[
FIRST QUARTER MODULE 1
     FORMATION OF HEAVIER
     ELEMENTS DURING STAR
    FORMATION AND EVOLUTION
Physical Science– Grade 11/12
Quarter 1 – Module 1: Formation of Heavier Elements during Star Formation and
                       Evolution
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Regional Director: Gilbert T. Sadsad
Assistant Regional Director: Jessie L. Amin
                             Development Team of the Module
 Writer: Rommel Carl R. Peralta
 Illustrator: Ray Daniel Peralta
 Layout Artist: Jose P. Gamas Jr.
 Language Editor: Diana Desuyo
 Editors/ Reviewers: Jocelyn Navera
                     Kristina Nieves
                     Brenly Mendoza
                     Bevelyn Nocomora
                                              ii
 Give evidence for and describe the
formation of heavier elements during
    star formation and evolution.
         (S11/12PS-IIIa-2)
                iii
   Supplementary Learning Module for Senior High School Learners
                        LESSON
         FORMATION OF HEAVIER ELEMENTS DURING
            STAR FORMATION AND EVOLUTION
                                    In the previous grade level, you understood
                            the different theories about the origin of the
                            universe. One of the most accepted theories on the
                            origin of the universe is the Big Bang Theory. In this
                            lesson, you will know deeper what Big Bang Theory
                            is and how the heavier elements formed during the
                            Big Bang.
      Knowing how the heavier elements formed
due to Big Bang will help us understand the
complexity of matter and energy. Hence, this
knowledge will give us the brightness to the gray
area of unknown. Do you want to want to see this
brightness? Read on and accomplish the tasks
prepared for you in this module.
      At the end of the module, you should be
able to:
                                •  Describe the evidences of Big Bang
                            theory.
                                • Describe the formation of heavier
                            elements during star formation and evolution
                                    1
                              Directions: Choose the letter of the correct answer.
1. What do you call to the process that creates new atomic nuclei from pre-
   existing nucleons, primarily protons and neutrons?
   a. Nucleosynthesis
   b. Big Bang
   c. Dark Matter
   d. Energy Transfer
2. What process is responsible for the creation of rarer elements heavier than iron
   and nickel?
   a. Stellar Nucleosynthesis
   b. Supernova Nucleosynthesis
   c. Big Bang Nucleosynthesis
   d. Cosmic Ray Spallation
3. What is the most abundant element in the universe?
    a. Hydrogen
    b. Helium
    c. Lithium
    d. Iron
4. Which of the following is one of the products of the reaction below?
                                             ____
        "
   a.   !𝐷       b. #"𝐻𝑒       c. $!𝑇        d. %$𝐿𝑖
5. What is the product of the reaction below:
                                                _____
    a. "!𝐷       b. #"𝐻𝑒       c. $!𝑇        d. %$𝐿𝑖
   Hi! How did you find the test?
        Please check your answers at the answer key
   section and see how you did. Don’t worry if you got a low
   score, this just means that there are more things that
   you can learn from this module. So, hop on!
                                        2
                                    JUMBLED LETTERS
                           Below are jumbled words about Big Bang
                      Theory. Arrange the letters and match it to
                      its description.
1. It is a remnant from an early stage of the universe also known as “relic radiation”
2. It is the increase in distance between any two given gravitationally unbound parts of the
         observable universe with time.
3. It is a form of matter thought to account for approximately 85% of the matter in the universe
          and about a quarter of its total mass-energy density or about 2.241 x 10-27 kg/m3
4. It is a form of energy that affects the universe on the largest scales.
5. It is a phenomenon where electromagnetic radiation (such as light) from an object
        undergoes an increase in wavelength.
6. In this era, a gravitational singularity before this time and it is hypothesized that the four
        fundamental forces all have the same strength, and are possibly even unified into one
        fundamental force.
7. The period after the formation of the first atoms and before the first stars.
8. Triggered by the separation of the strong nuclear force, the universe undergoes an
       extremely rapid exponential expansion.
