phy sci ppt.pptx

HOW THE ELEMENTS FOUND IN THE
UNIVERSE WERE FORMED
NUCLEOSYNTHESIS: THE BEGINNING OF ELEMENTS

HOW THE ELEMENTS FOUND IN THE UNIVERSE
WERE FORMED
Nucleosynthesis: The Beginning of Elements
Learning Competencies 1 to 4
1. The Formation of the Light Elements in the Big Bang Theory
2. The Formation of Heavier Elements during Star Formation
and Evolution
3. The Nuclear Fusion Reactions in Stars
4. How Elements Heavier than Iron are Formed

What is Big Bang Theory?
The big bang theory is a
cosmological model stating
that the universe started its
expansion about 13.8
billion years ago. Pieces of
evidence supporting this
theory are (1) occurrence of
redshift, (2) background
radiation, and (3)
The Formation of the Light Elements
in the Big Bang Theory

Redshift
In the 1910s, Vesto Slipher and Carl Wilhelm Wirtz
measured the wavelengths of light from spiral nebulae,
which are interstellar clouds of dust and ionized gases.
They discovered that the light from the nebulae increased
in wavelength. They explained their discovery as a Doppler
shift.

Redshift
The Doppler shift or Doppler effect explains that when an
object gets closer to us, its light waves are compressed into
shorter wavelengths (blueshifted). On the other hand, when
an object moves away from us, its light waves are
stretched into longer wavelengths (redshifted).

Redshift
Slipher and Wirtz then explained that the redshift or
increase in wavelength was due to the increase in the
distance between the Earth and the nebulae. They
concluded that the redshift occurred due to the expansion
of space.
In 1929, Edwin Hubble used the redshift of light from
galaxies to calculate the velocities and distances of these
galaxies from the Earth. He discovered that they were
moving away from the Earth and from each other. His
calculations supported the theory that the universe is

Cosmic Microwave Background Radiation
In 1965, Robert Wilson and Arno
Penzias discovered a low, steady
“hum” from their Holmdel Horn
antenna (an antenna built to
support NASA’s Project Echo).
They concluded that the noise is
Cosmic Microwave Background
Radiation (CMBR), the remains of
energy created after the big bang

Abundance of Light Elements
The observed abundance of light elements supports the big
bang theory. The theory predicts that the universe is
composed of 73% hydrogen and 25% helium by mass. The
prediction correlated to the measured abundances of
primordial material in unprocessed gas in some parts of the
universe with no stars.

Formation of Light Elements
Big bang nucleosynthesis is the process of producing the
light elements during the big bang expansion.
In the beginning, the universe was
very hot that matter was fully
ionized and dissociated. Few
seconds after the start of the big
bang, the universe was filled with
protons, neutrons, electrons,
neutrinos, and positrons. After the
first three minutes, the universe
cooled down to a point where

Protons and neutrons combined to form atomic nuclei such
as deuterium.
However, the temperature of the universe was still much
greater than the binding energy of deuterium. Binding
energy is the energy required to break down a nucleus into
its components. Therefore, deuterium easily decayed upon

When the temperature cooled down below 1010 K,
deuterium nuclei combined with other nuclei to form heavier
ones.
Helium-3 was formed from the fusion of two deuterium
nuclei and a release of a neutron.

Helium-4 was produced from the fusion of deuterium and
tritium.

Tritium or Hydrogen-3 was produced from the fusion of
two deuterium nuclei and a release of a proton.

Helium-4 was also synthesized from deuterium and helium-
3.

For the first three minutes, a substantial amount of
neutrons was converted into helium-4 nuclei, before their
decay. Helium then combined to other nuclei to form
heavier ones such as lithium-7 and beryllium-7.

Lithium-7 was synthesized from helium-4 and tritium.

Beryllium-7 was produced from helium-3 and helium-4.

Among the light elements formed, deuterium, helium-3,
helium-4, and lithium-7 were stable. Beryllium-7 was
unstable and decayed spontaneously to lithium-7.

