Lessons

2. Learning Objectives
1. Give evidence for and explain the formation of
the elements after the big bang and during star
formation and evolution; and,
2. Write the nuclear reactions involved in the
synthesis of new elements.

3. How did the
universe come
into being?

4. The Origin of the Universe

5. The Origin of Light Elements
The origin of all the naturally occurring elements fall
into two phases:
•Big Bang or Primordial Nucleosynthesis —the origin of
the “light” elements; and
•Stellar Nucleosynthesis— the origin and production
of the “heavy” elements.

6. Nucleosynthesis is the process that
creates new atomic nucleus from
preexisting nucleons, which is proton
neutrons.
Primordial or Big Bang Nucleosynthesis
refers to the process of producing the
“light elements” shortly after the Big
Bang.

7. The energy and temperature of the
universe are extremely high to cause
the neutrons and protons to combine
and form certain species of atomic
nuclei in a process called nuclear
fusion.

8. Protons Neutrons

9. Through Nuclear Fusion, the light
elements- Hydrogen (H), Helium (He), and
small amounts of lithium (Li) and
beryllium (Be) were formed.
The isotopes produced during the big
bang nucleosynthesis were H-1, H-2, H-3,
H-4, L-7.

10. An Isotope is a form of an element
that has the same atomic number
of the original element but with
different atomic mass or mass
number.

11. Origin of Heavier Elements
Heavy elements were formed only billions of
years after the formation of stars.
The density inside a star is great enough to
sustain fusion for extended time periods
required to synthesize heavy elements.

12. Stars are hot and dense enough to burn
hydrogen-1 (1H) to helium-4 (4He).
The formation of heavy elements by fusion
of lighter nuclei in the interior of stars is
called “stellar nucleosynthesis”.

13. There are many nuclear synthetic pathways or
nuclear fusions to produce heavy elements:
•Carbon-Nitrogen-oxygen cycle
•Proton-proton fusion
•Triple alpha process

14. Layers near core of stars have
very high temperatures enough
to nucleosynthesize heavy
elements such as silicon and iron.

15. Elements heavier than Iron
Elements heavier than iron cannot be
formed through fusion as tremendous
amounts of energy are needed for the
reaction to occur.
Heavy elements are formed in a supernova,
a massive explosion of a star.

16. The density inside a star is
great enough to sustain
fusion for extended time
periods required to
synthesize heavy elements.

17. A supernova is the
explosive death of a star

18. In supernova, neutron capture reaction
takes place, leading to formation of
heavy elements.
In a neutron capture reaction, heavy
elements are created by addition of more
neutrons to existing nuclei instead of
fusion of light nuclei.

19. Adding neutrons to a nucleus doesn’t
change an element. Rather, a more massive
isotope of the same element is produced.
Elements higher than iron requires
tremendous amount of energy to be
formed. Thus, they were produced from a
neutron capture reaction in a supernova.

20. Summary:
There are 3 reactions that led to the formation of the elements:
nucleusynthesis, fusion, and neutron capture reaction.
These reaction required a certain amount of energy to proceed, which was
obtained from the heat of the continuously expanding universe. Thus energy
in the form of heat does not only produce work but also the elements that
make up matter that we have today.
The reaction involved in the formation of these elements are dependent on
the atomic mass of the elements. More energy, and thus higher temperature,
is needed to form heavier elements.
Nucleuosynthesis formed light elements, whereas fusion in stars formed
elements with an atomic mass that is within the range of beryllium and iron.
Thus any element with an atomic mass higher than iron, which required
tremendous amount of energy to be formed was produced from a neutron
capture -reaction in supernova.