Nucleosynthesis and stellar lifecycles
Outline:
1. What nucleosynthesis is, and where it occurs
Stellar lifecycles
2. Molecular clouds
3. YSO & protoplanetary disk phase
4. Main Sequence phase
5. Old age & death of low mass stars
6. Old age & death of high mass stars
7. Nucleosynthesis & pre-solar grains
What nucleosynthesis is,
and where it occurs
Nucleosynthesis
Except for H, He
(created in Big Bang),
all other elements created
by fusion processes in
stars
Relative abundance
formation of elements
Stellar
Nucleosynthesis
Some H destroyed;
all elements with
Z > 2 produced
Various processes,
depend on
(1) star mass
(determines T)
(2) age (determines
starting composition)
Z = no. protons, determines element
p>
Beta Stability Valley.
Nucleons with right
mix of neutrons (n) to
protons (p) are stable.
Those that lie outside
of this mix are radioactive.
n>
p>
Beta Stability Valley.
too
many n
n>
Too many n:
beta particle (electron)
emitted, n converted
to p. (Beta Decay)
e.g. 26Al -> 26Mg + beta
e.g. 53Mn -> 53Cr + beta
Some stellar
nucleosynthesis
resulted in
n-rich nucleons
that are short-lived
nuclides.
Beta Stability Valley.
too
many p
p>
Too many p:
electron captured by
nucleus, p converted
to n.
n>
e.g.,
41Ca + electron -> 41K
Other stellar
nucleosynthesis
produced short-lived
p-rich nucleons.
Stellar lifecycles: from birth to death
low mass
star (< 5 Msun)
high mass
star (> 5 Msun)
Stellar lifecycles: low mass stars
3. Red Giant
2. Main Seq.
low mass
star (< 5 Msun)
4. Planetary nebula
4. White dwarf
Nucleosynthesis possible
if white dwarf in binary system
(during nova or supernova)
Stellar nucleosynthesis
1 & 5.
molecular
cloud
Stellar lifecycles: high mass stars
Stellar nucleosynthesis
2. Main Seq.
(luminous)
1 & 6.
molecular
cloud
3. Red Giant/
Supergiant
high mass
star (>5 Msun)
5. Neutron star
4. Supernova
5. Black hole
Track stellar evolution on H-R diagram of T vs luminosity
Luminosity: energy / time
Distribution of
stars on
H-R diagram.
When corrected for
intrinsic brightness,
there are MANY more
cool Main Sequence
stars than hot.
On main sequence, luminosity depends on mass
L ~ M3.5
Molecular clouds:
Where it begins & ends
molecular
cloud
Molecular clouds
cold, dense areas in
interstellar medium (ISM)
Horsehead Nebula
Mainly molecular H2,
also dust, T ~ 10s of K
Famous Eagle
Nebula image.
Cool dark clouds
are close to hot
stars that are
causing them to
evaporate.
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 larger
Interplanetary Dust
Particle (IDP)
2
atoms
3
atoms
4
atoms
5
atoms
6
atoms
7
atoms
2
atoms
3
atoms
4
atoms
H2
C3 *
c-C3H
C5 *
C5 H
C6 H
PN
NaCN
AlF
C2 H
l-C3H
C4 H
l-H2C4
CH2CHCN
SO
OCS
AlCl
C2 O
C3 N
C4Si
C2H4*
CH3C2H
SO+
SO2
C2**
C2 S
C3 O
l-C3H2
CH3CN
HC5N
SiN
c-SiC2
CH
CH2
C3 S
c-C3H2
CH3NC
CH3CHO
SiO
CO2*
CH+
HCN
C2H2*
CH2CN
CH3OH
CH3NH2
SiS
NH2
CN
HCO
NH3
CH4*
CH3SH
c-C2H4O
CS
H3+*
CO
HCO+
HCCN
HC3N
HC3NH+
H2CCHOH
HF
H2D+, HD2+
SH*
SiCN
HD
AlNC
FeO?
SiNC
CO+
HCS+
HCNH+
HC2NC
HC2CHO
CP
HOC+
HNCO
HCOOH
NH2CHO
SiC
H2O
HNCS
H2CNH
C5 N
HCl
H2S
HOCO+
H2C2O
l-HC4H* (?)
