01. PS SHS Unit 1 Nucleosynthesis (Study Guide).pdf

Unit 1 Nucleosynthesis: The Beginning of the Elements Table of Contents Table of Contents 1 Introduction 3...

Unit 1 Nucleosynthesis: The Beginning of the Elements Table of Contents Table of Contents 1 Introduction 3 Essential Questions 4 Review 4 Lesson 1.1: The Big Bang Theory and the Formation of Light Elements 5 Objective 5 Warm-Up 5 Learn about It 6 Key Points 14 Web Links 15 Check Your Understanding 15 Challenge Yourself 17 Lesson 1.2: Stellar Evolution and the Formation of Heavier Elements 18 Objectives 18 Warm-Up 18 Learn about It 20 Key Points 25 Web Links 26 Check Your Understanding 26 Challenge Yourself 27 Lesson 1.3: The Nuclear Fusion Reactions in Stars 28 Objective 28 Warm-Up 28 Learn about It 30 Key Points 35 Web Links 36 Check Your Understanding 36  Challenge Yourself 38 Lesson 1.4: How Elements Heavier Than Iron Were Formed 39 Objective 39 Key Points 43 Web Links 44 Check Your Understanding 44 Challenge Yourself 45 Laboratory Activity 46 Performance Task 50 Self Check 52 Key Words 52 Wrap Up 53 Photo Credits 54 References 54 2 GRADES 11/12 | PHYSICAL SCIENCE Unit 1 Nucleosynthesis: The Beginning of Elements “There is a fundamental reason why we look at the sky with wonder and longing—for the same reason that we stand, hour after hour, gazing at the distant swell of the open ocean. There is something like an ancient wisdom, encoded and tucked away in our DNA, that knows its point of origin as surely as a salmon knows its creek. Intellectually, we may not want to return there, but the genes know, and long for their origins—their home in the salty depths. But if the seas are our immediate source, the penultimate source is certainly the heaven. The spectacular truth is—and this is something that your DNA has known all along—the very atoms of your body—the iron, calcium, phosphorus, carbon, nitrogen, oxygen, and on and on—were initially forged in long-dead stars. This is why, when you stand outside under a moonless, country sky, you feel some ineffable tugging at your innards. We are star stuff. Keep looking up.” Neill de Grass Tyson, astronomer From the book, "Astronomical Tidbits: A Layperson’s Guide to Astronomy" (2010) 3  Essential Questions At the end of this unit, you should be able to answer the following questions. How were the light elements formed during the big bang and what evidence supports this? How do stars evolve? How were the heavier elements formed during star formation and what evidence supports this? How do nuclear fusion reactions take place? How are elements heavier than iron formed? Review Atoms are composed of protons, neutrons, and electrons. Protons and neutrons are in the nucleus while electrons are in the orbitals surrounding the nucleus. The atomic number (Z) of an element is the number of protons in the nucleus. The mass number (A) of an element is the total number of protons (Z) and neutrons (N). 4  Lesson 1.1: The Big Bang Theory and the Formation of Light Elements Objective In this lesson, you should be able to: give evidence for and explain the formation of the light elements in the big bang theory. According to the big bang theory, the universe went through a big explosion fourteen billion years ago. It was made up of hot particles with high energies. Through expansion, everything cooled down, making way for the formation of protons and neutrons. What do you think happened when these protons and neutrons combined? Warm-Up Big Bang Nucleosynthesis Before there were heavier elements, there were only hydrogen and helium. In this activity, you will observe how these elements came about from mere protons and neutrons. Materials: red marbles blue marbles Procedure: 1. Secure the required materials. Neutrons will be represented by blue marbles while protons will be represented by red marbles. 2. All players start with seven red marbles in their left hand, and one blue marble in their right hand. 3. Move around the room and “react” with each person you meet. Allowed reactions are shown below. Use only one particle each. 5  4. Do one round of rock-paper-scissors to see who gets to keep the new particle. 5. Keep all the particles you collected in your right hand. 6. The first one to get He-4 wins. Guide Questions: 1. Observe all nuclear reactions in the figure. Why is a proton-proton interaction not allowed? 2. Which two elements were formed as final products in the game? 3. Why do you think are these reactions allowed? Learn about It Cosmology Have you ever wondered where did Earth come from? Why are there stars, and what is beyond our planet’s atmosphere? Have you also wondered how the universe exactly looks like, what it is made of, and how do these components of the universe interact with one another? These questions can be answered by cosmology, the body of science that studies the origin, evolution, and eventual fate of the universe. Cosmological studies are conducted by groups of scientists across disciplines of chemistry, physics, and astronomy. 6 The origin of the universe has been explained in several ways. Religious or mythological cosmology explains the origin of the universe and life based on religious beliefs of a specific tradition. The concept of creatio ex nihilo which favors the belief that God, a creator deity, created all matter in the universe has been adapted by Greeks who established early Christianity. Since then, Christianity explains that the universe is created by God. The creation of Earth is outlined in the book of Genesis, as well as the creation of mankind and all forms of life. Physical cosmology, on the other hand, explains the origin of the universe based on scientific insights, studies, and experiments. Astrophysicists, chemists, geologists, and other scientists study physical cosmology, trying to figure out the physical origin of the universe. Early philosophers and scientists established most of the laws of the universe you currently know. Nicolaus Copernicus proposed that all systems in the universe, including our solar system, is heliocentric in nature. In this model, the sun is the center of the universe, and planets revolve around it. After a decade, Isaac Newton refined the Copernicus model and introduced the law of universal gravitation which extended the laws of classical physics in Earth to that of the universe. Back then, the universe was considered static and unchanging. This idea was not questioned until Albert Einstein published his final theory in general relativity. Advance modeling of the universe using Einstein’s mathematical concepts and models suggested that the universe is dynamic and constant changing. Nicolaus Copernicus Isaac Newton Albert Einstein (1473–1543) (1643–1727) (1879–1955) 7 Nowadays, modern physical cosmology supports the big bang theory, which explains that the universe is constantly expanding. Scientists believe that the formation of the universe, as well as the elements, can be explained with this theory. The Discovery of the Big Bang Theory The big bang theory is a cosmological model that describes how the universe started its expansion about 13.8 billion years ago. According to this theory, the universe is very dynamic. Scientific evidence shows that it is continuously moving and expanding. The idea that the universe is dynamic was first observed by Vesto Slipher and Carl Wilhelm Wirtz in 1910 where they discovered that most spiral galaxies were moving away from Earth. This phenomenon is known as redshift. Later in 1927, Georges Lemaître, a Belgian Catholic priest, suggested that these galaxies were not moving but instead proposed that the universe is expanding. In 1929, Edwin Hubble used the redshift of light to calculate the velocities of galaxies. He calculated how distant galaxies were from Earth. He described how distant galaxies were moving away from Earth and from each other. His calculations supported the theory that the universe is expanding. 