Chapter 18 by gegeshandong

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									Evolution from the Main Sequence
                                 Chapter 18
                          Evolution from the Main Sequence
UVU Survey of Astronomy
                                           Chapter 18
                                Evolution from the Main Sequence
UVU Survey of Astronomy




                          During its stay on
                          the main
                          sequence, any
                          fluctuations in a
                          star’s condition
                          are quickly
                          restored; the star
                          is in equilibrium.
                                         Chapter 18
                              Evolution from the Main Sequence
                          Again to follow the post-main-sequence evolution
                          of a star we will resort to the stage method. Not
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                          every star adheres to this sequence but it serves to
                          describe the steps that many stars take
                                          Chapter 18
                               Evolution from the Main Sequence
                          Even while on the main sequence,
                          Stage 7, the composition of a star’s
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                          core is changing.
                          Eventually, as hydrogen in the core
                          is consumed, the Star leaves the
                          main sequence, Stage 8.
                          Its evolution from then on depends
                          very much on the mass of the star:
                          Low-mass stars go quietly.
                          Medium-mass stars struggle.
                          High-mass stars go out with a
                          bang!
                                           Chapter 18
                                Evolution from the Main Sequence

                          When the fuel in the
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                          core is used up the
                          fusion ceases. The
                          result is a contraction
                          of the Star the and the
                          formation of a new
                          fusion furnace in a
                          shell around the
                          helium core. This is
                          Stage 8
                                          Chapter 18
                               Evolution from the Main Sequence
                          Stage 9: The Red-Giant
                          Branch. The now much
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                          larger surface of the
                          furnace causes outer
                          layers of the star to
                          expand and cool. It is
                          now a red giant,
                          extending out beyond
                          the orbit of Mercury.
                          Despite its cooler
                          surface temperature, its
                          luminosity increases
                          enormously due to its
                          large size.
                                         Chapter 18
                              Evolution from the Main Sequence

                          Stage 10: Helium fusion. Once the core temperature
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                          has risen to 100,000,000 K, the helium in the core can
                          fuse, through a three-alpha process:




                           The 8Be nucleus is highly unstable, and will decay in
                           about 10–12 sec unless an alpha particle fuses with it
                           first. This is why high temperatures and densities are
                           necessary.
                                        Chapter 18
                              Evolution from the Main Sequence
                          The helium flash:
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                          The pressure within the helium core is almost totally
                          due to “electron degeneracy” – two electrons cannot
                          be in the same quantum state, so the core cannot
                          contract beyond a certain point. This pressure is
                          almost independent of temperature so when the
                          helium starts fusing, the pressure cannot adjust and
                          the core explodes completely disrupting the
                          surrounding shell furnace.
                          Helium begins to fuse extremely rapidly; within hours
                          to days the enormous energy output is over, but the
                          star is now on its way to White Dwarf, Stage 13
                                             Chapter 18
                                 Evolution from the Main Sequence
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                          Stage 10. Disruption of the
                          hydrogen furnace throws
                          the star out of equilibrium
                          and it starts to shrink, but it
                          has much heat to dissipate
                          from the Helium Flash.
                          The result is the surface
                          gets smaller as the surface
                          temperature gets higher,
                          causing movement across
                          the graph toward the blue
                          while maintaining nearly
                          the same brightness.
                                         Chapter 18
                              Evolution from the Main Sequence

                          Stages 11 and 12 depend very much on the mass of
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                          the star.

                          From .5 to 1.4 solar masses the transition from the
                          horizontal branch White Dwarf goes smoothly.

                          From 1.4 to about 5.5 solar masses they must shed
                          the extras mass to get down to the Chandrasakar
                          limit of 1.4 solar masses, then they can transition to
                          White Dwarf.
                                 Chapter 18
                          Evolution from the Main Sequence
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                                          Chapter 18
                               Evolution from the Main Sequence

                           Horizontal Branch, Stage 10.
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                          The Helium Flash has disrupted
                          the nuclear shell and the star is
                          dying. There is much heat to be
                          shed before the star can
                          become a White Dwarf. The star
                          shrinks by gravity but remains
                          bright by the surface getting
                          hotter. Stars over 1.4 Solar
                          masses also must rid
                          themselves of the extra mass.
                                           Chapter 18
                               Evolution from the Main Sequence

                           The Instability Strip, still Stage
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                          10 .. As the dying star moves
                          along the horizontal branch it
                          encounters a region,
                          discovered by Hertsprung,
                          called the Instability Strip. The
                          star becomes a variable star
                          changing brightness slightly in
                          a very few days.
                                         Chapter 18
                              Evolution from the Main Sequence

                                                                             It was later shown
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                                                                             that there are two
                                                                             families of Cephieds




                                                        Absolute Magnitude
                          Henrietta Levitt discovered
                          a direct relationship of
                          Period to Luminosity of the
                          Cephied Variables and the
                                                                                   Period
                          RR Lyra Variables
                                            Chapter 18
                                 Evolution from the Main Sequence
                          Stars with more than about 5.5
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                          solar masses have a different
                          problem. The Helium flash
                          becomes a permanent nuclear
                          furnace. The Helium core fuses
                          helium to carbon and the shield
                          furnace continues to fuse
                          Hydrogen to Helium and the
                          star is now in a some what
                          stable state. Many stars go into
                          a new Red Giant condition for a
                          period. This is the Asymptotic
                          Giant Branch, Stage 11
                                 Chapter 18
                          Evolution from the Main Sequence
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                              End Chapter 18
                                 Chapter 18
                          Evolution from the Main Sequence
UVU Survey of Astronomy

								
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