Post-Main%20Sequence%20Evolution%20of%20Massive%20Stars

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Post-Main Sequence Evolution of Massive Stars

• Stars of more than 8 solar masses leave behind
  neutron stars and black holes and typically
  explode as supernovae.

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But first, NOVAE from WDs

• Back to binary star evolution
• More massive star leaves MS first, becomes RG,
  then WD
• Less massive star swells as it leaves MS, fills
  its Roche lobe
• Mass then flows from companion star onto WD
  through inner Lagrangian point.
• This mass forms an ACCRETION DISK around the WD
• The mass stream hits the outer part of the
  accretion disk and makes a HOT SPOT

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Accretion Disk in Close Binary w/ WD
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Accretion Disks Cataclysmic Variables

• Viscosity in the ACCRETION DISK (AD) causes its
  gas to
• lose its angular momentum and SPIRAL INTO THE
  WD
• as it does so, it gets very hot and EMITS
  ULTRAVIOLET RADIATION
• often, more radiation comes from the disk than
  from the (very small) WD
• INSTABILITIES in the AD can cause dramatic
  variations in rate of inflow
• therefore, AD Luminosity varies a lot --
• CATACLYSMIC VARIABLES produced this way.

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NOVAE

• An explosion on the surface of a WD Nova
  Herculis 1934 and typical light curve

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What makes a Nova explode?

• As H gas from companion builds up on WD surface
  it gets hotter and denser
• Eventually (typically after 103 or 104 yrs) it
  IGNITES
• This THERMONUCLEAR DETONATION (of pp chains to
  He, mainly)
• produces a huge burst of POWER -- this is a
  NOVA.

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Novae and Recurrent Novae

• Most of the gas is expelled in a rapidly
  expanding shell.
• Luminosity rises between 5 and 12 magnitudes
  (100--63,000 times) in just a few days
• rapid decline over a couple of weeks is followed
  by slow decline to original low L in a few years.
• While most of the accreted H is blasted off in
  nova explosions some (plus some created He) does
  remain on the WD surface and the WD's mass
  increases.
• As long as mass continues to flow from the
  companion, many such explosions can occur --
  RECURRENT NOVAE

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Ejected Shells Novae Persei Cygni
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Thought Question

• What happens to a white dwarf when it accretes
  enough matter to reach the 1.4 MSun limit?
• A. It explodes
• B. It collapses into a neutron star
• C. It gradually begins fusing carbon in its
  core

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Thought Question

• What happens to a white dwarf when it accretes
  enough matter to reach the 1.4 MSun limit?
• A. It explodes
• B. It collapses into a neutron star
• C. It gradually begins fusing carbon in its
  core

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Life Stages of High-Mass Stars

• Late life stages of high-mass stars are similar
  to those of low-mass stars
• Hydrogen core fusion (main sequence)
• Hydrogen shell burning (supergiant)
• Helium core fusion (supergiant)

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How do high-mass stars make the elements
necessary for life?
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Big Bang made 75 H, 25 He stars make
everything else
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Helium fusion can make carbon in low-mass stars
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CNO cycle can change C into N and O
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Helium Capture

• High core temperatures allow helium to fuse with
  heavier elements to make Oxygen, Neon, Magnesium,
  etc.

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Helium capture builds C into O, Ne, Mg,
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Massive Stars Cook Heavy Elements

• After MS, H shell burning, He core burning, He
  shell burning, stars w/ Mgt8M? have so much
  gravity that
• the C core crushed until it reaches T gt 7 x 108 K
  so
• Carbon can also fuse
• 12C 4He ? 16O ? some
• 16O 4He ? 20Ne ? also some
• 12C 12C ? 24Mg ?
• These fuels produce less energy per mass, so each
  is burned up faster and faster
• Most will fuse Oxygen and Neon too
• 16O 16O ? 32S ? 20Ne 4He ? 24Mg ?