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 was the cosmic microwave background radiation (CMBR), which spread out across space evenly. This radiation was believed to be energy remains. In 2014, astronomers and scientists used the refined model of the big bang and calculated that the universe is 13.8 billion years old and that approximately only five percent of it is ordinary matter. The formation of these matter was explained by the model. 8 The Big Bang Theory According to the theory, the universe began as a point called singularity. It is a hot, dense point containing all space, time, matter, and energy. There is “nothing” around the singularity, but in this nothingness is where the singularity expanded rapidly in a process known as inflation. Space was believed to first expand at speeds faster than light. Energy started expanding after and created matter and antimatter, although some of these pairs cancel each other in a process known as annihilation which brings back energy. As the universe expanded, it cooled down. Matter in the form of proton, neutron, electron, and photon is scattered in a highly energetic soup termed the plasma soup. This soup is where nuclei of light atoms start to form via nucleosynthesis or nuclear fission between protons and neutrons. Later on, electrons started to mingle with these nuclei in a primordial chemical process known as recombination. These particles, which are now called atoms, continued moving in space until energy, in the form of gravity, acted on these particles and collapsed them to form celestial bodies such as stars and galaxies. You will learn in succeeding lessons that heavier nuclei and matter form from these cosmological units. The universe continues to expand until today. Space continues to travel faster than matter and energy, increasing the distance between galaxies and matter. Big Bang Nucleosynthesis Big bang nucleosynthesis (BBN), also known as primordial nucleosynthesis, is the process of producing light elements during the big bang expansion. The American cosmologist Ralph Alpher was able to prove BBN with his calculations. He was able to calculate the proportions of neutrons and protons present in the early universe when the Big Bang started. With the right knowledge of these proportions and the energy present in the early universe, he was able to predict that elements such as hydrogen and helium can be formed. 9 Rapid cooling occurs as the singularity expanded rapidly, slowing the subatomic particles. BBN began 100 seconds after the big bang, and one process lasts for approximately three minutes, producing two stable isotopes of hydrogen, two isotopes of helium, some lithium atoms, and beryllium isotopes. How these light elements form are explained in detail below. 1. Deuterium (D), an isotope of hydrogen that has one proton and one neutron, was first formed from the fusion of a proton and a neutron, accompanied by the emission of a high-energy photon ( ). Binding energy is the energy required to break down a nucleus into its components. Deuterium (2H) was easy to break up because of its low binding energy. In the first few seconds after the big bang, deuterium was always destroyed by high-energy photons brought by intense temperatures. This situation is known as the deuterium bottleneck. It was not until after one hundred seconds that the temperature cooled down and became favorable for deuterium. The formation of this very small hydrogen isotope marked the beginning of the BBN cascade that ultimately produce heavier light elements such as helium, lithium, and beryllium. 2. Tritium (T), a radioactive isotope of hydrogen with one proton and two neutrons, was formed from the fusion of two deuterium nuclei, accompanied by a release of a proton. 10 3. Helium-3, an isotope of helium with one neutron and two protons, was formed from the fusion of two deuterium nuclei and a release of a neutron. 4. Helium-4, which has two neutrons and two protons, has a binding energy equivalent to 28 MeV. Further fusion of helium-4 was rare because the resulting atoms had lower binding energies than helium-4. It was produced from several nuclear reactions. He-4 can be initially formed when a proton fuses with a tritium atom. Aside from that, He-4 can also be formed when deuterium fuses with a tritium atom as shown below. Lastly, He-4 can be formed when deuterium fuses with a He-3. 11 5. Lithium-7, an unstable nucleus with three protons and four neutrons, was produced from the nuclear fusion of helium-4 and tritium. Lithium-7 decayed spontaneously to form two stable helium nuclei. 6. Beryllium-7, an unstable isotope of beryllium with four protons and three neutrons, was produced from the nuclear fusion of helium-3 and helium-4 accompanied by the emission of high energy photon. Beryllium-7 also reacts with a neutron and decays to the unstable lithium-7, with the subsequent release of a proton. 12 Big Bang Nucleosynthesis: The Bigger Picture The big bang nucleosynthesis predicted the formation of deuterium, tritium, helium-3, helium-4, lithium-7, and beryllium-7. The nuclear processes are summarized in the diagram below. The main reactants and products are presented in blocks, while other reactants and products in the format of (a,b) are presented above the arrows. The particle a reacts with the previous reactant producing or removing b along the process. The nuclear reaction proceeds according to the direction of the arrow. Fig. 1. The nuclear reactions predicted by big bang nucleosynthesis. To summarize, the early conditions of the universe has allowed the formation of these elements via BBN. At around one hundred seconds after the big bang, with the temperature cooled near 100 K (or -173.15oC), there were numerous protons and neutrons, present in a 7:1 ratio. The energy of the universe was also high enough for an exchange between protons and neutrons. As the temperatures cool further, neutrons quickly combined with protons to produce deuterium and then helium according to the processes previously discussed. 