How were elements heavier than beryllium formed?
The Formation of Heavier Elements during
Star Formation and Evolution
Light elements – hydrogen, helium, lithium, and beryllium,
were formed during the big bang nucleosynthesis.
Elements heavier than beryllium are formed through
stellar nucleosynthesis. The abundances of these
elements change as the stars evolve.
Stellar nucleosynthesis is the process by which
elements are formed within stars.

Evolution of Stars
The star formation theory proposes that stars form due to
the collapse of the dense regions of a molecular cloud. As the
cloud collapses, the fragments contract to form a stellar core
called protostar.
Due to strong gravitational force, the protostar contracts and
its temperature increases. When the core temperature
reaches about 10 million K, nuclear reactions begin.

Evolution of Stars
The reactions release positrons and neutrinos which
increase pressure and stop the contraction. When the
contraction stops, the gravitational equilibrium is reached,
and the protostar has become a main sequence star.

Evolution of Stars
In the core of a main sequence star, hydrogen is fused into
helium via the proton-proton chain. When most of the
hydrogen in the core is fused into helium, fusion stops, and
the pressure in the core decreases.
Gravity squeezes the star to a point that helium and hydrogen
burning occur. Helium is converted to carbon in the core while
hydrogen is converted to helium in the shell surrounding the
core. The star has become a red giant.

Evolution of Stars
When the majority of the helium in the core has been
converted to carbon, then the rate of fusion decreases.
Gravity again squeezes the star.

In a low-mass star (with mass less than twice the Sun’s
mass), there is not enough mass for a carbon fusion to occur.
The star’s fuel is depleted, and over time, the outer material
of the star is blown off into space. The only thing that remains
is the hot and inert carbon core. The star becomes a white
dwarf.
Evolution of Stars

Evolution of Stars
However, the fate of a massive star is different. A massive
star has enough mass such that temperature and pressure
increase to a point where carbon fusion can occur.
The star goes through a series of stages where heavier
elements are fused in the core and in the shells around the
core. The element oxygen is formed from carbon fusion; neon
from oxygen fusion; magnesium from neon fusion: silicon
from magnesium fusion; and iron from silicon fusion.

Evolution of Stars
The fusion of elements continues until iron is formed by
silicon fusion. Elements lighter than iron can be fused
because when two of these elements combine, they produce
a nucleus with a mass lower than the sum of their masses.
The missing mass is released as energy.
Therefore, the fusion of elements lighter than iron releases
energy. However, this does not happen to iron nuclei. Rather
than releasing energy, the fusion of two iron nuclei requires

Evolution of Stars
Therefore, elements lighter than and including iron can be
produced in a massive star, but no elements heavier than iron
are produced.
When the core can no longer produce energy to resist gravity,
the star is doomed. Gravity squeezes the core until the star
explodes and releases a large amount of energy.
The star explosion is called a supernova.

The Nuclear Fusion Reactions in Stars
Stellar nucleosynthesis is the process by which elements
are formed in the cores and shells of the stars through
nuclear fusion reactions. Nuclear fusion is a type of
reaction that fuses lighter elements to form heavier ones. It
requires very high temperatures and pressures. It is the
reaction that fuels the stars since stars have very high
temperatures and pressures in their cores.

Hydrogen is the lightest element and the most abundant in
space. Thus, the formation of heavier elements starts with
hydrogen. There are two dominant hydrogen burning
processes, the proton-proton chain and carbon-nitrogen-
oxygen (CNO) cycle.

The proton-proton chain is a series of thermonuclear
reactions in the stars. It is the main source of energy
radiated by the sun and other stars. It happens due to the
large kinetic energies of the protons. If the kinetic energies
of the protons are high enough to overcome their
electrostatic repulsion, then proton-proton chain proceeds.
Proton-Proton Chain

The chain starts when two protons fuse.
When the fused proton breaks, one proton is
transmuted into a neutron.
The proton and neutron then pairs, forming
an isotope of hydrogen called deuterium.
Another proton collides with a deuterium
forming a helium-3 nucleus and a gamma
ray.
Finally, two helium-3 nuclei collide, and a
helium-4 is created with the release of two
protons.