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Multiple Shell Burning

• Advanced nuclear burning proceeds in a series of
  nested shells
• High Mass Star Evolution

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Element Formation Abundances

• The more common heavy elements have an even
  number of protons built up by 4He nuclei (alpha
  process)
• H and He alone were made in the BIG BANG.
• All other elements (up to iron) are made in
  PRE-SUPERNOVAE stars.
• Anything heavier than Fe (unstable) made in
  Supernovae

Fe is endpoint of fusion it has the minimum mass
per nucleon energy would be absorbed -- not
given off -- to go to heavier nuclei
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Alpha Process Builds Middle Elements
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Elemental Abundances H Rules!
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How Does a High-Mass Star Die?
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Approach to Supernova

• The iron core collapses excess neutrons build up
• 16O 16O ? 31S n
• Si fusion up to Fe takes lt 1 day to complete!
• Mass of Fe core grows exceeds Chandrasekhar mass
• Yielding CORE COLLAPSE
• Key details high energy photons are absorbed,
  causing a pressure drop photodisintegration!
• 56Fe ? ? 13(4He) 4 n
• 4He ? ? 2p 2n
• Only when density gt 109g/cm3 (500 cars/teaspoon)
• and T gt 5x109K

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Neutronization and Collapse

• This PHOTODISINTEGRATION occurs in lt 0.1 second!
• Further NEUTRONIZATION (production of excess
  neutrons) occurs when electrons are crushed
  into protons
• p e ? n ? (weak nuclear reaction)
• Atoms disappear and become nuclear matter,
  with density about 4 x 1014 g/cm3 !
• The core collapses!
• (Whole sun into a city size -- a billion
  tons/teaspoon!)

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Supernova Explosion

• Core electron degeneracy pressure goes away
  because electrons combine with protons, making
  neutrons and neutrinos
• Neutrons collapse to the center, forming a
  neutron star

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Core Collapse, continued

• Once NEUTRONIZATION is nearly complete,
• the core collapse is halted by a combination of
  NEUTRON DEGENERACY PRESSURE
• and the REPULSIVE PART OF THE STRONG NUCLEAR
  FORCE.
• The core, with radius about 10 km, becomes a
  NEUTRON STAR (NS -- to which we'll return shortly)

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FORMATION of a TYPE II SUPERNOVA

• Ca, Si, S, Mg, Ne, O, C layers continue to burn
  and collapse onto the NS core.
• BUT huge NEUTRINO PRESSURES build up, and, in
  addition, the NS is so stiff that matter
  hitting it BOUNCES from a SHOCK.
• Bounce works better if star is rotating (and has
  big magnetic fields)
• EXPLOSIVE NUCLEOSYNTHESIS produces ELEMENTS
  HEAVIER THAN IRON
• Also helps BLAST OFF MOST OF THE STAR'S ENVELOPE.
  
• This rapidly expanding star gets very luminous,
  very fast since the radius is so big A SUPERNOVA

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Energy and neutrons released in supernova
explosion enable elements heavier than iron to
form, including Au (gold) and U
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Type II (massive star) SN formation, illustrated
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PROPERTIES of TYPE II SUPERNOVAE

• Luminosities equal more than that of 109
  ordinary stars for a few days, while at peak
  power
• Timescales rise, 1 day peak, 1 week hump, 2 to
  3 months slow decline, 2 years (powered by
  56Co).

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More SN Type II Properties

• Most of the star's mass is EJECTED at velocities
  from 10,000-30,000 km/s (3-10 of the speed of
  light!!!).
• Spectra are rich in H lines much hydrogen
  expelled

Crab nebula photos 14 years apart dont line
up Illustrates fast outward motion
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Supernova Remnants (bright nebulae)

• The expelled gas interacts with the ISM to make a
  SUPERNOVA REMNANT, a BRIGHT NEBULA which glows
  for 105 years Crab Nebula Views
• CRAB SN was seen in 1054 CE and its expanding SNR
  is beautiful now Crab Nebula Movie

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SN 1987A

• The nearest, recent Type II SN in the Large
  Magellanic Cloud (50 kpc away)
• Neutrinos (17) were detected only ones not from
  the Sun great confirmation of massive star
  evolution theory!

