Goals

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• Goals
• Explain why stars evolve
• Explain how stars of different masses evolve
• Describe two types of supernova
• Explain where the heavier elements come from
• Describe how star clusters support stellar
  evolution theories

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Most stars spend a majority of their lives (90)
on the main sequence (about 10 billion years for
our Sun) Virtually all the low mass stars ever
formed still exist. None of them have left the
main sequence. On the other hand, massive O and
B stars leave the main sequence after a few 10s
of millions of years. Most of the high mass
stars that have ever existed perished a long time
ago. The stars in between are in the stages of
evolving into?
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During the main sequence of a stars life the
outward force of energy from burning Hydrogen is
balanced by the inward pull of gravity
(hydrostatic equilibrium). As the star burns
more Hydrogen eventually the reaction will slow
down and the amount of energy released will
diminish. Gravity then begins to compress the
star more.
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The most important factor determining the fate of
a star is its mass. Low mass stars die gently
while high mass stars die catastrophically. The
dividing line is about 8 solar masses.
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Evolution of a Sun-like Star
As hydrogen is consumed in the core the Helium
waste becomes concentrated. Eventually, hydrogen
becomes completely depleted at the center until
the nuclear fires cease. The hydrogen burning
moves to higher levels.
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As soon as the hydrogen becomes substantially
depleted the helium core begins to shrink under
the increased pressure of the unbalanced
gravity. The increased pressure and heat causes
the hydrogen shell to burn even faster causing
the star to get brighter as the helium core
continues to shrink and heat up. The star is
becoming a red giant. This stage takes 100
million years.
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The change in energy production and radius causes
the star to move off of the main sequence towards
the red giant branch. At the end of this phase
the stars luminosity is hundreds of times
greater and its radius is 100 solar radii.
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As a Solar mass star evolves it is massive enough
to fuse Helium into carbon.
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The simultaneous shrinking orf the core and
expanding of the outer layers cannot continue
forever. A few 100 million years after a solar
mass star leaves the main sequence, Helium
begins to burn. The initial burning of the
helium is very fast and is called the helium
flash and last for a few hours. In response the
star again begins to move on the HR diagram.
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A star like our Sun is massive enough to burn
helium and convert it into carbon. A similar
process of core surrounded by a helium shell
develops just like it did when helium began to
burn. Now, however, the star swells to an even
large size. It is now a red supergiant. Our Sun
will engulf the Earth.
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If our Sun were massive enough it could burn
carbon. But, it cant, so now our Sun begins to
die. As the hydrogen and helium continue to burn
in the outer shell the outer envelope becomes
unstable and is ejected into space. The
remaining hot core ionizes the ejected atmosphere
and we now have a planetary nebula.
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The remaining core continues to evolve. It
continues to shrink until the degenerate
electrons prevent it from shrinking any more. We
now have a white dwarf star. As the white dwarf
continues to cool it eventually will become a
cold, dense, burned-out ember in space, or a
black dwarf.
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Some white dwarf stars are part of a binary star
system. If the stars are close enough together
then material from one star can be pulled off by
the other star. The material then forms an
accretion disk before the material falls to the
surface. If enough hydrogen gets dumped on a
white dwarf star, then eventually the material
will explosively ignite and we will have a nova.
Once a nova explodes it is ready to repeat the
process and we get recurrent nova.