Nuclear Fusion – Energy for Stars Activity

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Plymouth State University *

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Chemistry

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Jan 9, 2024

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Name: ________Jack Tortolani_____________________ Date: ______11/7/23_______________ Nuclear Fusion – Energy for Stars (50 points) A Nuclear Furnace Go to: http://www.seasky.org/cosmic/sky7a01.html and answer the following questions using the “A Nuclear Furnace” section. 1. The nuclear reactions inside a star, such as our sun, convert hydrogen into helium by means of a process known as _____nuclear fusion__________________________________ . 2. How many protons does a standard hydrogen atom have in its nucleus? ___1_______ 3. The two isotopes of hydrogen are _________Deuterium________________ and ________tritium___________ . 4. What gives a star its energy? Include in your explanation how the isotopes create the energy. Answer: A star derives its energy primarily from nuclear fusion in its core. In the core of a star, hydrogen nuclei (protons) combine to form helium nuclei through a process known as nuclear fusion. This fusion process releases a tremendous amount of energy in the form of light and heat. The key steps in this process involve the conversion of hydrogen isotopes: 1. **Proton-Proton Chain:** In most stars, such as our Sun, the primary fusion process is the proton-proton chain. It involves the conversion of hydrogen-1 (1H) isotopes into helium-4 (4He) isotopes through a sequence of nuclear reactions. In these reactions, protons fuse together, and some are transformed into neutrons through the weak force. This chain of reactions releases energy in the form of gamma rays, which eventually reaches the star's surface and is emitted as visible light. 2. **CNO Cycle (in some stars):** In more massive stars, another fusion process called the CNO (Carbon-Nitrogen- Oxygen) cycle can occur alongside the proton-proton chain. This process involves the conversion of carbon, nitrogen, and oxygen isotopes as catalysts in the fusion of hydrogen into helium. The energy generated through these nuclear fusion processes is what makes a star shine brightly and provides the heat and light that sustain the star's existence. This energy balance, where the outward pressure from the energy generation counteracts the gravitational forces trying to collapse the star, is what maintains a star's stable existence over billions of years. 5. What happens when all the hydrogen inside a star is used up? Answer: When all the hydrogen inside a star is used up, the star goes through a series of significant changes depending on its mass. Here are the typical evolutionary stages for a star like our Sun: 1. **Expansion into a Red Giant:** As hydrogen in the core gets depleted, the star's core contracts and heats up, while the outer layers expand. The star becomes a red giant, with the outer envelope expanding to many times its original size. During this phase, the star fuses hydrogen in a shell around the helium-rich core. 2. **Helium Fusion:** In the core of the red giant, helium fusion begins. Helium nuclei fuse together to form carbon and oxygen through the triple-alpha process. This releases energy, causing the star to temporarily stabilize. Helium fusion occurs in a shell around the core.

3. **Helium Shell Burning:** The star experiences a series of helium shell flashes, leading to expansions and contractions. These helium shell flashes may cause the outer layers to be expelled in a series of pulsations, forming a planetary nebula. 4. **Formation of a White Dwarf:** Eventually, the core becomes mostly carbon and oxygen, and the outer layers are expelled, leaving behind a hot, dense core called a white dwarf. A white dwarf is supported by electron degeneracy pressure and gradually cools over billions of years, becoming a faint white dwarf. For more massive stars, the process is different. They can go through successive fusion stages, creating heavier and heavier elements in their cores, ultimately leading to a supernova explosion, and what's left behind may be a neutron star or a black hole. The exact fate of a star depends on its mass. Lower-mass stars like the Sun end their lives as white dwarfs, while more massive stars have more complex evolutionary paths, including the possibility of a supernova explosion. 7. What elements do massive stars have the potential to convert in to? Answer: Massive stars have the potential to convert lighter elements into heavier ones through a series of nuclear fusion reactions in their cores. This process, which occurs during different stages of a massive star's life, leads to the production of various elements. 1. **Hydrogen:** Massive stars start with hydrogen fusion, converting hydrogen into helium through a series of reactions. 2. **Helium:** Once the core is mostly helium, massive stars can fuse helium into heavier elements, such as carbon, oxygen, and neon. 3. **Carbon and Oxygen:** These elements can be fused into even heavier elements like magnesium, silicon, sulfur, and iron. 4. **Iron:** Iron is the culmination of this fusion process. Fusion reactions beyond iron do not release energy but instead require energy input. Massive stars can produce a range of elements, with iron being the limit of their core fusion. Elements heavier than iron are primarily formed in supernova explosions and through other astrophysical processes. 8. Do massive stars convert into lighter or heavier elements? (See a periodic table if necessary). Answer: Massive stars convert lighter elements into heavier ones through a series of nuclear fusion reactions in their cores. This process starts with the conversion of hydrogen into helium and progresses to even heavier elements like carbon, oxygen, and iron. Fusion in Stars Video Lesson Visit the Fusion in Stars Lesson Once you open the lesson, watch the lesson video (8:47). Answer the questions that follow using the video and/or the video transcript located directly below the video. Highlight your answer. 1. Once the hydrogen in the core of a star runs out, what element does the star begin to fuse next?

a. Lithium b. Beryllium c. Helium d. Carbon e. Boron 2. Nuclear fusion only happens in the of _____ a star, where the ________ and _______ are high enough to allow nuclear fusion to occur. a. outer layers, pressure, viscosity b. outer layers, temperature, pressure c. core, temperature, pressure d. core, temperature, viscosity e. core, temperature, gravitational potential 3. Once the hydrogen and the helium in the core of a star runs out, what element does the star begin to fuse next? a. Beryllium b. Carbon c. Boron d. Iron e. Lithium 4. Which two of the following elements are not formed in a main sequence star that gives out radiation at a steady rate? a. Carbon and Francium b. Plutonium and Oxygen c. Iron and Francium d. Plutonium and Carbon e. Plutonium and Francium 5. Which of the following elements can only be formed in a supernova? a. Hydrogen

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b. Helium c. Uranium d. Iron e. Carbon 6. What process is the source of energy in main sequence stars? a. Nuclear fission b. Nuclear fusion c. Oxidization 7. Which element do main sequence stars primarily use for nuclear fusion? a. Helium b. Oxygen c. Iron d. Carbon e. Hydrogen 8. Plutonium-239 is an isotope of plutonium with a half-life of approximately 24,000 years. It can be formed in a nuclear reactor, but it is not found naturally. Which of the following reasons explains why this is? a. Heavy elements such as plutonium are only formed naturally in supernovas. All of the heavy elements in our solar system were created in a supernova that occurred before the solar system was formed. Since this was more than 4 billion years ago, all of the plutonium that was created in the supernova has decayed. b. Heavy elements such as plutonium were only created during the big bang. Since the big bang occurred 13.8 billion years ago, all of the plutonium that was created in it has decayed. c. Creating plutonium requires far more energy than any natural process can provide. Plutonium can only be created in nuclear reactors. 9. What is the most common element in the universe? a. Carbon b. Oxygen c. Helium d. Hydrogen

e. Neon 10. The heat generated through nuclear fusion in a star’s core exerts an outward force on the material around it. This would cause the star to expand, but it is balanced by another force acting upon the material in the star, which keeps it stable. What is the other force acting on the matter in the star? a. Gravity b. Electrostatic attraction c. Electrostatic repulsion d. Pressure

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