Nucleosynthesis and stellar lifecycles

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Nucleosynthesis and stellar lifecycles
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• Outline
• What nucleosynthesis is, and where it occurs
• Molecular clouds
• YSO protoplanetary disk phase
• Main Sequence phase
• Old age death of low mass stars
• Old age death of high mass stars
• Nucleosynthesis pre-solar grains

Stellar lifecycles
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What nucleosynthesis is, and where it occurs
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Nucleosynthesis formation of elements Except
for H, He (created in Big Bang), all other
elements created by fusion processes in stars
Relative abundance
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Stellar Nucleosynthesis Some H destroyed all
elements with Z gt 2 produced Various
processes, depend on (1) star mass (determines
T) (2) age (determines starting composition)
Z no. protons, determines element
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Beta Stability Valley. Nucleons with right mix
of neutrons (n) to protons (p) are stable. Those
that lie outside of this mix are radioactive.
p gt
n gt
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Beta Stability Valley. Too many n beta
particle (electron) emitted, n converted to p.
(Beta Decay) e.g. 26Al -gt 26Mg beta e.g. 53Mn
-gt 53Cr beta Some stellar nucleosynthesis resul
ted in n-rich nucleons that are
short-lived nuclides.
too many n
p gt
n gt
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Beta Stability Valley. Too many p electron
captured by nucleus, p converted to
n. e.g., 41Ca electron -gt 41K Other
stellar nucleosynthesis produced
short-lived p-rich nucleons.
too many p
p gt
n gt
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Stellar lifecycles from birth to death
low mass star (lt 5 Msun)
high mass star (gt 5 Msun)
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Stellar lifecycles low mass stars
Stellar nucleosynthesis
2. Main Seq.
3. Red Giant
low mass star (lt 5 Msun)
1 5. molecular cloud
4. Planetary nebula
4. White dwarf
Nucleosynthesis possible if white dwarf in binary
system (during nova or supernova)
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Stellar lifecycles high mass stars
Stellar nucleosynthesis
3. Red Giant/ Supergiant
2. Main Seq. (luminous)
1 6. molecular cloud
high mass star (gt5 Msun)
5. Neutron star
4. Supernova
5. Black hole
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Track stellar evolution on H-R diagram of T vs
luminosity Luminosity energy / time
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Distribution of stars on H-R diagram. When
corrected for intrinsic brightness, there are
MANY more cool Main Sequence stars than hot.
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On main sequence, luminosity depends on mass
L M3.5
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Molecular clouds Where it begins ends
molecular cloud
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Molecular clouds cold, dense areas
in interstellar medium (ISM)
Horsehead Nebula
Mainly molecular H2, also dust, T 10s of K
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Famous Eagle Nebula image. Cool dark clouds are
close to hot stars that are causing them
to evaporate.
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Dust in ISM consists of -- ices, organic
molecules, silicates, metal, graphite, etc. --
some of these preserved as pre-solar grains
organic components in meteorites
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A larger Interplanetary Dust Particle (IDP)
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2 atoms 3 atoms 4 atoms 5 atoms 6 atoms 7 atoms
2 atoms 3 atoms 4 atoms 5 atoms 6 atoms 7 atoms
PN NaCN
H2 C3 c-C3H C5 C5H C6H
AlF C2H l-C3H C4H l-H2C4 CH2CHCN
AlCl C2O C3N C4Si C2H4 CH3C2H
C2 C2S C3O l-C3H2 CH3CN HC5N
CH CH2 C3S c-C3H2 CH3NC CH3CHO
CH HCN C2H2 CH2CN CH3OH CH3NH2
CN HCO NH3 CH4 CH3SH c-C2H4O
CO HCO HCCN HC3N HC3NH H2CCHOH
SO OCS
SO SO2
Molecules in ISM as of 12 / 2004 Note
many C-compounds
SiN c-SiC2
SiO CO2
SiS NH2
CS H3
HF H2D, HD2
SH SiCN
CO HCS HCNH HC2NC HC2CHO
CP HOC HNCO HCOOH NH2CHO
SiC H2O HNCS H2CNH C5N
HCl H2S HOCO H2C2O l-HC4H (?)
KCl HNC H2CO H2NCN l-HC4N
HD AlNC
FeO? SiNC
O2 ?
8 atoms 9 atoms 10 atoms 11 atoms 12 atoms 13 atoms
NH HNO H2CN HNC3
NO MgCN H2CS SiH4
NS MgNC H3O H2COH
CH3C3N CH3C4H CH3C5N (?) HC9N C6H6 (?) HC11N
HCOOCH3 CH3CH2CN (CH3)2CO
CH3COOH (CH3)2O (CH2OH)2 (?)
C7H CH3CH2OH H2NCH2COOHGlycine ?
NaCl N2H c-SiC3
OH N2O CH3
H2C6 HC7N CH3CH2CHO
CH2OHCHO C8H
All molecules have been detected (also) by
rotational spectroscopy in the radiofrequency to
far-infrared regions unless indicated otherwise.
indicates molecules that have been detected by
their rotation-vibration spectrum, those
detected by electronic spectroscopy only.
l-HC6H (?)
CH2CHCHO (?)
http//www.ph1.uni-koeln.de/vorhersagen/molecules/
main_molecules.html
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Photochemistry can occur in icy mantles to
create complex hydrocarbons from simple molecules
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Gravity in molecular clouds helps
promote collapse of cloud and sometimes
is assisted by a trigger
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Young stellar objects (YSOs) protoplanetary
disks (proplyds)
YSOs
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YSOs Proplyds Molecular cloud fragments that
have collapsed no fusion yet
lt Protoplanetary disk around glowing YSO
in Orion
Solar nebula the Protoplanetary disk out of
which our solar system formed
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• Herbig-Haro
• Objects--
• YSOs with
• disks bipolar
• outflows

