nucleosynthesis and stellar lifecycles. outline: 1.what nucleosynthesis is, and where it occurs...
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Nucleosynthesis and stellar lifecycles
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Outline:
2. What nucleosynthesis is, and where it occurs
3. Molecular clouds
4. YSO & protoplanetary disk phase
5. Main Sequence phase
6. Old age & death of low mass stars
7. Old age & death of high mass stars
8. Nucleosynthesis & pre-solar grains
Ste
llar
lifec
ycle
s
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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 createdby fusion processes instars
Re
lati
ve
ab
un
da
nce
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StellarNucleosynthesis
Some H destroyed;all elements withZ > 2 produced
Various processes,depend on
(1) star mass (determines T)
(2) age (determinesstarting composition)
Z = no. protons, determines element
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Beta Stability Valley.
Nucleons with rightmix of neutrons (n) toprotons (p) are stable.
Those that lie outsideof this mix are radioactive.
n >
p >
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Beta Stability Valley.
Too many n:beta particle (electron)emitted, n convertedto p. (Beta Decay)
e.g. 26Al -> 26Mg + betae.g. 53Mn -> 53Cr + beta
Some stellarnucleosynthesisresulted inn-rich nucleonsthat are short-livednuclides.
n >
p > too
many n
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Beta Stability Valley.
Too many p:electron captured bynucleus, p convertedto n.
e.g.,41Ca + electron -> 41K
Other stellarnucleosynthesisproduced short-livedp-rich nucleons.
n >
p >
toomany p
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Stellar lifecycles: from birth to death
low massstar (< 5 Msun)
high massstar (> 5 Msun)
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Stellar lifecycles: low mass stars
1 & 5.molecularcloud
low massstar (< 5 Msun)
3. Red Giant2. Main Seq.
4. Planetary nebula
4. White dwarf
Stellar nucleosynthesis
Nucleosynthesis possibleif white dwarf in binary system(during nova or supernova)
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Stellar lifecycles: high mass stars
1 & 6.molecularcloud
high massstar (>5 Msun)
3. Red Giant/ Supergiant
2. Main Seq.(luminous)
4. Supernova
5. Black hole
5. Neutron star
Stellar nucleosynthesis
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Track stellar evolution on H-R diagram of T vs luminosity
Luminosity: energy / time
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Distribution ofstars onH-R diagram.
When corrected forintrinsic brightness,there are MANY morecool Main Sequencestars 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
molecularcloud
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Molecular cloudscold, dense areas ininterstellar medium (ISM)
Horsehead Nebula
Mainly molecular H2,also dust, T ~ 10s of K
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Famous EagleNebula image.
Cool dark cloudsare close to hotstars that arecausing them toevaporate.
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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 largerInterplanetary DustParticle (IDP)
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2 atoms
3 atoms
4 atoms
5 atoms
6 atoms
7 atoms
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+ H2CCHOHCO+ 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-HC4NNH HNO H2CN HNC3
NO MgCN H2CS SiH4*
NS MgNC H3O+ H2COH+
NaCl N2H+ c-SiC3
OH N2O CH3*
2 atoms
3 atoms
4 atoms
5 atoms
6 atoms
7 atoms
PN NaCN
SO OCS
SO+ SO2
SiN c-SiC2
SiO CO2*
SiS NH2
CS H3+*
SH* SiCN
HD AlNC
FeO? SiNC
O2 ?
8 atoms
9 atoms
10 atoms 11 atoms12 atoms
13 atoms
CH3C3N CH3C4H CH3C5N (?) HC9N C6H6* (?) HC11N
HCOOCH3 CH3CH2CN (CH3)2CO
CH3COOH (CH3)2O (CH2OH)2 (?)
C7H CH3CH2OHH2NCH2COOHGlycine ?
H2C6 HC7N CH3CH2CHO
CH2OHCHO C8H
l-HC6H* (?)
CH2CHCHO (?)
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.
http://www.ph1.uni-koeln.de/vorhersagen/molecules/main_molecules.html
Molecules inISM as of12 / 2004
Note manyC-compounds
HF H2D+, HD2+
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Photochemistry can occur in icy mantles to createcomplex hydrocarbons from simple molecules
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Gravity in molecularclouds helps promotecollapse of cloud
…and sometimes isassisted 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
< Protoplanetary disk around glowing YSO in Orion
Solar nebula:the Protoplanetary diskout of which our solar
system formed
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Herbig-HaroObjects--
• YSOs withdisks & bipolaroutflows
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Magnetic fields aroundYSOs can create polarjets and X winds
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Collapse of molecular cloud fragments occurs rapidly
~105 to 107 yrs,depending on mass
Protostellar diskphase lasts ~106 yrs
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Single collapsing molecular cloud produces manyfragments, 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 -> 13N 13N -> 13C
13C + p -> 14N
14N + p -> 15O 15O -> 15N
15N + p -> 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
Planetary nebula
White dwarf
Red Giant
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There are different types of Red Giant Stars
1) RGB (Red Giant Branch)2) Horizontal branch3) AGB (Asymptotic Giant Branch)
These differ in position on H-R diagram and ininterior 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 coreRed Giant (AGB) star: He burning in shell
AGB star
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Convective dredge-ups bring productsof fusion to surface
Red Giant includes: s-process nucleosynthesis
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s-processnucleosynthesis:
slow neutronaddition
beta decaykeeps pacewith n addition
No.
pro
tons
(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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Note: planetary nebula have nothing to do with planets!
Planetary nebulas
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Nuclear fusionstops whenthe star becomesa white dwarf—
It gradually cools down
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Old age & death of high mass stars
SupernovaBlack hole
Super Giant
Neutron star
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High-mass stars: Progressive core fusionof elements heavier than C
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Includes: s-process nucleosynthesis as Supergiant,r-process nucleosynthesis during core collapse
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r-processnucleosynthesis:
rapid neutron addition
beta decay does notkeep pace withn addition
No.
pro
tons
(Z
)
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End for high mass star comes as it tries to fuse core Fe into heavier elements– andfinds 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 Nebulasupernovaremnant.
A spinningneutron star(pulsar) occursin the centralregion.
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There are different types of Supernovae
1) Type 2 (kept upper H-rich portion)2) Type 1b (lost H, but kept He-rich portions)3) Type 1c (lost both H & He portions)4) 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 systemswhen material from companion falls onto whitedwarf
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Nucleosynthesis &pre-solar grains
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process main commentproducts
H-burning 4He main seq.
He-burning 12C, 16O Red Giant
C-O-Ne-Si 20Ne, 28Si, 32Si, Supergiantsburning up to 56Fe
s-process many elements Red Giants, Supergiants
r-process many heavy supernovaelements
Summary of nucleosynthesis processes
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material suggested astrophysical site
Ne-E exploding novaS-Xe Red Giant or SupergiantXe-HL supernovaeMacromolecular C low-T ISM
SiC C-rich AGB stars, supernovaeCorundum Red Giant & AGB starsNanodiamond supernovaeGraphite, Si3N4 supernovae
Pre-solar material in meteorites
Solar system formed out of diverse materials.
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