For more massive and hotter stars, the carbon-nitrogen-
oxygen cycle is the more favorable route in converting
hydrogen to helium.
Carbon-Nitrogen-Oxygen (CNO) Cycle
Unlike the proton-proton chain, the CNO cycle is a catalytic
process. Carbon-12 acts a catalyst for the cycle. It is used
in the initial reaction and is regenerated in the final one.

How Elements Heavier than Iron are Formed
Nucleosynthesis is the process by which new nuclei are
formed from pre-existing or seed nuclei.
If the stellar nucleosynthesis produced only elements up to iron, then what type of
nucleosynthesis produced the elements heavier than iron?
The big bang nucleosynthesis produced hydrogen and
helium, whereas the stellar nucleosynthesis produced
elements up to iron in the core of the stars.

The stellar nucleosynthesis produced nuclei that are
heavier than helium-4 by nuclear fusion. It started by fusing
two helium-4 nuclei to form beryllium-8 accompanied by a
release of energy in the form of gamma radiation (γ). This
process continues until nickel-56.

However, nickel-56 is unstable and undergoes positron
(0
+1β) emission. Recall that positron emission results in a
nucleus with lower atomic number.

Nickel-56 radioactively decomposes to a more stable iron-
56 through subsequent emission of two positrons.

The fusion reactions cannot produce nuclei higher than
iron-56 because fusion reaction becomes unfavorable. This
is because the nuclear binding energy per nucleon, the
energy that holds the nucleus intact, decreases after iron-
56. Therefore, different pathways are needed for the
synthesis of heavier nuclei.
Synthesis of heavier nuclei happens via neutron or proton
capture processes.

In neutron capture, a neutron is added to a seed nucleus.
The addition of neutron produces a heavier isotope of the
element.
Neutron Capture
For example, iron-56 captures three neutrons to produce
iron-59.

The generated isotope, when unstable, undergoes beta (0
-
1β) decay. This decay results in an increase in the number
of protons of the nucleus by 1. Hence, a heavier nucleus is
formed.
Neutron Capture
Beta decay results in the formation of a new element. For
example, the unstable iron-59 undergoes beta decay to
produce cobalt-59.

Neutron capture can either be slow or rapid.
Neutron Capture
Slow neutron capture or s-process happens when there
is a small number of neutrons. It is termed slow because
the rate of neutron capture is slow compared to the rate of
0
-1β decay. Therefore, if a 0
-1β decay occurs, it almost
always occurs before another neutron can be captured.

Neutron Capture
Rapid neutron capture or r-process, on the other hand,
happens when there is a large number of neutrons. It is
termed rapid because the rate of neutron capture is fast
that an unstable nucleus may still be combined with
another neutron just before it undergoes 0
-1β decay.

Neutron Capture
The r-process is associated with a supernova. The
temperature after a supernova is tremendously high that
the neutrons are moving very fast. Because of their speed,
they can immediately combine with the already heavy
isotopes. This kind of nucleosynthesis is also called
supernova nucleosynthesis.

Proton Capture
Proton capture (p-process) is the addition of a proton in
the nucleus. It happens after a supernova, when there is a
tremendous amount of energy available. It is because the
addition of a proton to the nucleus is not favorable because
of Coulombic repulsion, which is the repulsive force
between particles with the same charge.

Proton Capture
Proton capture produces a heavier nucleus that is different
from the seed nucleus.
For example, molybdenum-94 undergoes proton capture to
produce technetium-95.

WHAT IS THE MISSING PART OF THE
REACTION?
1.
2.
3.

REACTION?
4.
5.

REACTION?
6.
7.

phy sci ppt.pptx

More Related Content

What's hot (20)

Similar to phy sci ppt.pptx (20)

Recently uploaded (20)

phy sci ppt.pptx

Editor's Notes