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Lecture on the Origins of the Solar Systems
Origin of the elements and
Standard Abundance Distribution
Clementina Sasso
Lotfi Yelles Chaouche
Nucleosynthesis:
Study of the nuclear processes responsible for the formation of the
elements which constitute the baryonic matter of the Universe
Contemporary nucleosynthesis theory associates the production of certain
elements/isotopes or group of elements with:
The cosmological Big Bang
Stars
Supernovae
Cosmological Big Bang
1
H, 2 H, 3 He,4 He,7 Li
There are naturally occurring elements as heavy as Uranium.
Some elements (e.g., Carbon, Nitrogen, Oxygen) are rather plentiful (1
atom in every 105 atoms).
WHERE DO THE OTHER ELEMENTS COME FROM?
Nucleosynthesis theory predicts that these elements
were formed in the cores of stars
BB
1,2H 3,4He 7Li
Intergalactic medium
Galaxy formation
Interstellar medium
Star
formation
Mass
loss
Stars
White
Dwarfs
Neutron
stars
SN
Black
holes
Sites for nucleosynthesis
• Intermediate mass stars (2< M/M . <10)
• Massive stars and associated type II supernovae
(M/M . >10)
• Exploding CO white dwarfs in binary stellar
sistems (type Ia supernovae)
Overview of nucleosynthesis mechanisms:
(depending on the stellar mass)
•
•
•
•
•
H-burning
He-burning
C-burning
O-burning
Si-burning
• ENS (Explosive NucleoSynthesis)
• Neutron capture
s-process
r-process
• p-process
Fuel
H
H
He
C
O
Si
Process
Products
p-p
He
CNO
He
3α
C, O
C+C
O, Ne, Na, Mg
O+O
Mg, S, Si, P
Nuc. eq.
Co, Fe, Ni
Tthreshold (10
~4
15
100
600
1000
3000
6
K)
H-burning
T  107 K
*
*
**
**
H-burning
The total abundance of C, N, O (and F) nuclei is constant in time
H-R Diagram
• Stars on the Main Sequence derive
essentially all their energy from the
conversion of H to He by nuclear fusion
in their core.
• In the course of this long phase the stellar
configuration achieve both hydrostatic and
thermal equilibrium.
Evolution to Red Giant
• Once the star uses up all the H in its convective core, nuclear fusion
ceases, convection is quenched.
The star is no longer in hydrostatic equilibrium.
– Gravity wins out over pressure, and the core
begins to collapse and heats up.
– As the core shrinks, the energy of the inward
falling material is converted to heat.
– Just outside the core, the hydrogen was
almost, but not quite, hot enough to undergo
fusion.
– The added energy from the collapse of the
core heats the hydrogen surrounding the core to the point that it can
undergo fusion. A shell of hydrogen begins to fuse to helium just outside the
collapsing core.
Evolution to Red Giant
• The energy produced in this burning shell flows to
the outer regions of the star, causing them to expand.
– As the surface expands, it cools down
and becomes redder in color.
– The luminosity increases.
– On the H-R diagram, the star leaves the
main sequence and moves to the upper
right, becoming a red giant.
• The envelope becomes convectively
unstable and H burning ashes move to the
surface (dredge-up).
After this point, the evolution depends
strongly on the mass of the star.
1) INTERMEDIATE MASS STARS
2) MASSIVE STARS
Evolution of Intermediate Mass Stars
• Once the helium core reaches a T=108 K, the 3α reaction can take place:
+4He  8Be
4He + 8Be  12C
4He
Three helium atoms can fuse to
form one carbon atom releasing
energy in the process.
Evolution of Intermediate Mass Stars
A carbon atom reacts with a helium atom to produce
oxygen:
12C+ 4He
 16O
16O
4He
Evolution of Intermediate Mass Stars
He burning occurs in a convective core (inner part of the larger He core).
The composition of the inner core is constantly mixed and turns gradually from He
to C and O.
When He is depleted, convection is quenched.
• Once the star has converted all its He to C, it can no longer maintain
hydrostatic equilibrium.
– Gravity begins to win, the core contracts
again.
• Heat released by the contracting core flows
into a shell of He just outside the core. The
heated He shell begins to fuse into C.
• Outside this He burning shell is a He shell,
and then a H burning shell.
Evolution of Intermediate Mass Stars
• The C-O core heats up.
• The envelope expands and cools, and convection sets in again
throughout it.
The convective envelope overlaps the boundary of the new
extinguished H burning shell and the processed material (Ni and
He), is once more dredged up and mixed in the envelope.
Evolution of Intermediate Mass Stars
•
•
Nuclear burning takes place in two shells.
The great difference between the two nuclear burning processes do not
allow a steady state to develop. The two shells do not supply energy
concomitantly but in turn and the mass of the He layer changes periodically.
•
Particularly noteworthy is the dredge-up
of processed material into the convective
envelope by the moving inner boundary of
the convective zone.
The lasting result of each cycle is the growth
of the C-O core.
Evolution of Intermediate Mass Stars
• In these stars, the contracting carbon
core will not reach temperatures
sufficient to burn the C into heavier
elements.
• No further nuclear reactions are
possible.
• The inert carbon core that remains
(WD) will simply cool down over
billions of years.
The envelope mass decreases, mainly
because of mass loss at the surface (stellar
wind and superwind).
2) Evolution of Massive Stars
• Once the star exhausts the He in its core, the carbon-oxygen core
begins to contract and heat up.
• The carbon core will become sufficiently hot to ignite the C.
• The carbon core can fuse into oxygen, neon, sodium, magnesium,
silicon, etc.
Carbon burning
T = 5 x 108 K
20Ne
+
23Mg
+n
24Mg
23Na
+p
16O
+ 2
Oxygen burning
T ~ 109 K
32S
+ +
31S
+ n
31P
+ p
16O
16O
28Si
24Mg
+