13 The correlation between the predicted and observed cosmic abundances of hydrogen and helium was the major proof of the big bang theory. Theoretical physicists calculated the abundances of primordial material based on the big bang theory and the big bang nucleosynthesis. Within a few minutes after BBN has started, they predicted that almost all available neutrons have combined with protons, forming 24% 4He by mass. About 93% of protons (hydrogen nuclei) or around 74% H by mass remained not combined. Collectively, hydrogen and helium were calculated to be the most abundant element in the universe, accounting for 98% of all matter by mass. Most of the nuclear reactions in BBN stopped as the temperature significantly dropped and prevented nuclear fusion. The relative abundances of hydrogen and helium remain nearly fixed until today. Only the spontaneous decay of tritium and lithium-7 happens after and continues to change elemental abundances. To verify their predictions, astronomers had measured abundances of primordial material in unprocessed gas in some parts of the universe with no stars. Parts of primitive asteroids known as chondrites commonly fall to Earth and provides scientists and astronomers insights on the origin, elemental composition, and age of the solar system as well as of the universe. They found out that indeed, hydrogen and helium are the most abundant elements in the universe. Key Points The big bang theory is a cosmological model that describes how the universe started its expansion about 13.8 billion years ago. Big bang nucleosynthesis (BBN), also known as primordial nucleosynthesis, is the process of producing light elements during the big bang expansion. The correlation between the predicted and observed cosmic abundances of hydrogen and helium was the major proof of the big bang theory. Theoretical physicists calculated the abundances of primordial material based on the big bang theory. Astronomers had measured abundances of primordial material such as chondrites and unprocessed gas in some parts of the universe with no stars. 14  Web Links For further information, you can check the following web links: Read about the most popular theory of our universe's origin centers on a cosmic cataclysm unmatched in all of history—the big bang. National Geographic. 2015. ‘Origins of the Universe’’ https://www.nationalgeographic.com/science/space/universe/origins-of-the-universe/ 3D Animation of The Big Bang Theory 3D Animator. 2010. ‘My 3D Animation of Big Bang Theory’ https://www.youtube.com/watch?v=YJJK9x1Ffhw Check Your Understanding A. Identify what is being described in the following statements. 1. These are the two most abundant elements in the universe. 2. This is a phenomenon termed for the movement of most spiral galaxies were away from Earth. 3. This radiation was believed to be the remains of energy created after the big bang. 4. This is the energy required to break down a nucleus into its components. 5. This is a radioactive isotope of hydrogen with one proton and two neutrons. B. Write T if the statement is true. Otherwise, change the underlined word to make it true. 1. Deuterium is easy to break because of its high binding energy. 2. Heavy elements were formed during primordial nucleosynthesis. 3. The correlation of the observed and predicted abundance of H and He in space is the major proof of the big bang theory. 4. He-4 is an isotope of helium with 2 protons and 4 neutrons. 5. Li-7 decayed spontaneously to produce helium nuclei. 15 C. Complete the table below. Write the number of protons and neutrons produced after the reaction. No. of protons No. of neutrons H 2 H + 3H 2 H + 3He 2 He + 4He 4 He + 3H 4 D. Complete the following diagram to summarize the synthesis of the elements during the big bang expansion. 16  Challenge Yourself Answer the following questions briefly and clearly. 1. Explain why further fusion of two He-4 is rare. 2. Explain how H and He were formed during primordial nucleosynthesis. 3. Explain why a neutron is released during the fusion of two deuteriums. 4. Explain how two He-4 nuclei are formed from He-4 and tritium. 5. What are your thoughts on the big bang theory? Do you have doubts or do you agree with it? 17  Lesson 1.2: Stellar Evolution and the Formation of Heavier Elements Objectives In this lesson, you should be able to: describe the evolution of stars; and give evidence for and describe the formation of heavier elements during star formation and evolution. Stars, which are giant balls mostly made up of hydrogen and helium, act as sites for nuclear reactions in the universe. Through the process, they are able to fuse light elements to form heavier elements. These reactions also involve light emission, which is the reason why stars are so bright. Where did you think atoms making up all living things originate from? Warm-Up Nucleosynthesis Game The evolution of stars is accompanied by many different reactions that lead to the formation of different elements from hydrogen and helium. Play this game with your classmates to demonstrate a few examples of these reactions. Materials: blue marbles red marbles ball magnet two six-sided dice the chart of the nuclides, which can be accessed via the link below The Chart of the Nuclides Joint Institute for Nuclear Astrophysics Center for the Evolution of Elements (JINA-CEE). 2017. ‘Chart of Nuclides’ http://www.jinaweb.org/outreach/marble/Marble%20Nuclei%20Project %20-%20Quick%20Reference%20Sheet.pdf 18 Procedure: 1. Secure the required materials. Neutrons will be represented by blue marbles while protons will be represented by red marbles. 2. Each team starts with a single hydrogen nucleus: 1 red marble stuck to a ball magnet. 3. On each turn, a team member rolls two dice and do the corresponding reactions: Die Reaction Instructions Example roll 3-4 Hydrogen Add one proton fusion 5-6 Absorb a Add one neutron neutron 7-8 Radioactive Refer to the chart of the decay nuclides: black box - do nothing pink diamond - remove 1 proton, add 1 neutron blue circle - remove 1 neutron, add 1 proton yellow triangle - remove 1 proton green checkerboard - remove 2 protons and 2 neutrons 9-10 Free choice Add either 1 proton or 2 neutrons 11 Helium fusion Add two protons and two neutrons 19  2 or Bombardment In each hand, hold your 12 nucleus and your opponent’s nucleus. Position your hands three feet apart and 6 inches off the ground. Drop simultaneously. Only the marbles attached to the silver magnet will be retained. 4. The first team to build a nucleus that has 8 protons or higher wins. Guide Questions: 1. Which nuclear reactions have your nuclei undergone? 2. Which reactions best helped you reach your goal nucleus? Why? 3. Which reactions set you back? Why? Learn about It The Synthesis of Heavier Elements The big bang model was able to explain how light elements such as hydrogen, helium, lithium, and beryllium were produced. As the universe cooled down, protons and neutrons started reacting to form deuterium, which consequently reacts in a cascade of reactions to form tritium and heavier elements. Few minutes after the big bang, the universe is filled with an abundant number of hydrogen and helium atoms, and some lithium and beryllium. No elements heavier than beryllium are formed during BBN because of the relatively short period of time before the temperature dropped significantly. At very low temperatures, there is not enough energy to fuse more neutrons to existing nuclei. But you know that the periodic table of elements does not end on beryllium. There are a lot of elements far bigger than beryllium. How are these heavier elements formed? Just like the universe continued to expand, the story of nucleosynthesis did not end at big bang nucleosynthesis. 