9. The temperature of the universe falls to the point (about a billion degrees) where atomic
       nuclei can begin to form as protons and neutrons combine through nuclear fusion to
       form the nuclei of the simple elements of hydrogen, helium and lithium.
10. 8.5 - 9 billion years. Our Sun is a late-generation star, incorporating the debris from many
    generations of earlier stars, and it and the Solar System around it form roughly 4.5 to 5
    billion years ago (8.5 to 9 billion years after the Big Bang).
                                           CHOICES
   a.   DKAR MTATRE                                           f. PCLKAN POECH
   b.   CSOICM CIMROEAWV ACBGRKONUD                           g. ILATOINNFARY HOECP
   c.   KARD NEERYG                                           h. DKRA AEG
   d.   XPEIANNGD UVINEESR                                    i. NCULOSEYNHETSSI
   e.   ASLOR SSETMY RTAMIONOF                                j. DERSIFTH
    Good job in finishing the activity! Take note of the key concepts you had
           written. These words might appear on the next activities.
                                                3
                           BIG BANG THEORY
        The Big Bang theory is a cosmological model of the observable universe from the
earliest known periods through its subsequent large-scale evolution. The model describes how
the universe expanded from an initial state of extremely high density and high temperature,
and offers a comprehensive explanation for a broad range of observed phenomena, including
the abundance of light elements, the cosmic microwave background (CMB) radiation, and
large-scale structure.
        Crucially, the theory is compatible
with Hubble's law – the observation that the
farther away galaxies are, the faster they are
moving away from Earth. Extrapolating this
cosmic expansion backwards in time using
the known laws of physics, the theory
describes a high density state preceded by a
singularity in which space and time lose
meaning. There is no evidence of any
phenomena prior to the singularity. Detailed
measurements of the expansion rate of the
universe place the Big Bang at around
13.8 billion years ago, which is thus
considered the age of the universe.
        After its initial expansion, the universe
cooled sufficiently to allow the formation of
subatomic particles, and later atoms. Giant
clouds of these primordial elements – mostly
24% of the universe’s ordinary matter is
currently comprised of helium, about 74% hydrogen, and 2% of other elements (relative
abundance)-later coalesced through gravity, forming early stars and galaxies, the
descendants of which are visible today. Besides these primordial building materials,
astronomers observe the gravitational effects of an unknown dark matter surrounding
galaxies. Most of the gravitational potential in the universe seems to be in this form, and the
Big Bang theory and various observations indicate that it is not conventional baryonic matter
that forms atoms. Measurements of the redshifts of supernovae indicate that the expansion of
the universe is accelerating, an observation attributed to dark energy's existence.
         Georges Lemaître first noted in 1927 that an expanding universe could be traced
back in time to an originating single point, which he called the "primeval atom". For several
decades, the scientific community was divided between supporters of the Big Bang and the
rival steady-state model, but a wide range of empirical evidence has strongly favored the Big
Bang, which is now universally accepted. Edwin Hubble concluded from analysis of galactic
redshifts in 1929 that galaxies are drifting apart; this is important observational evidence for
an expanding universe. In 1964, the CMB was discovered. This was a crucial evidence of the
Big Bang Theory which predicted a uniform background radiation throughout the universe.
                                                     Source: https://en.wikipedia.org/wiki/BigBang.
                                                 4
                TIME LINE OF THE BIG BANG
         Since the Big Bang, 13.7 billion years ago, the universe has passed through many
different phases or epochs. Due to the extreme conditions and the violence of its very early
stages, it arguably saw more activity and change during the first second than in all the
billions of years since.
        From our current understanding of how the Big Bang might have progressed, taking
into account theories about inflation, Grand Unification we can put together an approximate
timeline as follows:
   •   Planck Epoch (or Planck Era), from zero to approximately 10-43 seconds (1 Planck
       Time). This is also known as singularity epoch. This is the closest that current physics
       can get to the absolute beginning of time, and very little can be known about this period.
