Instructions
Answer    each    question    based    on    your    knowledge    of    Earth    Science    and    the    information    provided.    
The     Atom    

Answer    each    of    the    following    questions    based    on    the    information    provided within “The    Atom” section    of    the    
reading and    your    knowledge    of    Earth    science.       
1. What    is    an    atom?    
2. What    are    the    three    subatomic    particles    found    in    an    atom    and    the    charges    of    each?
3. Which    subatomic    particle    is    responsible    for    the    charge    of    the    nucleus    of    an    atom?    
4. How    is    the    atomic    number    of    an    atom    determined?   

What    is    Nuclear    Fusion?    
Answer    each    of    the    following    questions    based    on    the    information    provided within “What    is    Nuclear    Fusion?”
section    of    the    reading and    your    knowledge    of    Earth    science.        
1. Define    nuclear    fusion.    
2. Identify    the    type    of    energy    that    is    released    as    a    result    of    nuclear    fusion.
3. Why    is    high    temperature    needed    in    order    for    nuclear    fusion    to    occur?
4. Determine    the    two    types of    forces    that    are    responsible    for    the    energy    released    during    nuclear    fusion.    
5. How    does    the    size    of    an    atom    influence    the    coulomb    force?    

Nuclear    Fusion    in    the    Stars
Answer    each    of    the    following    questions    based    on    the    information    provided within “Nuclear    Fusion    in    the    
Stars” section    of    the    reading and    your    knowledge    of    Earth    science.        

1. Identify    the    force    that    holds    a    star together.
2. Where    does    nuclear fusion    take    place    within    a    star?    
3. Describe    what    happens    to    the    gaseous materials    within    a    star    due    to    the    extreme    heat    and    pressure    
conditions.
 
5. When is    a    star    no    longer    able    to    carry    out    nuclear    fusion?    
Types    of    Nuclear    Fusion
Answer    each    of    the    following    questions    based    on    the    information    provided within “The    Types    of    nuclear    
fusion” section    of    the    reading and    your    knowledge    of    Earth    science.        
   
2. What    determines    the    type    of    nuclear    fusion    that    will    occur    in    a    star?
  
5. Why    are    larger    main    sequence    stars    able    to    carry    out    nuclear    fusion    for    a    longer    period    of    time    than    
smaller    main    sequence    stars?    

Nuclear    Fusion    and    the    Sun
Answer    each    of    the    following    questions    based    on    the    information    provided within “Nuclear    fusion    and    the    
Sun” section    of    the    reading and    your    knowledge    of    Earth    science.        
1. Identify    the    type    of    star    our    Sun    is.
2. What    type    of    nuclear    fusion    occurs    in    our    Sun?
3. What    is    the    approximate    age    of    our    Sun    and    what    is    the    estimated    amount    of    time    left    in    its    current    
evolutionary stage?
4. How    far    away    is    the    Sun    from    Earth?
5. Why    is    nuclear    fusion    important    for    life    on    Earth?

 

ARTICLE:

The Universe
What is Nuclear Fusion?

The Atom
All objects in the universe are composed of the smallest known unit of matter in the universe: the atom. An atom is an extremely small particle that contains three subatomic particles: Protons, Neutrons, and Electrons. 
Each subatomic particle is identified based on its overall charge. Protons and neutrons are the largest of the three subatomic particles. Both subatomic particles are located within the Nucleus, and each has a mass of 
approximately 1.674 x 10-27 kg. This means that most of the mass of an atom is the found within the nucleus of an atom. 

Each subatomic particle serves a specific function within the 
atom. Protons are positively charged particles that are found 
within the nucleus. These small particles are responsible for 
identifying an atom’s atomic number as well as the specific 
element that the atom is. Each element is associated with a 
specific number of protons. For example, nitrogen, as seen in 
Figure 1, contains seven protons within its nucleus. This means that 
it has an atomic number of 7 and all nitrogen atoms will contain 
seven protons within its nucleus. If the number of protons were to 
be changed an entirely new atom with different properties would be 
produced. 

The smallest subatomic particle, the electron, does not add much 
mass to the atom. The main function of the electrons is to 
determine how the atom will bound with other atoms to form 
compounds. Each compound will then have a positive or negative charge, depending on the ratio between protons and electrons. If an atom has more electrons than protons, it will have a negative charge and will need 
to bound with an atom that has a positive charge, or more protons than electrons. The goal of each compound is to create neutrally charged atoms in which each individual atom has a complete other electron shell. Two atoms 
and/or compounds will then bound together to form a neutrally charged compound. Neutrons do not carry a charge at all. Instead, the number of neutrons within the nucleus determines the stability of the atom. For each element a stable element is one with an equal number of protons and neutrons. 
However, some atoms may contain additionally neutrons within its nucleus. Each variation of an atom is known as an Isotope. Occasionally, the unequal number of protons and neutrons in the nucleus creates an unstable 
isotope. These isotopes will eventually undergo a process in which one or more of the neutrons will transform into protons. This is known as Radioactive Decay and results in a new stable atom. 

Atoms can also be transformed when exposed to a tremendous amount of heat and/or pressure. Under specific conditions, typically only found with the stars in the Universe, smaller atoms will combine to create new larger
atoms. This process is known as Nuclear Fusion and continues within the star until it has run out of smaller elements that can be used to carry out nuclear fusion. 
     