KCl
HNC
H2CO
H2NCN
l-HC4N
NH
HNO
H2CN
NO
MgCN
NS
7
atoms
Note many
C-compounds
O2 ?
9
atoms
HNC3
CH3C3N
CH3C4H
H2CS
SiH4*
HCOOCH3
CH3CH2CN
MgNC
H3O+
H2COH+
CH3COOH
(CH3)2O
NaCl
N2H+
c-SiC3
OH
N2O
CH3*
http://www.ph1.uni-koeln.de/vorhersagen/molecules/main_molecules.html
6
atoms
Molecules in
ISM as of
12 / 2004
8
atoms
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.
5
atoms
C7 H
H2C6
CH3CH2OH
HC7N
CH2OHCHO
C8 H
l-HC6H* (?)
CH2CHCHO (?)
10
atoms
11
atoms
CH3C5N (?)
HC9N
(CH3)2CO
(CH2OH)2 (?)
H2NCH2COOH
Glycine ?
CH3CH2CHO
12
atoms
C6H6* (?)
13
atoms
HC11N
Photochemistry can occur in icy mantles to create
complex hydrocarbons from simple molecules
Gravity in molecular
clouds helps promote
collapse of cloud
…and sometimes is
assisted 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 disk
out of which our solar
system formed
Herbig-Haro
Objects-• YSOs with
disks & bipolar
outflows
Magnetic fields around
YSOs can create polar
jets and X winds
Collapse of molecular cloud fragments occurs rapidly
~105 to 107 yrs,
depending on mass
Protostellar disk
phase lasts ~106 yrs
Single collapsing molecular cloud produces many
fragments, 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
13C
+ p -> 14N
14N
+ p -> 15O
15N
+ p -> 12C + 4He
13N
-> 13C
15O
-> 15N
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
Red Giant
Planetary nebula
White dwarf
There are different types of Red Giant Stars
1) RGB (Red Giant Branch)
2) Horizontal branch
3) AGB (Asymptotic Giant Branch)
These differ in position on H-R diagram and in
interior structure
Red Giant (RGB) star: H burning in shell
Red Giant (Horizontal branch) star: He fusion in core
Red Giant (AGB) star: He burning in shell
AGB star
Convective dredge-ups bring products
of fusion to surface
Red Giant includes: s-process nucleosynthesis
slow neutron
addition
beta decay
keeps pace
with n addition
No. protons (Z)
s-process
nucleosynthesis:
An AGB can lose its outer layers—
Ultimately a planetary nebula forms,
leaving a white dwarf in the center
Planetary nebula
White dwarf
Planetary nebulas
Note: planetary nebula have nothing to do with planets!
Nuclear
fusion
stops when
the star
becomes
a white
dwarf—
It gradually
cools down
Old age & death of high mass stars
Super Giant
Neutron star
Supernova
Black hole
High-mass stars:
Progressive core fusion
of elements heavier than C
Includes: s-process nucleosynthesis as Supergiant,
r-process nucleosynthesis during core collapse
rapid neutron addition
beta decay does not
keep pace with
n addition
No. protons (Z)
r-process
nucleosynthesis:
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
--Fe core turns into dense neutrons
--Supernova forms because overlying star falls
onto dense core & bounces off of it
Supernova remnants
Crab Nebula
supernova
remnant.
A spinning
neutron star
(pulsar) occurs
in the central
region.
There are different types of Supernovae
1)
2)
3)
4)
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)
Type 2 supernovae had intact upper layers
Type 1b & c supernovae had lost upper layers
Type 1a supernovae occur in binary systems
when material from companion falls onto white
dwarf
Nucleosynthesis &
pre-solar grains
Summary of nucleosynthesis processes
process
main
products
comment
H-burning
4He
main seq.
He-burning
12C, 16O
Red Giant
C-O-Ne-Si
burning
20Ne, 28Si, 32Si,
Supergiants
s-process
many elements
Red Giants,
Supergiants
r-process
many heavy
elements
supernova
up to 56Fe
Pre-solar material in meteorites
material
suggested astrophysical site
Ne-E
S-Xe
Xe-HL
Macromolecular C
exploding nova
Red Giant or Supergiant
supernovae
low-T ISM
SiC
Corundum
Nanodiamond
Graphite, Si3N4
C-rich AGB stars, supernovae
Red Giant & AGB stars
supernovae
supernovae
Solar system formed out of diverse materials.