Chapter 5: Cosmic foundations for origins of life stars

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Chapter 5 Cosmic foundations for
origins of life - stars
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Stellar evolution forming the elements for
biolmolecules and planets.

• Stars are fusion reactors that convert lighter
  elements into heavier ones, liberating energy
  (from Emc2)
• They therefore continuously evolve as their
  fuels are used up. H burns to He, He burns to C,
  etc
• Stellar end-states white dwarfs, neutron
  stars, or black holes.
• In all of these cases, significant fraction of
  stellar mass, ejected into interstellar medium
• Planets, and biomolecules made out of these
  materials

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Energy liberated when light atomic nuclei undergo
fusion! (eg. Proton-proton reaction)

• Two protons colliding at high enough speed,
  undergo fusion.
• Products a deuteron (heavy water), a positron
  (positively charged electron), and a neutrino
  (very weakly interacting particle) energy
  release
• Special Relativity Energy release per fusion
  is proportional to mass difference between
  products and reactants

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Energy production in the Suns core
the proton-proton chain
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Net result of p-p burn

• For each Helium-4 nucleus produced
• - Consume 4 protons
• - Liberate energy and 2 neutrinos
• - Neutrinos arise from weak interaction.
• eg. they arise during conversion of a proton
  to a neutron in building a deuteron
• Nuclear reactions yield predictable neutrino
  fluxes from the Sun that directly reflect
  reaction rates

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Stellar temperatures

• Stars have different colours
• Corresponds to different temperatures from black
  body curves
• Note huge (and harmful) UV fluxes produced by
  massive stars
• (life possible on planets around them?)

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Spectral Classification of Stars Consequence
ofstellar temperatures

• - Stellar spectra can be divided into spectral
  classes of stars O,B,A,F,G,K,M
• Atomic theory this represents a sequence of
  decreasing temperature - hot stars are more
  completely ionized than cool stars so see fewer
  absorption features.
• - The Sun is a G2 star.

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Hertzsprung-Russell Diagram Plotting L vs. T

• Luminosity L and temperature T of a star are
  independent physical properties of a star.
• Temperature correlates with colour of a star (hot
  is blue, cool is red). L varies by factor of 100
  million!
• Plot L of a star vs. its colour on a diagram
  find that these are correlated with one another.
  Known as colour-magnitude diagram.
• - Most stars occur along main-sequence, where
  they burn hydrogen.

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H-R Diagrams (L vs. T) of Nearest, and brightest
stars
Stars within 5pc of Sun
100 brightest stars in the sky
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• STELLAR RADII
• Range from 0.08 of the Sun, to 630 times the
  Suns radius (Betelgeuse)
• Giants radii of 10 100 solar radius
• (Mira is Red Giant)
• Supergiants up to 1000 solar radii

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• Main Sequence Stars confined to well defined
  band from top left (high T, high L), to bottom
  right (low T, low L).
• Temperature range over main sequence 3,000K (M
  type) 30,000 K (O type) 1 decade in
  temperature
• Range in luminosities over 8 decades!
• - partly explained by black body relation
• At top end stars are hot and large blue
  supergiants
• At bottom end stars are cool and small red
  dwarfs
• O and B stars extremely rare one in 10,000
• Stars spend most of their life on main-sequence
  burning hydrogen

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• Off the main sequence
• Red giants (upper right of H-R diagram high L,
  low T) and white dwarfs (lower left low L,
  high T).
• Red giants burn hydrogen in a shell
• White dwarfs hard to detect very faint
• Sun will go through red-giant phase and end up as
  a cooling white dwarf.
• Red giant will swell to orbit beyond Earth
  consequences for life!

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Main sequence is a mass sequence ie stellar mass
determines stellar properties
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Structure of Red Giant star furious hydrogen
burning occurs in a shell gradually moving out
through unburned material. Non-burning He ash
accumulates in core.
After 10 billion years, core of solar mass star
uses up H, and consists of He. Fusion ceases at
centre of core, and it begins to contract. Star
leaves main sequence.
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Red Giant Branch

• Subgiant Branch Stage 7 Stage 8
• H burns in a shell, He ash accumulates in core.
• Red Giant Branch Stage 8 stage 9
• - Outer layers of star so cool that convection
  throughout star occurs so ascend a vertical
  track

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• Tip of Giant Branch
• Radius is 100 solar radii (size of Mercurys
  orbit)
• He core is 1/1000 size of star - few times
  larger than the Earth. 25 of stellar mass locked
  up in core
• 10,000 times the luminosity of the Sun.
• Core density, about 100 million kg/ cubic metre.
• Envelope density, about 1/1000 kg/ cubic metre

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Helium fusion the Triple-Alpha Process
fine tuning!