       General relativity proposes a gravitational singularity before this time (although even
       that may break down due to quantum effects), and it is hypothesized that the four
       fundamental forces (electromagnetism, weak nuclear force, strong nuclear force and
       gravity) all have the same strength, and are possibly even unified into one fundamental
       force, held together by a perfect symmetry which some have likened to a sharpened
       pencil standing on its point (i.e. too symmetrical to last). At this point, the universe
       spans a region of only 10-35 meters (1 Planck Length), and has a temperature of over
       1032°C (the Planck Temperature).
   •   Grand Unification Epoch, from 10–43 seconds to 10–36 seconds:
       The force of gravity separates from the other fundamental forces (which remain
       unified), and the earliest elementary particles (and antiparticles) begin to be created.
   •   Inflationary Epoch, from 10–36 seconds to 10–32 seconds. Triggered by the separation
       of the strong nuclear force, the universe undergoes an extremely rapid exponential
       expansion, known as cosmic inflation. The linear dimensions of the early universe
       increases during this period of a tiny fraction of a second by a factor of at least 1026 to
       around 10 centimeters (about the size of a grapefruit). The elementary particles
       remaining from the Grand Unification Epoch (a hot, dense quark-gluon plasma,
       sometimes known as “quark soup”) become distributed very thinly across the universe.
   •   Electroweak Epoch, from 10–36 seconds to 10–12 seconds. As the strong nuclear force
       separates from the other two, particle interactions create large numbers of exotic
       particles, including W and Z bosons and Higgs bosons (the Higgs field slows particles
       down and confers mass on them, allowing a universe made entirely out of radiation to
       support things that have mass).
   •   Quark Epoch, from 10–12 seconds to 10–6 seconds. Quarks, electrons and neutrinos
       form in large numbers as the universe cools off to below 10 quadrillion degrees, and
       the four fundamental forces assume their present forms. Quarks and antiquarks
       annihilate each other upon contact, but, in a process known as baryogenesis, a surplus
       of quarks (about one for every billion pairs) survives, which will ultimately combine to
       form matter.
   •   Hadron Epoch, from 10–6 seconds to 1 second. The temperature of the universe cools
       to about a trillion degrees, cool enough to allow quarks to combine to form hadrons
       (like protons and neutrons). Electrons colliding with protons in the extreme conditions
       of the Hadron Epoch fuse to form neutrons and give off massless neutrinos, which
       continue to travel freely through space today, at or near to the speed of light. Some
       neutrons and neutrinos re-combine into new proton-electron pairs. The only rules
                                               5
    governing all this apparently random combining and re-combining are that the overall
    charge and energy (including mass-energy) be conserved.
•   Lepton Epoch, from 1 second to 3 minutes. After the majority (but not all) of hadrons
    and antihadrons annihilate each other at the end of the Hadron Epoch, leptons (such
    as electrons) and antileptons (such as positrons) dominate the mass of the universe.
    As electrons and positrons collide and annihilate each other, energy in the form of
    photons is freed up, and colliding photons in turn create more electron-positron pairs.
•   Nucleosynthesis, from 3 minutes to 20 minutes. The temperature of the universe falls
    to the point (about a billion degrees) where atomic nuclei can begin to form as protons
    and neutrons combine through nuclear fusion to form the nuclei of the simple elements
    of hydrogen, helium and lithium. After about 20 minutes, the temperature and density
    of the universe has fallen to the point where nuclear fusion cannot continue.
•   Photon Epoch (or Radiation Domination), from 3 minutes to 240,000 years:
    During this long period of gradual cooling, the universe is filled with plasma, a hot,
    opaque soup of atomic nuclei and electrons. After most of the leptons and antileptons
    had annihilated each other at the end of the Lepton Epoch, the energy of the universe
    is dominated by photons, which continue to interact frequently with the charged
    protons, electrons and nuclei.