20 Stellar Nucleosynthesis and the Formation of Stars Have you ever heard of the saying that humans are made from stars? It might be absurd to think that humans are children of the stars, but partly, the idea is true. Most elements that compose all living and nonliving things on Earth do come from stars. These elements are actually produced through processes that occur inside these stars and are carried all throughout the universe. Elements heavier than beryllium were formed through stellar nucleosynthesis. Stellar nucleosynthesis is the process by which elements are formed within stars. Hydrogen and helium formed from BBN begin combining in nuclear fusion reactions, releasing tremendous amounts of light, heat, and radiation. These nuclear fusion reactions will be discussed in the next lesson. According to the star formation theory, stars are formed when gravity started acting on matter and particles expanding with the universe. These dense regions of molecular clouds, known as stellar nurseries, collapse to form young stellar objects known as protostars which eventually become mature stars. The abundance of the elements in a star change as it evolves. Stellar evolution is the process by which a star changes during its lifetime. The primary factor that determines how stars evolve is mass. Formation of Main Sequence Stars All stars are born from clouds of gas and dust called nebulae or molecular clouds that collapsed due to gravity. As a cloud collapses, it breaks into smaller fragments which contract to form a superhot stellar core called a protostar. The protostar continues to accumulate gas and dust from the molecular cloud and continues to contract while the temperature increases. When the core temperature reaches about 10 million K, nuclear fusions and other nuclear reactions begin. Hydrogen will start combining with one another in a series of proton-proton fusion reactions. This cascade of proton reactions will be discussed in detail in the next lesson. The nuclear 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. 21 The sun is said to in the middle of its main sequence phase of stellar evolution and will continue to be in this phase for about five more billion years. Red, small stars called red dwarfs stay on the main sequence phase for hundreds of billions of years or longer. In these stars, hydrogen fuses slowly and core energy is stabilized. Formation of Red Giants Sooner, the proton-proton chain reactions will exhaust all hydrogen in the core of a main sequence star. Helium, which is the product of these nuclear fusion reactions, will become the major component of the core. Hydrogen fusion becomes significant on the outer shell, while some of it is also fused to the core’s surface. 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-burning occurs. Helium is converted to carbon in the core via alpha processes, increasing the star’s core density. These processes, which involve helium atoms (also called alpha particles), will be discussed in detail in the next lesson. Meanwhile, hydrogen is converted to helium in the shell surrounding the core, increasing the temperature up to 10 million kelvins. This increase in temperature is accompanied by an increase in pressure that pushes inert hydrogen away from the core. The star, at this point, has become a red giant. Fig. 2. Fusion of elements in a red giant. 22 Formation of a White Dwarf from Low Mass Stars When the majority of the helium in the core has been converted to carbon, the rate of alpha fusion processes decreases. Gravity again squeezes the star. The star’s fuel is depleted and over time, the outer material of the star is blown off into space as planetary nebula. The only thing that remains is the hot and inert carbon core. The star becomes a white dwarf. Fig. 3. White dwarf with inert carbon core. The composition of a white dwarf depends on how much mass is in it before it becomes such. The white dwarf discussed previously is assumed to have come from the main sequence low mass stars. White dwarf from stars with a size similar to most main sequence stars such as that of the sun does not contain enough energy to fuse carbon, and is thereby composed of inert carbon and oxygen atoms. On the other hand, a star of less than half the mass of the sun will produce a white dwarf that is mainly made up of helium and some unfused hydrogen. Formation of a Multiple-Shell Red Giant from Massive Stars Unlike low mass stars, the fate of a massive star (or high mass 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; silicon from neon fusion; and iron from silicon fusion. The star then becomes a multiple-shell red giant. 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. The fusion of two elements lighter than iron therefore releases energy. However, the fusion of two iron nuclei requires an input of energy. As a consequence, no elements heavier than iron are produced in the stars. 23  Fig. 4. Multiple-shell red giant. Formation of a Supernova 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 explosion also releases massive amounts of high-energy neutrinos which, in turn, breaks nucleons and release neutrons. These neutrons are picked up by nearby stars and lead to the creation of elements heavier than iron. Fig. 5. The supernova Cassiopeia A. 24 Evidences of Stellar Evolution and Nucleosynthesis The crucial piece of evidence that supports the stellar evolution and nucleosynthesis theory includes the discovery of the interstellar medium of gas and dust during the early part of the 20th century. Other pieces of evidence come from the study of different stages of formation happening in different areas in space and piecing them together to form a clearer picture. Energy in the form of infrared radiation (IR) is detected from different stages of star formation. For instance, astronomers measure the IR released by a protostar and compare it to the IR from a nearby area with zero extinction. Extinction in astronomy means the absorption and scattering of electromagnetic radiation by gases and dust particles between an emitting astronomical object and an observer. The IR measurements are used to approximate the energy, temperature, and pressure in the protostar. Key Points Stellar nucleosynthesis is the process by which elements are formed within stars. The primary factor that determines how stars evolve is mass. The star formation theory proposes that stars form due to the collapse of the dense regions of a molecular cloud. Stellar evolution is the process by which a star changes during its lifetime. ○ All stars are born from clouds of gas and dust called nebulae or molecular clouds that collapsed due to gravity. ○ As a cloud collapses, it breaks into smaller fragments which contract to form a superhot stellar core called a protostar. ○ The protostar continues to accumulate gas and dust from the molecular cloud and continues to contract while the temperature increases, forming a main sequence star. ○ Main sequence star transforms into red giants if hydrogen atoms successfully fuse to form the helium core. ○ When the core can no longer produce energy to resist gravity, the star undergoes an explosion, called a supernova. 