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Rings Around Supernova 1987A

• The supernovas flash of light caused rings of
  gas (ejected from star earlier) around the
  supernova to glow

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Impact of Debris with Rings

• More recent observations are showing the inner
  ring light up as debris crashes into it

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Type I (White Dwarf) Supernovae

• Type I SN are usually even more luminous
• peak M -19 This is about as luminous as an
  ENTIRE GALAXY!
• Very close to a STANDARD CANDLE, I.e, all Type
  Ia (WD) SNe are nearly equally bright
• Rise in 1 day fast decline, months slow
  decline, over years
• Spectra are devoid of H lines (no H envelope)
• Often found in Pop II (low metalicity) regions,
  while Type II SN are associated only with Pop I
  (composition like sun) stars.

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Type Ia SN formation, illustrated
WD driven over the Chandrasekhar limit thanks
to accretion in a binary system
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Origin of Type I SNe

• Majority, at least, arise from WDs in binary
  systems (Type Ia).
• If WD starts out massive and close to
  Chandrasekhar limit, then if it accretes much
  mass from companion it can be pushed over the
  maximum mass electron degeneracy pressure can
  support.
• This usually leads to a collapse and immediate
  DETONATION EXPLOSION, which usually COMPLETELY
  DISRUPTS the star (i.e. no NS left behind).
• Some Type I SNe may have a WD core collapse to a
  Neutron Star and most (but not all) gas
  expelled.
• Exploding layers shine even brighter since they
  don't have to push out overlying mass that high
  mass (Type II) SN have. No envelope also explains
  the missing H lines.

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Explosive Nucleosynthesis

• Nearly all elements heavier than iron are
  produced in SN explosions
• The s-process (slow) adds neutrons to build
  elements via intermediate decays (AGB stars
  SN)
• 56Fen?57Fe 57Fen?58Fe 58Fen?59Fe
• These neutron-rich isotopes decay to elements
  with more protons that are stable 59Fe?59Coe-?
• The r-process adds lots of neutrons very fast to
  produce the heaviest stable (and unstable)
  elements (those above Bismuth) only during SN
  explosions
• These elements then pollute (enrich?) the ISM
• Power for SN light curves comes from Ni-56 and
  Co-56 decays

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Role of Mass

• A stars mass determines its entire life story
  because it determines its core temperature
• High-mass stars with gt8MSun have short lives,
  eventually becoming hot enough to make iron, and
  end in supernova explosions
• Low-mass stars with lt2MSun have long lives,
  never become hot enough to fuse carbon nuclei,
  and end as white dwarfs
• Intermediate mass stars can make elements heavier
  than carbon but end as white dwarfs

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• Low-Mass Star Summary
• Main Sequence H fuses to He in core
• Red Giant H fuses to He in shell around He core
• Helium Core Burning
• He fuses to C in core while H fuses to He in
  shell
• Double Shell Burning
• H and He both fuse in shells
• 5. Planetary Nebula leaves white dwarf behind

Not to scale!
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• Reasons for Life Stages
• Core shrinks and heats until its hot enough for
  fusion
• Nuclei with larger charges require higher
  temperatures for fusion
• Core thermostat is broken while core is not hot
  enough for fusion (shell burning)
• Core fusion cant happen if degeneracy pressure
  keeps core from shrinking

Not to scale!
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• Life Stages of High-Mass Star
• Main Sequence H fuses to He in core
• Red Supergiant H fuses to He in shell around He
  core
• Helium Core Burning
• He fuses to C in core while H fuses to He in
  shell
• Multiple Shell Burning
• Many elements fuse in shells
• 5. Supernova (Type II) leaves neutron star behind

Not to scale!
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Good Stars! (They Recycle)

• ISM
• Star formation
• Stellar Evolution
• Explosions and enrichment of the
• ISM (do again!).
• About 3 solar masses / year are recycled in the
  Milky Way