- Hydrogen and helium were formed in the early universe after the Big Bang, while all other elements have been synthesized through nuclear fusion processes inside stars. - Stars generate energy through nuclear fusion reactions like the proton-proton chain or CNO cycle, which fuse hydrogen into helium. Further reactions like the triple-alpha process fuse helium into carbon. - As stars evolve and age, heavier elements are produced through successive nuclear fusion processes up to iron, which has the highest binding energy. Elements heavier than iron are produced through s-process and r-process neutron capture. - When stars run out of nuclear fuel and can no longer fuse elements, their cores collapse in supernova explosions, seeding the

1. Nucleosynthesis :An important nuclear
Astrophysics phenomenon:
Stellar Nucleosynthesis
By Tarun P. Roshan
IISER MOHALI, India

2. Cosmic Abundances:
The relative proportion of the elements in the Universe is
“COSMIC ABUNDANCE”
Almost all natural elements are found on Earth and on other
bodies of The Solar system.
Scientists recognize 93 natural elements.
Same 93 elements are also found everywhere in the
Universe.

4. Hydrogen and Helium
in the known Universe
Hydrogen is the most abundant element in the
known Universe; helium is second
Heavier elements constitute less than 1% of the total
matter in the known Universe.
Cosmological observations suggest that only 4.6% of the
universe comprises the visible baryonic matter which
constitutes stars, planets and living beings.
The rest is made up of dark energy (72%) and dark
matter (23%)

5. All the Elements except, Hydrogen and Helium, have been
synthesized in the Stars during their evolution.
The NUCLEOSYNTHESIS process is related with the evolution of
Stars.
Elements are formed inside the Stars since birth and death of the
stars is a continues process.

6. Universe started with aUniverse started with a
BIG BANG.BIG BANG.
After this time, the Universe was in an extremely hotAfter this time, the Universe was in an extremely hot
and dense state and began expanding rapidly.and dense state and began expanding rapidly.

7. The first seconds after the BIG BANG
it was very hot
then the Universe started
to cool down.

8. Right after the BIG BANG, the p+
, the n0
and the e-
were
flying around without control.
When the Universe started to cool down the quarks
started making primitive elements.

9. In 1938, Hans Albrecht Bethe and Weizsacker analyzed two process
in stars, p-p chain and CNO cycle and believed to be the source of
energy in Stars. They showed the possibility to converting hydrogen
into helium through nuclear reaction.
These reactions take place at high temperature and high densities, such
conditions are readily available in the interior of Stars.

10. The process of creation new nuclear species by fusion
reaction in stars is Nucleosynthesis.
Major Processes by which elements are synthesized:
1.) Hydrogen burningHydrogen burning -a first stage of NUCLEOSYNTHESIS.NUCLEOSYNTHESIS.
The p-p chain reaction, which causes fusion of four hydrogenThe p-p chain reaction, which causes fusion of four hydrogen
nuclei to form helium.nuclei to form helium.
Sun and other similar stars generate their energy by this reaction.Sun and other similar stars generate their energy by this reaction.

11. Hydrogen Burning:
The first stage of Nucleosynthesis is the fusion of Hydrogen nuclei
and consequent formation of Helium, called the p-p chain.
Sun and other similar stars generate their energy by this process.

12. CNO cycle
Helium as its end product.
It starts with carbon and acts as a catalyst for the reaction.
It produces energy in main sequence stars.
The main part of the cycle involves C and N, while the ON cycle
usually contributes little energy.

13. Helium Burning: the triple-alpha reaction.
Simplest reaction in a helium gas should be the fusion of two
helium nuclei.
There is no stable configuration with A=8.
For example the beryllium isotope 8
Be has a lifetime of
only 2.6×10-16
s
But a third helium nucleus can be added to 8
Be before decay, forming
12
C by the “triple-alpha” reaction

14. Thus helium burning proceeds in a two-stage reaction,
and energy released is
Q3α
= [ 3ΔM( 4
He) − ΔM(12
C)]c 2
= 7.275MeV
In terms of energy generated per unit mass 5.8 ×1013 JKg-1≡
(I.e. 1/10 of energy generated by H-burning).

15. Carbon and oxygen burning:
Fusion of two Carbon nuclei requires temperature above
5×108
K ,
and Oxygen nuclei requires temperature above 109
K.
.
Once carbon is formed by triple-α reaction in the core,
formation of heavier nuclei becomes possible.
.