25  Web Links For further information, you can check the following web links: Read about the birth, life, and death of a star. NASA. 2003. ‘Stellar Evolution’’ https://www.nasa.gov/audience/forstudents/9-12/features/stellar_evol_feat_912.html The Last Star in the Universe Kurzgesagt – In a Nutshell. 2016. ‘The Last Star in the Universe – Red Dwarfs Explained’ https://www.youtube.com/watch?v=LS-VPyLaJFM Check Your Understanding A. Identify what is being described by the following statements. 1. This stellar core is formed as fragments from the collapsing cloud contract. 2. This is the mechanism that explains how hydrogen is fused into helium in the core of a main sequence star. 3. This new element is formed from He in a red giant star. 4. It is the force that squeezes stars when mass, temperature or pressure is altered. 5. This is formed when a star becomes an inert carbon core. B. Write T if the statement is true. otherwise, change the underlined word to make it true. 1. The abundances of elements formed within the stars change as the stars evolve. 2. Nuclear reactions begin when the temperature gets extremely low. 3. A main sequence star is formed from a protostar. 4. When the majority of the helium in the core has been converted to carbon, the rate of fusion increases. 5. Carbon fusion can occur in low-mass stars. 26  C. Supply the missing information in the chart below. Challenge Yourself Answer the following questions briefly and clearly. 1. What important role does gravity play in star formation? 2. How does mass affect the fate of the stars? 3. Are nuclear fusion reactions of elements lighter than iron energy-requiring or energy-producing reactions? Why? 4. Why aren’t elements heavier than iron produced in the stars? 5. Explain how supernova happens. 27  Lesson 1.3: The Nuclear Fusion Reactions in Stars Objective In this lesson, you should be able to: write the nuclear fusion reactions that take place in stars. Fusion reactions occur in the stars and produce energy. This happens when light nuclei combine to form heavier ones. Hydrogen, helium, and lithium were produced in the Big Bang but the heavier elements were produced from nuclear reactions, making the stars “nucleus factories”. One famous example is the sun, which fuses hydrogen nuclei to make helium. Have you ever wondered what kinds of reactions happen in the stars? Warm-Up Stellar Fusion: the p-p chain Explore more fusion reactions that make our stars the “nucleus factory”. In this activity, you will encounter the reactions that produce the elements in our beloved star, the sun. You will again use marbles in a game to represent neutrons and protons. Discover how the cascade of fusion reactions known as the proton-proton chain works. Materials: six-sided dice blue and red marbles Procedure: 1. Secure the required materials. Neutrons will be represented by blue marbles while protons will be represented by red marbles. 2. All players start with 4 loose red marbles in their left hand and a six-sided die. 28  3. Find a partner, put one proton each on the table, and roll the die. If you got different numbers, the protons don’t stick and you take your proton back. If you got the same number, switch one of the protons with a neutron (blue marble). Roll the die again, and whoever gets a higher number keeps the marbles on the table in their right hand. 4. Find a new partner. If you both have loose protons, do step 3 again. If you have something else, perform any of the following reactions: 5. Continue doing step 4 until you get a He-4 (2 green + 2 yellow). Whoever gets it first wins. Guide Questions: 1. Which reactions frequently happened? Why do you think so? 2. Which reactions rarely happened? Why do you think so? 29  Learn about It Stellar Nucleosynthesis Stellar nucleosynthesis is the process by which elements are formed in the cores and overlying layers of the stars through nuclear fusion reactions. These reactions allow the formation of elements heavier than lithium, which is formed during the big bang nucleosynthesis (BBN). Arthur Eddington, George Gamow, and Hans Bethe are scientists known for their important contributions to the stellar nucleosynthesis theory. Arthur Eddington proposed that the stars get their energy from the nuclear fusion of hydrogen nuclei (based on the atomic mass measurements of F.W. Aston). He also proposed that heavier elements are formed in the stars. George Gamow derived a quantum mechanical formula for the probability of bringing two nuclei close enough such that the nuclear forces overcome the Coulomb barrier (also known as mutual electrostatic repulsion). He also derived the rate at which high-temperature nuclear reactions occur, much like in stellar cores. On the other hand, Hans Bethe studied how energy is produced in stars through hydrogen burning. Arthur Eddington Hans Bethe (1882–1944) (1906–2005) 30 Hydrogen and Helium Burning Hydrogen burning is a set of stellar processes that produce energy in the stars. It is a term used by astronomers for processes that result in the production of helium-4 from hydrogen. It has two dominant processes: first, the proton-proton chain reaction and second, carbon-nitrogen-oxygen cycle Proton-Proton Chain Reaction Proton-proton chain reaction is a chain reaction by which a star transforms hydrogen into helium. It occurs only when the kinetic energy of the proton is highly sufficient to overcome the Coulomb barrier. The main branch proton-proton chain has three steps. Fig. 6. Proton-proton chain reaction. First, two protons fuse to form a deuteron or deuterium nucleus. This reaction, which releases a positron or a positively charged electron and a neutrino, is called beta-plus decay. 31 Then, deuteron fuses with another proton to produce helium-3. This process is known as deuterium burning and consumes all deuterium produced in the previous step. A high energy photon is also produced in this fusion reaction. Lastly, two helium-3 nuclei fuse to form stable helium-4, with the release of two atoms of hydrogen. This set of reactions explains the formation of helium cores in main sequence stars and red giants. Carbon-Nitrogen-Oxygen Cycle The carbon-nitrogen-oxygen (CNO) cycle is the dominant source of energy in stars more massive than about 1.3 times the mass of the sun. This is also the main source of helium for such stars upon recycling 12C and finishing the whole cycle. The process is composed of six steps which involves repeated proton capture and beta-plus decay. Fig. 7. The C-N-O cycle: 12C → 13N → 13C → 14N → 15O → 15N → 12C. 32 First, carbon-12 fuses with hydrogen (also referred to as proton) to form nitrogen-13. This process is called proton capture. A release of high energy photons (or gamma rays) accompanies this fusion reaction. Second, nitrogen-13 undergoes a spontaneous beta-plus decay producing carbon-13 and subsequently releasing a positron and a neutrino. In this process, the proton is converted to a positron. Third, carbon-13 fuses with hydrogen to form nitrogen-14. A release of high energy photons (or gamma rays) accompanies this fusion reaction. Fourth, another proton capture happens where nitrogen-14 fuses with hydrogen to form oxygen-15. Gamma rays are also produced in this reaction. Fifth, oxygen-15 decays spontaneously to nitrogen-15. Similar to all beta-plus decay reactions, a positron and a neutrino are released as side products. Lastly, nitrogen-15 fuses with hydrogen to form carbon-12 and helium-4. This last proton capture reaction recycles 12C and produces 4He. Helium Burning Helium burning is a set of stellar nuclear reactions that uses helium to produce heavier elements such as beryllium, oxygen, neon, and iron. It involves two different processes: first, the triple-alpha process, and second, the alpha process. 