•   Recombination/Decoupling, from 240,000 to 300,000 years. As the temperature of
    the universe falls to around 3,000 degrees (about the same heat as the surface of the
    Sun) and its density also continues to fall, ionized hydrogen and helium atoms capture
    electrons (known as “recombination”), thus neutralizing their electric charge. With the
    electrons now bound to atoms, the universe finally becomes transparent to light,
    making this the earliest epoch observable today. It also releases the photons in the
    universe which have up till this time been interacting with electrons and protons in an
    opaque photon-baryon fluid (known as “decoupling”), and these photons (the same
    ones we see in today’s cosmic background radiation) can now travel freely. By the end
    of this period, the universe consists of a fog of about 75% hydrogen and 25% helium,
    with just traces of lithium.
•   Dark Age (or Dark Era), from 300,000 to 150 million years. The period after the
    formation of the first atoms and before the first stars is sometimes referred to as the
    Dark Age. Although photons exist, the universe at this time is literally dark, with no
    stars having formed to give off light. With only very diffuse matter remaining, activity in
    the universe has tailed off dramatically, with very low energy levels and very large time
    scales. Little of note happens during this period, and the universe is dominated by
    mysterious “dark matter”.
•   Reionization, 150 million to 1 billion years. The first quasars form from gravitational
    collapse, and the intense radiation they emit reionizes the surrounding universe, the
    second of two major phase changes of hydrogen gas in the universe (the first being
    the Recombination period). From this point on, most of the universe goes from being
    neutral back to being composed of ionized plasma.
•   Star and Galaxy Formation, 300 - 500 million years onwards. Gravity amplifies slight
    irregularities in the density of the primordial gas and pockets of gas become more and
    more dense, even as the universe continues to expand rapidly. These small, dense
    clouds of cosmic gas start to collapse under their own gravity, becoming hot enough
    to trigger nuclear fusion reactions between hydrogen atoms, creating the very first
    stars. The first stars are short-lived supermassive stars, a hundred or so times the
    mass of our Sun, known as Population III (or “metal-free”) stars. Eventually Population
                                            6
    II and then Population I stars also begin to form from the material from previous rounds
    of star-making. Larger stars burn out quickly and explode in massive supernova
    events, their ashes going to form subsequent generations of stars. Large volumes of
    matter collapse to form galaxies and gravitational attraction pulls galaxies towards
    each other to form groups, clusters and superclusters.
•   Solar System Formation, 8.5 - 9 billion years. Our Sun is a late-generation star,
    incorporating the debris from many generations of earlier stars, and it and the Solar
    System around it form roughly 4.5 to 5 billion years ago (8.5 to 9 billion years after the
    Big Bang).
•   Today, 13.7 billion years. The expansion of the universe and recycling of star materials
    into new stars continues
    Source: The Physics of Universe. https://www.physicsoftheuniverse.com/topics_bigbang_timeline.html
                                                  7
DIRECTIONS: Answer the following questions.
   1. Which of these is true about the Big Bang model?
      a.   The singularity is an established, well-defined part of the model.
      b.   Cosmic expansion stopped at some point in time.
      c.   Part of its proof is the amounts of H and He we have in the universe today.
      d.   The Big Bang was a big explosion that threw matter into many different directions
   2. Using one to two sentences each, explain the three pieces of evidence presented for
      the Big Bang Theory:
       Evidence 1: Redshift
       Evidence 2: Relative abundance
       Evidence 3: Cosmic microwave background
   3. Make a graphic organizer illustrating the timeline of Big Bang theory.
              Good job!
             Now, you already knew the evidences of
      Big Bang theory, the stages how it occurred from
      the singularity and how the light elements such
      as hydrogen and helium formed during the Big
      Bang. The question now is, how do the heavier
      elements formed after the Big Bang?
              On the next part of this module you will
      learn more about that elements heavier than
      beryllium    are   formed     through    stellar
      nucleosynthesis. Stellar nucleosynthesis is the
      process by which elements are formed within
      stars. The abundances of these elements change
      as the stars evolve.
              On the next part of the module, the
      formation of heavier elements will be discussed
      and emphasized.