+ 2α
Silicon burning
T = 3 x 109 K

28Si
7

PHOTODISINTEGRATION: Interaction between massive particles and energetic photons
Evolution of High Mass Stars
• In the late stages of the
life of a massive star…
– Helium converted into
heavier elements (carbon,
oxygen, …, iron)
– The star has an onion-like
structure
What’s special about iron?
Binding energy per nucleon
Elements Heavier than Iron …
• Once iron is formed, it is no longer possible to create
energy via fusion.
 Elements heavier than iron are not created via
nuclear fusion. (Iron is atomic number 26.)
•Elements heavier than iron are created by
neutron capture
•The neutron is added to the nucleus and converted
into a proton, increasing the atomic number to make
the next element in the periodic table.
•Proton capture can occur, but is less probable.
p-process:
Proton capture:
Ouf!
p
s-process:
56Fe
57X
Slow neutron capture:
b
Ouf!
n
56
26
Fe
57
26
Fe
57
27
X
Absorb n0, then … later … emit e- (b-particle).
Repeat. Progress up the valley of stability.
r-process:
Rapid neutron capture:
b
n
n
56
26
Fe
59
26
Fe
60
26
Fe
n
61
27
n
X
….
High n0 flux: absorb many n0s before b
emission.
These processes require energy.
Occur only at high r & T :
- Core & shell burning
- Supernovae
r-process and s-process
Temperature: ~1-2 GK
Density: 300 g/cm3
Neutron
capture
b-decay
Seed
(,n) photodisintegration
Neutron number
Equilibrium favors
“waiting point”
Speed Matters
r-process
departs from
valley of stability
s-process
Structure of an Evolved Massive Star
Evolution of High Mass Stars
• Without the ability to generate energy, the
iron core begins to collapse.
• Initially, the electron degeneracy pressure can
provide support for a short time.
• However, the silicon shell burning is
continually adding more and more iron onto
the core and eventually it will exceed the
Chandrasekhar limit of 1.4 MSun.
Beginning to Collapse
• Pressure and temperature rise as core collapses
• Photodisintegration
– light begins to break apart nuclei
• more energy loss
• Neutrino cooling is occurring
• Electrons and protons combine to make neutrons
– p+en
• Sources of energy to provide
pressure are disappearing
– core continues to collapse to
very dense matter
.
Type II Supernova
• Core collapses
• Degenerate core
– nuclei get so close
together the nuclear
force repels them
• Core bounces
– particles falling
inward sent back
outward
– up to 30,000 km/s
• Type II supernova
One heck of an explosion
• On July 4, 1054
A.D. a supernova
exploded in the
constellation Taurus.
Today, we see the
remnant as the Crab
Nebula.
SN 1987A
SN 1987A
Binary Star Systems
• Bigger star becomes a white dwarf
• Smaller star eventually becomes a
red giant
• Once smaller star fills its Roche
limit, it transfers mass to the white
dwarf
– if both are low mass, two white
dwarfs are formed
– if more mass is present, more
interesting stuff happens…
Type Ia Supernova
• Chandrasekhar limit
– a white dwarf must be less than 1.4
solar masses
• If a white dwarf reaches the
Chandrasekhar limit, it starts
burning carbon
• The whole dwarf burns in seconds!
• More energy released than the
whole 10 billion years on main
sequence!
• Glows very brightly for
weeks/months and fades away
Type Ia supernovae
occur about once a
century in the Milky
Way
Have a luminosity 10
billion times our Sun
Heavy Element Nucleosynthesis
S(Slow)-process tb<tncap
R(Rapid)-process tb>tncap
Tests for these processes ?
• •• Typical
Comparison
abundances
with
Analysis
of Isotopes
are
processes
calculated
in other
for
in meteorites
each
starstype of process,
and compared with
solar system
abundances