33 Triple-Alpha Process The triple-alpha process is a set of nuclear fusion reactions that start with three helium-4 nuclei (also called alpha particles) that are converted to carbon-12. It occurs in two stages. This triple-alpha process creates the inert carbon core found in white dwarfs and larger stars. Fig. 8. The triple-alpha process. First, two helium-4 nuclei fuse to form beryllium-8. This reaction is accompanied by a release of high energy gamma rays. Then, beryllium-8 fuses with another helium-4 nucleus to form the stable carbon-12. Beryllium-8 is a very unstable isotope, hence, it either decays or forms 12 C. Alpha Process The alpha process, also known as the alpha ladder, is a set of nuclear reactions that convert helium into heavier elements. The reactions consume helium, and the sequence ends at iron. Iron-56 is the most stable element, having the lowest mass to nucleon (the total number of protons and neutrons) ratio. Alpha processes increase the size and density of the core by forming heavier elements and are vital in transforming main sequence star to supergiants. 34 The nuclear reactions involved in the alpha process always involve the capture of an alpha particle. For example, carbon-12 captures an alpha particle (helium-4) to make oxygen-16. Oxygen-16 captures an alpha particle to produce neon-20. The process continues where the product captures an extra alpha particle, producing iron-52 as the ultimate product. The reactions always release high energy gamma rays. The series of alpha processes are shown below. Key Points Stellar nucleosynthesis is the process by which elements are formed in the cores and overlying layers of the stars through nuclear fusion reactions. Hydrogen burning is a set of stellar processes that produce energy in the stars. ○ Proton-proton chain reaction is a chain reaction by which a star transforms hydrogen into helium. ○ The carbon-nitrogen-oxygen (CNO) cycle is composed of six steps which involves repeated proton capture and beta-plus decay. 35  Helium burning is a set of stellar nuclear reactions that uses helium to produce heavier elements such as beryllium, oxygen, neon, and iron. ○ The triple-alpha process is a set of nuclear fusion reactions that start with three helium-4 nuclei (also called alpha particles) that are converted to carbon-12. ○ The alpha process, also known as the alpha ladder, is a set of nuclear reactions that convert helium into heavier elements. Web Links For further information, you can check the following web links: Read about the world’s first nuclear fusion plant. Austin, M. 2017. ‘The world’s first nuclear fusion plant is now halfway to ‘First Plasma’ https://www.digitaltrends.com/cool-tech/iter-nuclear-fusion-reactor-halfway-complete/ The race to create star on earth! Visit this link! Motherboard. 2017. ‘Nuclear Fusion Energy: The Race to Create Star on Earth’ https://www.youtube.com/watch?v=knrHPneSN10 Check Your Understanding A. Match Column A with Column B. Column A Column B 1. He proposed that heavy A. C-N-O cycle elements are formed in stars. 2. This has to be overcome in B. deuterium order for p-p chain to occur. 3. This fuses with another proton C. Hans Bethe to produce He-3. 36  4. He studied how energy is D. Coulomb barrier produced in stars through hydrogen burning. 5. This is the dominant source of E. hydrogen burning energy in massive stars. 6. This is also a product when N-15 F. Arthur Eddington fuses with C-12. 7. This involves alpha and G. helium burning triple-alpha process. 8. Two He-4 nuclei fuse to form H. alpha process this product. 9. This is a set of reactions I. He-4 converting He to heavier elements. 10. Energy is released in the C-N-O J. Be-8 cycle in the form of this radiation. K. proton-proton chain reaction L. gamma ray M. beta particle B. Provide the products of the following nuclear reactions. 1. → ______________ 2. → ______________ 3. → ______________ 4. → ______________ 5. → ______________ 6. → ______________ 7. → ______________ 8. → ______________ 9. → ______________ 10. → ______________ 37  Challenge Yourself Answer the following questions briefly and clearly. 1. How is He-4 formed via the proton-proton chain? 2. Why is the C-N-O cycle the dominant source of energy in massive stars? 3. What is the significance of hydrogen burning in the synthesis of elements lighter than iron? 4. How is C-12 formed via the triple-alpha process? 5. Why is triple-alpha process called as it is? 38  Lesson 1.4: How Elements Heavier Than Iron Were Formed Objective In this lesson, you should be able to: describe how elements heavier than iron are formed via neutron and proton capture. As previously discussed, stars can create heavier elements by nuclear fusion reactions but elements heavier than iron result from neutron capture processes. In overview, it happens when a stable nucleus absorbs a neutron, making it heavier and unstable so it releases energy. In the process, it turns a neutron into a proton, thus forming a different element. The process goes on forming other heavier elements. But how do heavier elements came about? S- and R-process Simulation Elements heavier than iron cannot be created by simple fusion reactions that form the lighter ones. In this activity, you will simulate the processes that happen in forming heavier elements. Materials: red marker neutron capture processes chart, which can be accessed via the link below Neutron Capture Processes Chart Joint Institute for Nuclear Astrophysics Center for the Evolution of Elements (JINA-CEE). 2017. ‘Neutron Capture Process Chart’. http://www.jinaweb.org/outreach/marble/Neutron%20capture%20process% 20chart.pdf 39 Procedure: A. Familiarizing the Chart 1. Familiarize yourself with the legends and conventions used in the chart. 2. Nuclides in gray boxes are stable. Their half-lives are almost forever. 3. Nuclides in blue circles and red diamonds are unstable. Their half-lives are written below the element symbol. B. Marking the Chart 1. If a nucleus undergoes neutron capture, it moves to the left. 2. If a nucleus undergoes beta minus decay, it moves up and to the left. 3. If a nucleus undergoes beta plus decay, it moves down and to the right. C. S-process Simulation Slow neutron capture processes (s-processes) occur every 10 years. If the half-life of a nucleus is shorter than 10 years, it undergoes decay. Otherwise, it undergoes neutron capture. First four steps for s-process simulation 1. Start with Fe-56. Since its half-life is infinity, it undergoes a neutron capture. Thus, you draw an arrow to the right. 2. Go to the nucleus where the arrow is pointing to and repeat the same assessment. 