                                              8
                                       LET US REACT!
                   Below are reactions of subatomic particles
             during the Big Bang. Determine and discuss the
             pattern on how the reactants yielded to such kind
             of isotopes.
           Using your discovered pattern, complete the nucleosynthesis reactions below:
1. p+ + n0            _ +γ
     #
2.   "𝐻𝑒   + "!𝐷      ____+ 𝑝&
     %
3.   $𝐿𝑖   +____       2 4He
             "         $
4. ___ +     !𝐷        !𝑇   + p+
                        %
5. ___ + ____           #𝐵𝑒   + γ
 Good job in finishing the activity! Take note of the key concepts you had
        written. These words might appear on the next activities.
                                                9
                 STELLAR NUCLEOSYNTHESIS
      Hydrogen and helium atoms in stars began combining in nuclear fusion reactions once
hydrogen-helium stars had formed from the action of gravity. This releases a tremendous
amount of light, heat, and radioactive energy. Fusion resulted in the formation of nuclei of new
elements. These reactions inside stars are known as stellar nucleosynthesis.
                    Figure 3. Equilibrium of the Sun and other main-sequece stars.
        The first fusion process occurs in the hydrogen core of stars such as the sun with a
temperature of less than 15 million K. These kinds of stars are called main-sequence stars.
In the process known as the main-branch proton-proton chain, Deuterium (D or 2H) forms
from proton fusion, with one proton turning into a neutron via beta-plus decay, giving off a
neutrino and a positron:
                                    1
                                        H + 1H → 2H + ν + e+
                   Figure 4. The main branch of the proton-proton chain reaction (p-p
                               chain) resulting in
                                                10the formulation of 4He.
       3
       He forms from deuterium and proton fusion, also known as deuterium burning. This
immediately consumes all deuterium produced.
                                        2
                                            H + 1H → 3He + γ
       Then 4He forms from 3He fusion.
                                   3
                                       He + 3He → 4He + 2 1H
                     Figure 5. A star with a very dense helium core and a hydrogen shell
                        expands into a red giant due to increased radiation pressure
                    Figure 6. The triple alpha process resulting in the formation of 12C.
                                               (Elert, 2015b)
         The entire three-step process releases about 26.7 MeV (megaelectronvolts) of energy.
The energy released is responsible for the thermal pressure that pushes against gravity. It is
also responsible for the light, heat and radiation emitted by the star. A different process
facilitates hydrogen fusion in main sequence stars with temperature greater than 15 million K.
                                                  11
        The core of a star becomes comprised of He as H is depleted, while H fusion only
occurs in a shell around it. Due to this process, the temperature and density of the core of the
star increases up to 100 million K. The star’s thermal pressure causes it to push out H gas.
The star balloons into a red giant.
          Several nuclear fusion processes occur in a red giant aside from hydrogen fusion. The
first is the triple alpha process. Alpha particles refer to 4He. This reaction involves the fusion
of three 4He atoms in the following steps:
                                          4
                                              He + 4He → 8Be
                                      8
                                          Be + 4He → 12C + γ
         The 8Be intermediate is unstable, so either it decays or forms 12C.
The star can keep growing into a supergiant as it accumulates mass.
Alpha fusion processes continue in the core via the alpha ladder. More
and more alpha particles are fused to create heavier elements all the way
to iron, making the core and star itself more massive.
        The main-sequence stars hotter than 15 million K could facilitate
the production of helium once carbon was present from alpha processes.
This happens through a process where 12C is used as a catalyst known
as the carbon fusion cycle or the CNO cycle. CNO cycle is the process
that involves repeated proton capture and beta-plus decay
                                                                                   Figure     7. Alpha
                                                                                   fusion      is    the
                                                                                   processes        that
                                                                                   continue in the core
                                                                                   via the alpha ladder.
                                                                                   The alpha particles
                                                                                   are fused to create
                                                                                   heavier elements all
                                                                                   the way to iron,
                                                                                   making the core.