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Magnetic fields around YSOs can create polar jets
and X winds
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Collapse of molecular cloud fragments occurs
rapidly
105 to 107 yrs, depending on mass Protostellar
disk phase lasts 106 yrs
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Single collapsing molecular cloud produces
many fragments, each of which can produce a star
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Main Sequence phase Middle age
Main sequence
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Star turns on when nuclear fusion occurs main
sequence star either proton-proton chain or CNO
cycle nucleosynthesis
P-P chain net 4 H to 1 He
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CNO cycle more efficient method, but requires
higher internal temperature, so only for stars
with mass higher than 1.1 solar masses 12C p
-gt 13N 13N -gt 13C 13C p -gt 14N 14N
p -gt 15O 15O -gt 15N 15N p -gt 12C
4He CNO cycle net reaction 4 H to 1 He
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Star stays on main sequence in stable condition
so long as H remains in the core
A more massive star must produce more energy to
support its own weight reason there is a
correlation of mass and luminosity on main
sequence
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But eventually the H runs out
Lifetime on main sequence fuel / rate of
consumption M / L M / M3.5 lifetime
1/M2.5 So a 4 solar mass star will have a main
sequence lifetime 1/32 as long as our sun
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• So, what happens when the core runs out of
  hydrogen?
• Star begins to collapse, heats up
• Core contains He, continues to collapse
• But H fuses to He in shell greatly inflating
  star
• ? RED GIANT (low mass)
• or SUPERGIANT (high mass)

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What happens next depends on stellar mass
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Old age and death of low mass stars
Red Giant
Planetary nebula
White dwarf
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• There are different types of Red Giant Stars
• RGB (Red Giant Branch)
• Horizontal branch
• AGB (Asymptotic Giant Branch)
• These differ in position on H-R diagram and in
• interior structure

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Red Giant (RGB) star H burning in shell
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Red Giant (Horizontal branch) star He fusion in
core Red Giant (AGB) star He burning in shell
AGB star
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Convective dredge-ups bring products of fusion to
surface
Red Giant includes s-process nucleosynthesis
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s-process nucleosynthesis slow
neutron addition beta decay keeps pace with n
addition
No. protons (Z)
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An AGB can lose its outer layers Ultimately a
planetary nebula forms, leaving a white dwarf in
the center
Planetary nebula
White dwarf
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Planetary nebulas
Note planetary nebula have nothing to do with
planets!
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Nuclear fusion stops when the star becomes a
white dwarf It gradually cools down
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Old age death of high mass stars
Super Giant
Neutron star
Supernova
Black hole
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High-mass stars Progressive core fusion of
elements heavier than C
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Includes s-process nucleosynthesis as
Supergiant, r-process nucleosynthesis during
core collapse
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No. protons (Z)
r-process nucleosynthesis rapid neutron
addition beta decay does not keep pace with n
addition
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End for high mass star comes as it tries to fuse
core Fe into heavier elements and finds this
absorbs energy STAR COLLAPSES EXPLODES AS
SUPERNOVA
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--Fe core turns into dense neutrons --Supernova
forms because overlying star falls onto
dense core bounces off of it
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Supernova remnants
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Crab Nebula supernova remnant. A
spinning neutron star (pulsar) occurs in the
central region.
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• There are different types of Supernovae
• Type 2 (kept upper H-rich portion)
• Type 1b (lost H, but kept He-rich portions)
• Type 1c (lost both H He portions)
• Type 1a (explosion on white dwarf in binary
  system)

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Type 2 supernovae had intact upper layers
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Type 1b c supernovae had lost upper layers
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Type 1a supernovae occur in binary systems when
material from companion falls onto white dwarf
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Nucleosynthesis pre-solar grains
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Summary of nucleosynthesis processes
process main comment products H-burning 4H
e main seq. He-burning 12C, 16O Red
Giant C-O-Ne-Si 20Ne, 28Si, 32Si, Supergiants bu
rning up to 56Fe s-process many elements Red
Giants, Supergiants r-process many
heavy supernova elements
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Pre-solar material in meteorites
material suggested astrophysical
site Ne-E exploding nova S-Xe Red Giant
or Supergiant Xe-HL supernovae Macromolecular
C low-T ISM SiC C-rich AGB stars,
supernovae Corundum Red Giant AGB
stars Nanodiamond supernovae Graphite, Si3N4
supernovae
Solar system formed out of diverse materials.
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