins
  gradually in a less massive red giant, it begins
  suddenly, in a process called the helium flash.
• After the helium flash, a low-mass star moves
  quickly from the red-giant region of the H-R
  diagram to the horizontal branch.

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Key Ideas

• Star Clusters and Stellar Populations The age of
  a star cluster can be estimated by plotting its
  stars on an H-R diagram.
• The clusters age is equal to the age of the
  main-sequence stars at the turnoff point (the
  upper end of the remaining main sequence).

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Key Ideas

• As a cluster ages, the main sequence is eaten
  away from the upper left as stars of
  progressively smaller mass evolve into red
  giants.
• Relatively young Population I stars are metal
  rich ancient Population II stars are metal poor.
  The metals (heavy elements) in Population I stars
  were manufactured by thermonuclear reactions in
  an earlier generation of Population II stars,
  then ejected into space and incorporated into a
  later stellar generation.

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Key Ideas

• Pulsating Variable Stars When a stars
  evolutionary track carries it through a region in
  the H-R diagram called the instability strip, the
  star becomes unstable and begins to pulsate.
• Cepheid variables are high-mass pulsating
  variables. There is a direct relationship between
  their periods of pulsation and their
  luminosities.
• RR Lyrae variables are low-mass, metal-poor
  pulsating variables with short periods.
• Long-period variable stars also pulsate but in a
  fashion that is less well understood.

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Key Ideas

• Close Binary Systems Mass transfer in a close
  binary system occurs when one star in a close
  binary overflows its Roche lobe. Gas flowing from
  one star to the other passes across the inner
  Lagrangian point. This mass transfer can affect
  the evolutionary history of the stars that make
  up the binary system.