        Figure 8. The CNO cycle which uses 12C as a catalyst to form more
                    4He in larger or hotter main-sequence stars.
       Then star will eventually be unable to generate energy to push against gravity due to
the formation of heavier elements, thus causing it to collapse on itself. It then undergoes a
supernova explosion that releases a tremendous amount of energy enough to synthesize
elements heavier than iron. Examples of these elements are uranium and thorium, which are
some of the heaviest known elements. This is done through the r-process that involves rapid
capture of neutrons by the atom. Other heavy elements are also synthesized through s-
process involving slow neutron capture in red giants. The r-process and s-process are
processes that change the atom’s atomic weight, after which the atom undergoes various
decay processes to change its identity.
                                                            Source: Commission on Higher Education,
                                                Teaching Guide for Senior High School Physical Science
                                                    12
DIRECTIONS: Answer the following questions.
  1. How do heavier elements formed during stellar nucleosynthesis? Explain your answer
     using the concept map below.
  2. What can you notice on the number of the atomic number patterns of the elements in
     alpha ladder being shown below. Discuss your findings.
                                    Very good!
                                     You already understood
                                 how the heavier elements
                                 formed through stellar
                                 nucleosynthesis.
                                     You are nearer to the
                                 finish line of this module.
                                          13
Directions: Write your thoughts about the following questions.
       1. How does Big Bang Theory affect us?
       2. Why is the era of nucleosynthesis so important?
       Discuss the few elements that were first discovered as man-made elements since
many of them did not emerge from the major nucleosynthesis reactions (or their minor
processes). You may refer on the figure below.
                      Source: Big Bang. Retrieved from https://en.wikipedia.org/wiki/BigBang.
                                             14
                                  The following terms used in this module are defined as
                                 follows:
Alpha process, also known as the alpha ladder, is one of two classes of nuclear fusion
      reactions by which stars convert helium into heavier elements, the other being the
      triple-alpha process.
Big Bang theory is a cosmological model of the observable universe from the earliest known
      periods through its subsequent large-scale evolution. It describes how the universe
      expanded from an initial state of extremely high density and high temperature, and
      offers a comprehensive explanation for a broad range of observed phenomena,
      including the abundance of light elements, the cosmic microwave background (CMB).
CNO Cycle' refers to the Carbon-Nitrogen-Oxygen cycle, a process of stellar nucleosynthesis
     in which stars on the Main Sequence fuse hydrogen into helium via a six-stage
     sequence of reactions.
Cosmic Microwave Background (CMB) is a remnant from an early stage of the universe
     also known as “relic radiation”
Dark Energy is a form of energy that affects the universe on the largest scales.
Dark Matter is a form of matter thought to account for approximately 85% of the matter in the
      universe and about a quarter of its total mass-energy density or about 2.241 x 10-27
      kg/m3
Expanding Universe is the increase in distance between any two given gravitationally
     unbound parts of the observable universe with time.
Main sequence is a continuous and distinctive band of stars that appears on plots of stellar
      color versus brightness
Proton–proton chain reaction is one of two known sets of nuclear fusion reactions by which
      stars convert hydrogen to helium.
Rapid neutron-capture process, also known as the r-process, is a set of nuclear reactions
      that is responsible for the creation of approximately half of the atomic nuclei heavier
      than iron; the "heavy elements", with the other half produced by the p-process and s-
      process
Red giant star is a dying star in the last stages of stellar evolution
Redshift- is a phenomenon where electromagnetic radiation (such as light) from an object
      undergoes an increase in wavelength.
Slow neutron-capture process, or s-process, is a series of reactions in nuclear astrophysics
      that occur in stars
Stellar nucleosynthesis is the process involving nuclear reactions through which fresh
       atomic nuclei are synthesized from pre-existing nuclei or nucleons.
Supernova is a large explosion that takes place at the end of a star's life cycle
                                               15
Directions: Answer the following questions.
     1. What are the evidences of Big Bang theory?
     2. How do heavier elements form during stellar nucleosynthesis?
      Great! You have completed your
learning episodes in this module!