40  3. If you reach a nucleus that you think is going to decay, you can identify the type of decay from the legends in Part B. 4. Continue until you reach the endpoint, which is the heaviest isotope of Sr. D. R-process Simulation Rapid neutron capture processes (r-processes) occur every 100 ms. If the half-life of a nucleus is shorter than 100 ms, it undergoes decay. Otherwise, it undergoes neutron capture. 1. Do the same procedure as in Part C except for this time, the capture time is way smaller so more nucleus is going to undergo capture rather than decay. 2. Continue until you reach the endpoint, which is the heaviest isotope of Sr. Guide Questions: 1. Did the s-process simulation produce nuclei that are close to the line of stable isotopes (gray) or far from it? 2. Were all the stable isotopes created from the s-process? 3. Which stable isotopes came from the r-process? Nucleosynthesis is the process by which new nuclei are formed from preexisting or seed nuclei. So far, you have learned that elements lighter than beryllium-7 were produced based on the processes of big bang nucleosynthesis, while those elements heavier than it was synthesized in the processes involved in stellar nucleosynthesis and evolution. However, the fusion reactions in stellar nucleosynthesis cannot produce nuclei higher than iron. Above iron, fusion reaction becomes unfavorable because the nuclear binding energy per nucleon, the energy that holds the nucleus intact, is smaller for these heavier elements. As a consequence, the reaction of iron capturing protons, neutrons, or alpha particles would require more energy. The reactions are nonspontaneous and different pathways are needed for the synthesis of heavier nuclei. Synthesis of heavier nuclei happens via neutron or proton capture processes. Recall that when supernova is formed, a massive amount of neutrinos are released in the 41 universe, which hit nucleons and kick off neutrons and protons. These neutrons and protons can be captured by nuclei present in nearby stars. Neutron Capture In neutron capture, a neutron is added to a seed nucleus. The addition of a neutron produces a heavier isotope of the element. where X is the seed nucleus with atomic number Z and mass number A, n is a neutron, and Y is the product nucleus. For example, iron-56 captures three neutrons to produce iron-59. The generated isotope, when unstable, undergoes beta decay. Beta-decay results in an increase in the number of protons of the nucleus by one. Hence, a heavier nucleus is formed. It is represented by the general reaction below. 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. Slow neutron capture or s-process happens when there is a small number of available neutrons. It is termed slow because the rate of neutron capture is slow compared to the rate of beta decay. Therefore, if a beta decay occurs, it almost always occurs before another neutron can be captured. S-processes occur mainly on red giant or supergiant stars. These are very slow processes, where each neutron capture takes a decade and the cascade of processes takes thousands of years. The main neutron sources are carbon-13 and neon-22, which upon taking up alpha particles, produce one neutron each and 42 elements of oxygen-16 and magnesium-25, respectively. The earlier fuels up the production of heavy elements such as strontium, yttrium, and even lead, while the latter fuels up the production of primarily iron. Rapid neutron capture or r-process happens when there is a large number of available 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 beta decay. 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 because the addition of a proton to the nucleus is not favorable because of Coulombic repulsion (the repulsive force between particles with the same charge). Proton capture produces a heavier nucleus that is different from the seed nucleus. For example, molybdenum-94 undergoes proton capture to produce technetium-95. Key Points Stellar nucleosynthesis fusion reactions cannot produce nuclei higher than iron. Synthesis of heavier nuclei happens via neutron or proton capture processes. 43  In neutron capture, a neutron is added to a seed nucleus. The addition of a neutron produces a heavier isotope of the element. Neutron capture can occur slowly or rapidly. ○ Slow neutron capture or s-process happens when there is a small number of available neutrons. This is usually associated with red giant or supergiant stars. ○ Rapid neutron capture or r-process happens when there is a large number of available neutrons. This is usually associated with supernovas. Beta-decay results in an increase in the number of protons of the nucleus by one. Hence, a heavier nucleus is formed. Proton capture or p-process is the addition of a proton in the nucleus. Web Links For further information, you can check the following web links: Read about how neutron capture can be used against cancer. Otake, T. 2016. ‘Japanese researchers to test new weapon on unbeatable cancers’ https://www.japantimes.co.jp/news/2016/04/06/national/science-health/japanese-rese archers-to-test-new-weapon-on-unbeatable-cancers/#.W1hHxdgzbOQ Proton therapy for cancer? Visit this link. University of California Television (UCTV). 2011. ‘Proton Therapy for Cancer’ https://www.youtube.com/watch?v=knrHPneSN10 Check Your Understanding A. Given the half-life, assess whether the following nuclides will undergo s-process, r-process, or decay. 1. Fe-70 (77 ms) 6. Zn-65 (243.93 d) 2. Sr-88 (stable) 7. Cu-69 (2.85 m) 3. Ge-77 (11.21 h) 8. Se-82 (stable) 4. Se-81 (18.45 m) 9. Co-74 (30 ms) 5. Kr-78 (stable) 10. Ga-85 (93 ms) 44 B. Complete the following nuclear reactions: 1. ______ 6. _____ 2. _____ 7. _____ 3. _____ 8. _____ 4. _____ 9. _____ 5. _____ 10. _____ Challenge Yourself Answer the following questions briefly and clearly. 1. When does a nucleus undergo beta-decay? 2. Why does proton capture happen after a supernova? For questions 3–5, refer to the figure below and answer the succeeding questions. 3. How does the average binding energy per nucleon change with increasing mass numbers? 4. Describe the position of iron-56 in the curve. What is its implication? 5. Which elements are synthesized via nuclear fusion reactions? 45  Laboratory Activity Activity 1.1 Fragmentation Box Objectives At the end of this laboratory activity, the students should be able to: setup a gravity-based marble accelerator; simulate acceleration, collision, mass/energy variation, nuclear interactions, and neutron capture using the marbles and accelerator. Materials and Equipment magnetic marbles (red for 3 straight PVC pipes protons and blue for neutrons) 1 90-degree PVC pipe ball magnets 2 Y-adapter fittings plastic box/basin metal mesh Procedure A. Acceleration 1. Connect the pipes and the fittings together to create the acceleration tube and then attach it to the plastic basin as shown in the figure above. 2. Put a metal mesh right in front of the accelerator exit tube at the top of the plastic box. 3. Drop a single blue marble from the higher hole. 4. Drop another blue yellow marble from the higher hole. 5. Observe the difference. 