Chapter 13: Stellar Evolution cont'

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Chapter 13 Stellar Evolution cont.

• Dr. Michelle Arnold
• November 9, 2009

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Death of Low Mass Stars

• (Less than 10 solar masses)

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Leaving the Main Sequence

• A star leaves the main sequence when it has fused
  all of the hydrogen in its core

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Red Giant Phase

• Core
• No fusion is occurring
• Collapses due to gravity
• Temp, pressure and density rise
• Shell of hydrogen fusion
• Envelope of non-fusing hydrogen

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Red Giant Phase cont.

• Surface
• Expands (due to the H fusion shell)
• Cools (redder) since expanding gasses cool
• Increases in luminosity since it has a larger
  surface (doorway) for energy to leave
• Moves up and right on the H-R diagram

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Horizontal Branch Star

• The core within a red giant continues to collapse
  (outer surface expands) until the temp. and
  pressure in the core are great enough for helium
  fusion to begin
• A helium flash occurs
• The star settles into a new equilibrium and is
  called a Horizontal Branch Star
• Outer surface shrinks some
• Heats up (less red)
• Decreased luminosity
• Star moves down and left on the H-R diagram

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Horizontal Branch Star cont.

• Structure
• Core fusing helium into carbon
• Shell of hydrogen fusing into helium
• Envelope of non-fusing hydrogen
• Cepheid variable horizontal branch stars can be
  used as standing candles (blinking frequency is
  known to be related to luminosity)
• Short life (50 million years for our Sun), not a
  lot of helium

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Asymptotic Giant Branch Star (AGB)

• Star runs out of helium in its core
• Core (of carbon) once again begins collapsing
  under gravity
• Temp., pressure and density rise
• Shell of helium fusing into carbon
• Shell of hydrogen fusing into helium
• Envelope of non-fusing hydrogen
• Surface
• Expands, cools (redder), increased luminosity
• Moves up and right on the H-R diagram

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Planetary Nebula

• Carbon core continues to collapse but temp. and
  pressure will never be high enough for carbon to
  fuse (in low mass stars)
• Star will begin to lose its gravitational hold on
  its ever expanding outer layer
• Surface drifts off, star has less gravity due to
  less mass, it looses more mass, etc.
• Outer layers peel off over thousands of years
• See hotter and hotter portions of the stars
  interior
• Red, yellow, blue, UV
• Moves from right to left on H-R diagram
• UV released from the interior causes the escaping
  gasses to glow or fluoresce

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Planetary Nebular cont.
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White Dwarfs

• Gas released from dying low mass stars contains
  hydrogen, helium and some carbon
• Our Sun will lose 40 of its mass
• The remaining core will be mostly carbon
• Gravity packs the atoms together as tightly as
  possible (degenerate state)
• Remaining 60 of our Sun will compress to the
  size of our Earth
• White dwarfs are hot balls of carbon that will
  cool over time (UV, blue, yellow, red, etc.)
• Small size small luminosity

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White Dwarfs cont.
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Summary, Low Mass Star Death