      You are now ready to start a new
learning adventure in the next module.
       Congratulations!
                                    16
TRY THIS
  1) a   2) b                3) a          4) a           5) b
DO THIS: Jumbled Letters
1)   b. Cosmic Microwave Background               6) f. Planck Epoch
2)   d. Expanding Universe                        7) h. Dark Age
3)   a. Dark Matter                               8) g. Inflationary Epoch
4)   c. Dark Energy                               9) i. Nucleosynthesis
5)   j. Red Shift                                 10) e. Solar System Formation
DO THIS: Let’s React
        When the reactants combined, they formed new element plus energy which is
mostly a gamma ray. The formation of new elements was due to the fusion of the lighter
elements and sub atomic particles such as neutron and proton.
     1) D      2) 4He        3) p+         4) n           5) 3He, 4He
What You Have Learned
  Refer to the discussion in Keep This Mind: Stellar Nucleosynthesis
Reflect
   1.      The Big Bang essentially formed the entire universe that we know and all the
elements, forces, stars, planets and other celestial bodies. More specifically, most of the
hydrogen and helium in the universe today was formed during the early days of Big Bang.
Hydrogen is the key element in the organic chemicals of life. For example, hydrogen helps
bond other chemicals to form the DNA that is in nearly every cell in our body.
    2.     The era of nucleosynthesis is important because during this time all the
primodial hydrogen and helium was created from the nuclear fusion process. Except for
the few percent of matter that stars later fused into heavier elements, the chemical
composition of the universe remains unchanged today.
Assess What You Have Learned
     1. Red shift, Cosmic Microwave Background (CMB) and relative abundance
     2. The burning of helium to produce heavier elements then continues for about 1
        million years. Largely, it is fused into carbon via the triple-alpha process in which
        three helium-4 nuclei (alpha particles) are transformed. The alpha process then
        combines helium with carbon to produce heavier elements, but only those with an
        even number of protons. The combinations go in this order:
              i.   Carbon plus helium produces oxygen.
             ii.   Oxygen plus helium produces neon.
            iii.   Neon plus helium produces magnesium.
            iv.    Magnesium plus helium produces silicon.
             v.    Silicon plus helium produces sulfur.
                                            17
            vi.    Sulfur plus helium produces argon.
           vii.    Argon plus helium produces calcium.
           viii.   Calcium plus helium produces titanium.
            ix.    Titanium plus helium produces chromium.
             x.    Chromium plus helium produces iron.
        Other fusion pathways create the elements with odd numbers of protons. Iron has
   such a tightly bound nucleus that there isn't further fusion once that point is reached.
   Without the heat of fusion, the star collapses and explodes in a shockwave.
Griffith, W. Thomas and Juliet Wain Brosing. The Physics of Everyday Phenomena: A
Commission on Higher Education. Teaching Guide for Senior High School Physical Science.
     (2016)
Conceptual Introduction to Physics, 6th ed. NY: McGraw Hill, 2009.
Hewitt, Paul G. Conceptual Physics 11th edition. San Francisco: Pearson, 2015.
March, Robert . Physics for Poets, 5th ed. NY: McGraw-Hill 2003.
Naylor, John. Out of the Blue: A 24-hour Skywatcher's Guide. England: Cambridge University
      Press, 2002.
Pasachoff, Jay and Alex Filipenko. The Cosmos: Astronomy in the New Millenium. California:
     Thomson-Brooks/Cole, 2007.
Shipman, James T., Jerry D. Wilson, and Charles A. Higgins. An Introduction to Physical
     Science. Singapore: Cengage Learning Asia Pte Ltd, 2013.
Spielberg, Nathan and Bryon D. Anderson. Seven Ideas that Shook the Universe, 2nd ed.
      New Jersey: John Wiley & Sons, 1995.
[Big Bang. Retrieved from https://en.wikipedia.org/wiki/BigBang. Accessed July 20, 2020
                                            18