Nucleosynthesis and stellar lifecycles. Outline: 1.What nucleosynthesis is, and where it occurs...
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Transcript of Nucleosynthesis and stellar lifecycles. Outline: 1.What nucleosynthesis is, and where it occurs...
Nucleosynthesis and stellar lifecycles
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
What nucleosynthesis is,and where it occurs
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
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
Beta Stability Valley.
Nucleons with rightmix of neutrons (n) toprotons (p) are stable.
Those that lie outsideof this mix are radioactive.
n >
p >
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
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
Stellar lifecycles: from birth to death
low massstar (< 5 Msun)
high massstar (> 5 Msun)
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)
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
Track stellar evolution on H-R diagram of T vs luminosity
Luminosity: energy / time
Distribution ofstars onH-R diagram.
When corrected forintrinsic brightness,there are MANY morecool Main Sequencestars than hot.
On main sequence, luminosity depends on mass
L ~ M3.5
Molecular clouds:
Where it begins & ends
molecularcloud
Molecular cloudscold, dense areas ininterstellar medium (ISM)
Horsehead Nebula
Mainly molecular H2,also dust, T ~ 10s of K
Famous EagleNebula image.
Cool dark cloudsare close to hotstars that arecausing them toevaporate.
Dust in ISM consists of:
-- ices, organic molecules, silicates, metal, graphite, etc.-- some of these preserved as pre-solar grains & organic components in meteorites
A largerInterplanetary DustParticle (IDP)
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+
Photochemistry can occur in icy mantles to createcomplex hydrocarbons from simple molecules
Gravity in molecularclouds helps promotecollapse of cloud
…and sometimes isassisted by a trigger
Young stellar objects (YSOs)& protoplanetary disks (proplyds)
YSOs
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
Herbig-HaroObjects--
• YSOs withdisks & bipolaroutflows
Magnetic fields aroundYSOs can create polarjets and X winds
Collapse of molecular cloud fragments occurs rapidly
~105 to 107 yrs,depending on mass
Protostellar diskphase lasts ~106 yrs
Single collapsing molecular cloud produces manyfragments, each of which can produce a star
Main Sequence phase:Middle age
Main sequence
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
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
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
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
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)
What happens next depends on stellar mass
Old age and death of low mass stars
Planetary nebula
White dwarf
Red Giant
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
Red Giant (RGB) star: H burning in shell
Red Giant (Horizontal branch) star: He fusion in coreRed Giant (AGB) star: He burning in shell
AGB star
Convective dredge-ups bring productsof fusion to surface
Red Giant includes: s-process nucleosynthesis
s-processnucleosynthesis:
slow neutronaddition
beta decaykeeps pacewith n addition
No.
pro
tons
(Z
)
An AGB can lose its outer layers—Ultimately a planetary nebula forms,leaving a white dwarf in the center
Planetary nebula
White dwarf
Note: planetary nebula have nothing to do with planets!
Planetary nebulas
Nuclear fusionstops whenthe star becomesa white dwarf—
It gradually cools down
Old age & death of high mass stars
SupernovaBlack hole
Super Giant
Neutron star
High-mass stars: Progressive core fusionof elements heavier than C
Includes: s-process nucleosynthesis as Supergiant,r-process nucleosynthesis during core collapse
r-processnucleosynthesis:
rapid neutron addition
beta decay does notkeep pace withn addition
No.
pro
tons
(Z
)
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
--Fe core turns into dense neutrons--Supernova forms because overlying star falls onto dense core & bounces off of it
Supernova remnants
Crab Nebulasupernovaremnant.
A spinningneutron star(pulsar) occursin the centralregion.
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)
Type 2 supernovae had intact upper layers
Type 1b & c supernovae had lost upper layers
Type 1a supernovae occur in binary systemswhen material from companion falls onto whitedwarf
Nucleosynthesis &pre-solar grains
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
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.