• At stage 9 tip of Giant Branch central
  temperatures are
• 100 million K, at densities of
  kg/cubic metre, conditions allow ignition of
  helium ash accumulating in stellar core
• Beryllium 8 highly unstable. Decays very
  quickly into 2 alpha particles again - about
  ! SLIGHT CHANGE IN STRENGTH OF
  NUCLEAR FORCE AND THIS REACTION IS IMPOSSIBLE!
• Resonant interaction between Be and alpha
  particle allow second reaction above to occur -
  carbon is the ash

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Horizontal Branch Helium Main-Sequence

• Helium Flash Explosive onset of He burning at
  tip of Red Giant Branch (RGB). (stage 9)
• He burning core (stage 10) known as Horizontal
  Branch.

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Ascending the Asymptotic Giant Branch the
Accumulation of Carbon

• When He in core of star on Horizontal Branch is
  used up He shell burn commences star moves
  off Horizontal Branch.
• Now have 2 burning shells, H, and deeper in, He
  with Carbon ash accumulating in core
• Star moves up asymptotic giant branch
  increasing in size and luminosity. Carbon core
  continues to contract

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• Horizontal Branch (stage 10) He core burn
  and H shell burn. The Main-Sequence for He
  burning.
• Asymptotic Giant Branch (AGB) stage 10 stage
  11 Shell burning for both He and H. Carbon
  ash accumulates in core.
• Produces much larger red star Red Supergiant
  500 solar radii swallows Mars!, surface
  temperature 4000 K, central T 250 million K.

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5 billion years into the future the fate of the
Sun

• Planetary nebula NGC 3132.
• - ejection of envelope of star leaving a
  degenerate stellar core (white dwarf).
• White dwarf
• Outer edge of envelope

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Evolution of Massive Stars

• Stars more massive than 8 solar masses lead to
  supernova explosions
• High mass stars move almost horizontally (rather
  than vertically) in post main-sequence evolution
• - luminosity of star stays fairly constant but
  radius increases,reducing surface temperature
• High mass stars fuse carbon, oxygen, and other
  elements

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• For massive stars (more than 8 solar masses)
  series of burning shells ash of burn above it
  igniting producing ash beneath it.
• Creates an onion-like series of burning
  layers. at bottom of which is iron ash.
• Carbon burns for 1000 yr, oxygen for a yr,
  silicon for a week.
• Iron core grows for less than a day!

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Iron is natures most stable element

• Small nuclei liberate energy by fusion
• Elements more massive than iron liberate energy
  by fission into smaller nuclei.
• IRON DOES NOT BURN!
• Degenerate iron core is end-state of nuclear
  fusion in interior.

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Carbon burning (a) occurs at T 600 million K,
while (b) occurs at 200 million K
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The route to iron
Oxygen Fusion
Silicon Burning Building up to Nickel (T 3
billion K !)
Nickel-56 quickly decays via cobalt-56 into
stable iron-56
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Synthesizing Elements Beyond Iron

• Occurs by neutron capture to iron (which just
  changes the isotope), followed by radioactive
  decay into stable element eg.
• Neutron capture occurs during supernova explosion
  (high density and temperature) either by
  rapid (r) or slow (s) process
• Elements produced during explosion much rarer
  because time available to produce them is so
  short

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Nucleosynthesis in stars explains abundances of
the elements

• Sharp drop in abundance as go to higher atomic
  number reflects increasing Coulomb barrier to
  fusion
• Peaks and troughs in distribution reflect
  stable closed shell nuclei, etc.

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Supernova remnant the Crab nebula (supernova
seen by Chinese astronomers in 1054 A.D.)
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Canadian Galactic Plane Survey (CGPS) the
interstellar medium stirred by supernova
explosions and stellar winds.Map of atomic
hydrogen. Midplane of Milky Way - near
constellation Perseus