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Lecture 30
Death of High Mass Stars
and Ages of Clusters
January 14b, 2014
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High Mass Stars (M > 5 M
)
• High mass stars have:
– More mass
– Greater gravity
– Higher temperatures and pressures in the core.
• Fusion reactions do not stop with Helium
burning in the core as they do in smaller
stars.
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4
• Star becomes giant as for small mass star.
– Helium burning ends in core.
– Core contracts.
– Temp and pressure in core increase.
– He shell burning begins.
– Core continues collapse.
• Then carbon fusion begins in the core.
Carbon fuses into higher mass elements.
• Process continues as core runs out of fuel.
• All fusion ends with silicon fusing into iron.
Iron cannot fuse to produce energy.
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• Fusion of
different
elements
continues
through neon,
oxygen, silicon
and finally iron.
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• Star expands to
become a
Supergiant.
• Star moves
back and forth
on the HR
diagram with
each type of
fusion.
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• Each stage of fusion lasts for a shorter
period of time
Fusion Temp
(million K)
Duration
H 40 7 mill. yrs
He 200
500000 yrs
C 600 600 yrs
Ne 1200 1 yr
O 1500 6 mo.
Si 2700 1 day
Which of these stars could be fusing
silicon?
A.Yellow giants
B.Instability strip giants
C.White dwarfs
D.Red supergiants
E.Main sequence blue stars
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Which of these stars could be fusing
silicon?
A.Yellow giants
B.Instability strip giants
C.White dwarfs
D.Red supergiants
E.Main sequence blue stars
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Stars like the Sun probably do not form
iron cores during their evolution because
A. all the iron is ejected when they become
planetary nebulas.
B. their cores never get hot enough for them to
make iron by nucleosynthesis.
C. the iron they make by nucleosynthesis is all
fused into uranium.
D. their strong magnetic fields keep their iron in
their atmospheres.
E. they live such a short time that it is impossible
for iron to form in their cores.
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Stars like the Sun probably do not form
iron cores during their evolution because
A. all the iron is ejected when they become
planetary nebulas.
B. their cores never get hot enough for them to
make iron by nucleosynthesis.
C. the iron they make by nucleosynthesis is all
fused into uranium.
D. their strong magnetic fields keep their iron in
their atmospheres.
E. they live such a short time that it is impossible
for iron to form in their cores.
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Death of High Mass Star
• Iron builds up in the core.
• Iron cannot be fused and produce more
energy.
• What keeps iron core from collapsing?
– First: electron degeneracy
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• After core has a mass greater than 1.4 M
(Chandrasekhar limit) the electron
degeneracy is not strong enough.
• Electrons are forced to combine with the
protons to create neutrons.
• Core collapses until pressure from physical
force of neutrons bouncing against each
other stops it.
• Core rebounds and runs into outer material
that is still falling inward.
Death of a High Mass Star
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Supernova
• Collision between expanding core and
material falling inward produces huge shock
wave pushing all material outward in an
immense explosion called a supernova.
• Explosion can be as bright as an entire
galaxy (billions of stars) for a few days
• Some of the energy creates elements
heavier than iron. These elements are
distributed to the rest of the galaxy.
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Supernova 1987a
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Types of Supernovae
• Type 2 supernova = extremely high mass
star becomes a supernova as previously
described.
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• Type 1 supernova
– Need binary star system with a white dwarf and
red giant orbiting each other
– Red giant expands until it starts to donate some
of its material to the white dwarf
– If mass of white dwarf becomes greater than
1.4 M
its core will collapse and create a
supernova explosion.
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• A large shell of slowly expanding material forms
a supernova remnant around a central core.
Figures 21.10 and 21.12,
Chaisson and McMillan,
5
th
ed. Astronomy Today,
© 2005 Pearson Prentice Hall
Crab nebula
Vela supernova remnant
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Star Clusters
• Open Clusters --loose clusters of 10-100 stars
• Globular Clusters -- Old, tightly bound group of
100’s or 1000’s of stars
• All stars in a cluster are formed at the same time.
• Age of a cluster can be determined by looking at
what point the stars leave the main sequence; the
“turn-off point”.
• Age of Cluster = Lifetime of star at turn-off point.
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• Young Cluster -- Hyades cluster
• Around 600 million years old
Figure 20.19,
Chaisson and McMillan,
5
th
ed. Astronomy Today,
© 2005 Pearson Prentice Hall
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• Old Cluster -- 47 Tucanae
• One of the oldest clusters,
about 12 to 14 billion years old
Figure 20.20,
Chaisson and McMillan,
5
th
ed. Astronomy Today,
© 2005 Pearson Prentice Hall
