life and evolution of a massive star m ~ 25 m sun
TRANSCRIPT
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Life and Evolution of aMassive Star
M ~ 25 MSun
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• Birth in a Giant Molecular Cloud
• Main Sequence
• Post-Main Sequence
• Death
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The Main Sequence
• Stars burn H in their cores via the CNO cycle
• About 90% of a star’s lifetime is spent on the Main Sequence
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The CNO Cycle• 12C + 1H 13N + γ• 13N 13C + e+ + νe
• 13C + 1H 14N + γ• 14N + 1H 15O + γ• 15O 15N + e+ + νe
• 15N + 1H 12C + 4He + γ
• Overall reaction:4 1H 4He
• Hotter core temp allows H to fuse with C, N, and O
• More possibilities means faster reaction rate
• Faster reaction rate means higher luminosity and a shorter life
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Supergiants
• H burning in core stops• He core contracts
• T high enough for He fusion, no need for degeneracy pressure
• No He Flash
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Supergiants
• Low mass stars cannot burn past He– Degeneracy pressure
prevents core from contracting enough
• High mass stars have more mass, so they can burn heavier elements– Degeneracy pressure
never plays a part
• As core burning element runs out, core contracts
• Shell burning rate increases, star expands
• Eventually core contracts enough for fusion of heavier element to begin
• Shell burning slows and star contracts
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The Most Important Graph in the Whole Course
E=mc2
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Supernova: Type II
• Fe core temporarily supported by electron degeneracy pressure
• Gravity is stronger in high mass stars, crushes star further
• Electrons combine with protons to form neutrons and neutrinos
• Core collapses until neutron degeneracy pressure causes core to rebound
• Tons of neutrinos push material out with a ton of energy (10,000 km/s)
• Extra energy can create heavier elements than Fe
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Neutron Stars
• Supported by neutron degeneracy pressure
• M ~ 1-2 Msun
• R ~ 10 km• ρ ~ 1014 g/cm3
• Vesc ~ 0.5c
• Rotational periods range from msec – sec– Angular momentum
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Pulsars
• Rapidly spinning neutron star
• Tightly bunched magnetic field lines direct radiation out from poles
• Magnetic axis not aligned with rotation axis; lighthouse effect
• All pulsars are neutron stars, not all neutron stars are pulsars
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Pulsars
• Nearly perfect clocks • Radiation takes angular momentum away, slowing down rotation
• Pulsar dies when rotation gets too slow
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Millisecond pulsars/X-ray binaries
• Gain angular momentum from material accreted from companion
• Can be recycled pulsars• Very strong gravity
makes disk very hot and bright in x-rays
• X-rays pulse due to rotation
• VIDEO
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X-Ray Bursters
• Accreted H builds up into layer
• Pressure below H layer is high enough for fusion, which makes He
• If T reaches 108 K, He fusion can ignite, releasing tons of energy– P ~ 100,000 Lsun
• Bursts last few seconds
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General Relativity
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Black Holes
• Escape velocity – the speed necessary to climb out of a gravitational potential
• Black holes have infinitely deep potential wells
• Schwarzchild radius – the point in the gravitational well where vesc = c
r
GMvesc
2
kmM
M
c
GMR
Suns 0.32
2
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Black Hole Formation
• Neutron star can hold itself up against gravity with neutron degeneracy pressure
• A star that is so massive that it collapses past the neutron degeneracy limit will become a black hole
• The result is a singularity
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Cygnus X-1
• X-ray binary system• 18 Msun star orbiting
with an unseen 10 Msun
object• 10 Msun is much more
massive than neutron degeneracy pressure can support
• It must be a black hole
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Gamma Ray Bursts
• First discovered in the 60s by US spy satellites looking for nuclear bomb tests
• Astronomers first thought GRBs were just more energetic versions of X-ray binaries– X-ray binaries are concentrated in the disk of the
Milky Way– GRBs are not, so they must be extragalactic
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Gamma Ray Bursts
• Extremely short, luminous burst followed by long afterglow of lower-energy radiation
• Most energetic outbursts in the Universe– Brighter than 1,000,000 Milky Ways
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Long GRBs
• Appear to be correlated with core-collapse SN or galaxies with active star formation– Suggests that
progenitors are SN from super massive stars
– Formation of black hole
• Burst lasts > 2 sec• Afterglow lasts several
days to a month
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Short GRBs
• Do not appear to come from SN explosions
• Theory suggests that a double neutron star binary or neutron star and black hole binary collision would produce the energies necessary to make a short GRB
• Burst lasts < 2 sec• No afterglow• Not well understood