Transcript Lec10_ch12_deathofstars

Ch 12--Life Death of Stars
9 Nov 2000
ASTR103, GMU, Dr. Correll
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What do you think?
• Will the Sun end its existence? If so, how?
• What is a nova?
• What is the origin of the carbon, silicon, oxygen,
iron, uranium, and other heavy elements on Earth?
• What is a cosmic ray?
• What is a pulsar?
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Low-Mass Stars
• Post Main Sequence
– hydrogen fusion shell
– helium flash/helium fusion core
– helium fusion shell (asymptotic giant branch)
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Post Main Sequence Giants
• Core and interior shells condense giving off more
energy causing the outer layers to expand and
cool
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Planetary Nebula
• Outer atmosphere of
stars is ejected and the
surrounding could of gas
is called a planetary
nebula
– has nothing to do with
planet formation
– a wide variety of planetary
nebula are seen!
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Bipolar Planetary Nebula
• The Hourglass nebula about
the star Eta Carinae is an
example of a bipolar
planetary nebula
– about 8000 ly away
– around a massive (~30Msun
star)
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Remains of the Low-Mass Star
• After outer layers ejected in
planetary nebula and solar
wind, all that remains is the
hot, dense carbon-oxygen
core--a white dwarf!
• Fusion has ceased, and core
cools down over billions of
years
• White dwarf mass < 1.4 Msun
due to Chandrasekhar limit
– Degenerate electrons support
only this much mass before
combining with protons to form
neutrons
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Novas
• Consider what happens to a
white dwarf in a binary
system
– As the larger star evolves and
expands some of its outer
layers fall onto the white dwarf
companion
– What happens when the
hydrogen layer becomes
dense enough?
• Hydrogen flash of fusion--a
Nova occurs
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High-Mass Stars
• Higher mass enables fusion
of heavier elements
– each successive shell burns in
a shorter time
• carbon for about 600 yrs
• neon for about 1 yr
• oxygen for about a month
• silicon fusion for a day
– final inert core is iron!
– A star the size of Jupiter’s orbit
is powered by a core the size
of the Earth!
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High-Mass Stars
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Death of Massive Stars
• As iron core grows, the immense weight is
supported by electron degeneracy pressure-ie, electron gas
• Eventually the electron gas is compressed to
the point that the electron degeneracy
pressure is overcome by gravity--electrons
combine with protons forming neutrons and
emitting gamma rays and neutrinos
• As neutrons form the core collapses to form a
neutron core. A rebounding shockwave
creates a powerful explosion blowing off
outer layers of the star --Supernova!
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Death of Massive Stars
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Supernova 1987A
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Supernova 1987A
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Supernova Types
• Type Ia--begins as a
carbon-oxygen rich white
dwarf in a close binary
system
– giant companion enlarges,
depositing gas onto white
dwarf
– as mass approaches the
Chandrasekhar limit,
carbon fusion ignites in the
core
• nuclear powered
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Supernova Types
• Type II--core collapse of
massive stars
– mass of core greater than
Chandrasekhar limit
– gravity overwhelms
electron degeneracy
pressure
– core falls into the center
• gravitational energy
– Outer gas layers give many
hydrogen lines in spectrum
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Neutron Stars
• What’s left after a type II
supernova
– for stars of mass of 8-25 Msun, when
core collapses and electron
combine with protons, we are left
with a neutron core--a neutron star!
– Core about 10-20 km in diameter!
– Gas layers completely blown away
– Star now supported by neutron
degeneracy pressure
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Neutron Stars
• Stars naturally seem to
have magnetic fields.
– What happens to the
magnetic fields when
stellar cores collapse?
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Pulsars
• The magnetic field
collapses also, creating
a very high energy
density electric dynamo
– polar axis of magnetic
field not always in line
with rotation axis
– field energizes and beam
out charged particle
– “lighthouse” model
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Pulsars
• We observe pulsars via
radio waves
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Pulsars--SS433
• SS433--a pulsar
fed by a close
binary companion
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X-Ray Bursters
• Analogous to hydrogen
flashes on white
dwarves (novas), on a
neutron star, the
hydrogen slowly fuses
into helium and then a
helium flash can occur
– rather than peaking in the
optical range, helium
flashes are hotter and in
the x-ray range
• x-ray burster
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Stellar Evolution Summary
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Stellar Evolution Summary
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What do you think?
• Will the Sun end its existence? If so, how?
– The Sun will shed its outer layers as a planetary nebula in about 7
billions years. Its remnant white dwarf, with fusion ceased, will dim over
the next several billion years
• What is a nova?
– A relatively gentle explosion of hydrogen gas on the surface of a white
dwarf
• What is the origin of the carbon, silicon, oxygen, iron, uranium, and
other heavy elements on Earth?
– These elements were generated during stellar evolution of massive
stars and in supernova explosions
• What is a cosmic ray?
– High energy (speed) particles accelerated in supernova explosions
• What is a pulsar?
– A rotating neutron star with a magnetic field creating a “lighthouse” of
radiation which sweep across space
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Questions for Thought
• Describe the evolution of main sequence star with a
mass of 20 Msun. Describe the various phases of
energy generation, migration off the main sequence,
and the eventual fate of the star to include what
forces are at balance throughout.
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