16. Carbon and Oxygen burning are very similar, in both cases a
compound is produced at an excited energy level.
These reactions produce p, n, α-particles, which are immediately
captured by heavy nuclei, thus many isotopes created by
secondary reactions.
12
C +12
C→24
Mg + γ , 16
O+16
O → 32
Si + γ
→23
Mg + n → 31
S + n
→23
Na + p →31
P + p
→20
Ne + α → 28
Si + α
→16
O + 2α →24
Mg + 2α

17. Neon Burning:
At temperature of about 3 x 108
K , We obtain nuclei of Neon
16
8
O + 4
2
He → 20
10
Ne + gamma ( before the onset of carbon burning)
This reaction can be reversed in presence of gamma ray photon
which split up the nuclei, this process is called
Photo-disintegration.
20
10
Ne + gamma →16
8
o + α-particles,
At temperature 1.5 x 109
, these alpha particles can react with nuclei of
Neon ( that have not undergone photo-disintegration) to form Magnesium.
This process is called as Neon Burning.
At temperature of about 2 x 109
K the Oxygen nuclei start to react to form
silicon by Oxygen burning.

18. Silicon Burning : Photo-disintegration-rearrangement
After oxygen burning, the core contracts yet and temperature rises
about 3 x 109
K, the photo-disintegration of silicon starts:
28
Si + gamma → 24
Mg + α-particles, these
alpha particles rapidly undergo fusion reactions with silicon and with
subsequent products of fusion,
28
Si + α → 32
S + gamma,
32
S + α → 36
Ar + gamma,
36
Ar + α → 40
Ca + gamma,
This reaction will proceeds as far as producing elements with atomic
masses up to A-56, such as Iron , chromium, cobalt and nickel.

19. Every time a particular type of nuclear fuel is used up completely,Every time a particular type of nuclear fuel is used up completely,
Core contraction due to gravitation takes place. As aCore contraction due to gravitation takes place. As a
consequence, the temperature is raised of the core, thusconsequence, the temperature is raised of the core, thus
another reaction becomes possible.another reaction becomes possible.
This cycle continues till all the nuclei in the core have become
iron nuclei.
This means that with increasing mass number, the nuclei are
more tightly bound and are more stable.
Iron cannot combine with other nuclei to to produce heavier
nuclei.
Then, How did the elements heavier than iron form ?

20. Every time a particular type of nuclear fuel is used up completely,Every time a particular type of nuclear fuel is used up completely,
Core contraction due to gravitation takes place. As aCore contraction due to gravitation takes place. As a
consequence, the temperature is raised of the core, thusconsequence, the temperature is raised of the core, thus
another reaction becomes possible.another reaction becomes possible.
This cycle continues till all the nuclei in the core have become
iron nuclei.
This means that with increasing mass number, the nuclei are
more tightly bound and are more stable.
Iron cannot combine with other nuclei to to produce heavier
nuclei.
Then, How did the elements heavier than iron form ?

21. s- and r-processes
The elements heavier than iron probably synthesized by the
absorption of one neutron at a time.
S-process : The process of absorption of one neutron at a time is
slow process. In this way, all the elements up to Bi209
are formed.
This process stops at Bi209.
The elements heavier than Bi209
are unstable and emit beta- particles
before they can absorb a neutron.
R-process: If neutrons become available in large number, the nuclei
can absorb neutrons rapidly.
Elements right up to uranium are synthesized in stars by these
processes.

22. What happens after all the nuclear reactions have stopped and the
stellar core consist of iron only ?
Iron has highest binding energy per nucleon.
Thus, the core is forced to contract.
The gravitational energy heats the core resulting in the
disintegration of iron into nuclei of helium, even the helium cannot
remain intact as nuclei, they break up into protons and neutrons.

23. Energy transfers from the core to the envelope, very high temperature is
produced. Finally, the envelope explodes.
The explosion is called Supernova.
The elements built inside the star are thrown into the interstellar medium.
The new stars born from this enriched medium contain a small proportion
of heavy elements, these stars are called second generation stars.
Well, the envelope explodes but what happens to the core ?

24. The core keeps collapsing......................
If the initial mass of the star is 12MS
to 3MS ,
the matter in such
stars is mostly neutrons.
At the extremely high density in the core, the neutrons become
degenerate.
The pressure exerted by the degenerate neutrons is sufficiently
high to halt the collapse .
The core stabilizes in the form of a Neutron star
If the initial mass of the exploding star is close to 15MS
or more,
the core is left behind a mass more than 3MS
.
The core of this mass cannot attain equilibrium. It keeps
contracting, its gravitational field becomes so strong that even light
cannot escape it. It becomes a Black Hole

27.
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