46 B. Collision 1. Build a carbon-12 nucleus by attaching 6 red marbles and 6 blue marbles to a ball magnet center as shown in the figure below. C-12 target nucleus 2. Stick the center of the C-12 nucleus to the nail hanging through the metal mesh as shown in the figure below. This will be the “target nucleus”. Target nucleus position 3. Drop a single blue marble from the higher hole. 4. Observe and describe the result. 5. Reset your target nucleus if necessary. 6. Drop another single blue marble from the higher hole. 7. Observe and describe the result. 47 C. Mass/Energy Variation 1. Build a He-4 nucleus by attaching 2 red marbles and 2 blue marbles to a ball magnet center as shown in the figure below. He-4 target nucleus 2. Reset the target nucleus from Part C. 3. Instead of using a single proton marble, drop the He-4 nucleus from the lower hole. 4. Observe and describe the results. 5. Reset the target nucleus and the He-4 if necessary. 6. Drop the He-4 nucleus from the higher hole. 7. Observe and describe the results. D. Glancing Collision 1. Reset and reposition the target nucleus so that it is not directly in front of the accelerator exit tube as shown in the figure below. Target nucleus position 48  2. Drop the He-4 nucleus from the lower hole. 3. Observe and describe the results. 4. Reset the target nucleus and the He-4 if necessary. 5. Drop the He-4 nucleus from the higher hole. 6. Observe and describe the results. E. Neutron Capture 1. Use the He-4 nucleus as the target instead of C-12 and place it directly in front of the accelerator exit tube as shown in the figure below. Target nucleus position 2. Drop a single blue marble from the higher hole. 3. Observe if the target nucleus “captures” the blue marble. 4. Reset your target nucleus if necessary. 5. Do steps 2-4 ten times and record how many times the capture was successful. 6. Do steps 2-5 but using C-12 as the target nucleus. Data and Results A. Collision observations Mass/energy Glancing Acceleration Collision variation collision Low energy Marble dropped from the lower hole 49  High energy Marble dropped from the higher hole B. Neutron capture percentage chance No. of capture Percentage chance Target Nucleus (out of 10 times) (%) He-4 C-12 Guide Questions 1. What could have caused the differences in dropping the marble from different heights? 2. What could have caused the differences from accelerating a He-4 nucleus instead of a single proton marble? 3. What could have caused the differences from the impact parameters? (Direct vs. glancing collision) 4. Which has a higher percentage chance of neutron capture between He-4 and C-12? Why is this so? Performance Task Historical Development of the Atom Goal Your task is to make a creative representation of the historical development of an atom in a timeline. Role Your job is to research about a specific atom of your own preference and creatively make a timeline of its historical development. Audience The target audience is your classmates and teacher. 50 Situation The challenge involves making sufficient research and application of the knowledge you learned in this chapter to effectively and creatively convey the timeline. Product, Performance, and Purpose Your work will be judged by your teacher and your classmates upon presentation. You should also be able to answer their questions about it. Standards and Criteria Your performance will be graded by the following rubric. Needs Successful Exemplary Below Expectations, Criteria 0% to 49% Improvement Performance Performance 50% to 74% 75% to 99% 100% Content and Details not Details are Details are Details are presented. presented but not presented in an presented in an Creativity. Content is not organized. There organized manner. organized matter Detailed facts are related to the task. are some content Content is related that can be easily presented well. that is not related to the task. understood. Content related to the to the task. Content is related task. to the task. Additional supporting details are presented. Communication The presentation The presentation The presentation The presentation was not done. was done but in a was done was done clearly. Skills. disorganized and smoothly but the Concepts were The presentation was illogical manner. concepts are presented in a done in a clear and presented in such logical manner logical manner. a way that should and easily be rearranged for understandable by better the audience. understanding. 51  Self Check After studying this unit, can you now do the following? Check I can… give evidence for and explain the formation of the light elements in the big bang theory. describe the evolution of stars. give evidence for and describe the formation of heavier elements during star formation and evolution. write the nuclear fusion reactions that take place in stars. describe how elements heavier than iron are formed via neutron and proton capture. Key Words Big bang theory It is a cosmological model that describes how the universe started its expansion about 13.8 billion years ago. Big bang It is the process of producing the light elements during nucleosynthesis the big bang expansion. Extinction In astronomy, it means the absorption and scattering of electromagnetic radiation by gases and dust particles between an emitting astronomical object and an observer. Hydrogen burning It is a set of stellar processes that produce energy in the stars. 52 Nucleosynthesis It is the process by which new nuclei are formed from pre-existing or seed nuclei. Proton capture It is also called p-process and it is the addition of a proton in the nucleus. Rapid neutron It is also called r-process and it happens when there is a capture large number of available neutrons. Star formation theory It proposes that stars form due to the collapse of the dense regions of a molecular cloud. Stellar It is the process by which elements are formed within nucleosynthesis stars. The abundances of these elements change as the stars evolve. Stellar evolution It is the process by which a star changes during its lifetime. Slow neutron capture It is also called s-process and it happens when there is a small number of available neutrons. Supernova It is the star explosion. Wrap Up Nucleosynthesis of Elements 53  Photo Credits Unit Photo. Milky Way-100 billion stars by NASA, ESA is licensed under public domain via Wikimedia Commons. References Clayton, D.D. 1968. Principles of Stellar Evolution and Nucleosynthesis. Chicago, USA: University of Chicago Press. Constan, Z. “Learn Nuclear Science with Marbles.” National Science Foundation 2017. Accessed July 13, 2018. http://www.jinaweb.org/outreach/marble/Marble%20Nuclei%20Project%20- %20Activities%20Student%20Worksheet.pdf. Langer, N. “Nucleosynthesis.” Bonn University SS 2012. Accessed December 8, 2016. https:// astro.uni-bonn.de/~nlanger/siu_web/nucscript/Nucleo.pdf. National Aeronautics and Space Administration. “The Big Bang.” Accessed December 8, 2016. http://science.nasa.gov/astrophysics/focus-areas/what-powered-the-big-ban g/. National Geographic. “Origins of the Universe—An Expanding World.” Accessed December 8, 2016. http://science.nationalgeographic.com/science/space/universe/origins-univer se-article/. Overton, Tina, et al. 2010. Shriver and Atkins’ Inorganic Chemistry. 5th ed. London: Oxford University Press. 54