What is Nuclear Fusion?
Nuclear fusion is the process of two or more nuclei “fusing” together to produce a new, larger atom. This process will only occur under the extreme heat and temperature conditions that are found within the stars in our 
universe. It is essentially the “fuel source” of the stars because a tremendous amount of energy is required for the process to occur as well as a product of the collision between two or more atoms. The type of energy 
involved in both the process and end results of nuclear fusion is known as Electromagnetic Energy. 
Electromagnetic energy is energy that radiates outwards in all directions from a given source, usually a star. Stars are the only known place in our universe where the extreme environmental conditions needed for nuclear 
fusion to occur is found. Our Sun carries out a type of nuclear fusion in which two hydrogen atoms combine to form a helium atom. This is modeled above in Figure 2. During the fusion of the atoms electromagnetic energy is
released into the environment and serves as an energy source everything in the universe. You can view a nuclear fusion reaction like a car collision you might see in a movie. Two cars slam into each other and get stuck permanently together while little pieces of them go flying off in every direction. The process of nuclear fusion is much the same where the cars are atomic nuclei and the little pieces are various particles, such as a neutron, and electromagnetic energy waves are released into the environment during the collision.
The energy released during nuclear fusion is the result of two opposing forces within lighter atoms: the Nuclear Force and the Coulomb Force. The nuclear force combines protons and neutrons together in the nucleus of an 
atom and the Coulomb force causes protons to repel each other due to their positive charge. This occurs best in lighter atoms because they have fewer protons and therefore it is easier to overcome the repulsion that occurs 
due to the positive charge of each proton. Larger atoms have a stronger Coulomb force, so energy is required in order for nuclear fusion to occur. 

Nuclear Fusion in the Stars      
Stars are large, luminous spheres of gases that is held together by the forces of gravity. They are the largest celestial objects found throughout the 
Universe as well as the only known place in the Universe where nuclear fusion will occur naturally. 
This makes stars extremely important.  A star is composed of layers with the Core at its center, as seen in Figure 3. Due to the force of gravity, the matter within all layers of the star is constantly being pulled inwards towards its core. 
This creates extreme high pressure and temperature conditions within the center of the star which are ideal for carrying out nuclear fusion. 
Under the extreme environmental conditions within a star’s core, atoms are able to carry out nuclear fusion, or the process of smaller atoms 
combining to form new larger atoms. This is because the extreme high temperatures will transform the gaseous materials into a new state 
of matter: plasma. When in the plasma state, atoms have the energy needed to overcome the coulomb force that typically hold an atomic nucleus together. At the same time, the high pressure forces atoms to move close enough to 
one another to become fused into new atoms. This gives the atoms the 
ability to fuse together to form a new atom, carrying out the process of nuclear fusion. 
A star will continue to carry out nuclear fusion until it runs out of fusible atoms. When this occurs, the balance that existed between the outward release of energy and the inward pull of gravity shifts and the force of gravity will cause the star to collapse upon itself. Typically, the collapse of a star is not the end of its life. If the star is in its early or middle evolutionary stage prior to a collapse, the matter of former star will evolve into a different star 
type. This new star will contain larger atoms than the previous star. Therefore, a different type of nuclear fusion will occur that will produce even larger atoms. 

Types of Nuclear Fusion
There are three main types of nuclear fusion that occurs within the stars in our universe: Carbon Fusion Cycle,Triple-Alpha Process, and Proton-Proton Fusion. The type of nuclear fusion carried out by a star depends on The type of atoms that are found within the star. This changes with the star’s mass, environmental conditions (temperature and pressure), as well as the age of the star. These factors also influence the how the classification 
of a given star. Most known stars today are classified as Main Sequence Stars that carry out Proton-Proton Fusion. This is due to the significant amount of single-proton hydrogen atoms found within Main Sequence Stars. 
    
In proton-proton fusion, as seen in Figure 4, hydrogen atoms are fused together through several steps to eventually become a helium-4 atom. Initially two hydrogen atoms collide forming a single deuterium atom, a positron, and a neutrino. Therefore, Deuterium is an isotope of hydrogen because it still 
contains one proton. The three new particles will only exist 
within the specific conditions associated with nuclear fusion 
and therefore have specific characteristics. A positron is a 
particle with the same mass and magnitude of charge as an 
electron but has a positive charge instead of a negative one and 
a neutrino is a charge-less particle with a mass of nearly zero. 
After the initial collision of the hydrogen atoms, the deuterium 
atom fuses with a proton to form a helium-3 atom and releases
gamma rays as a result. Finally, the helium-3 atom fuses with 
another helium-3 atom that was created in the same process 
from another two hydrogen atoms. This creates a single helium4 atom and two protons. One of the biggest factors that determines the type of nuclear fusion that occurs within a star is the age of the 
star. This is because the number of atoms that are able to be fused together is not infinite. Young stars, classified as Main Sequence Stars, are filled with small atoms that can frequently carry out nuclear fusion. These 
stars will carry out nuclear fusion for billions of years before moving onto their next life stage. In fact, most stars that have been found in the universe are Main Sequence stars and still contain enough atoms to carry out nuclear fusion. 
While many of the known stars today, including our Sun, are Main sequence stars, not all Main Sequence Stars are the same size and temperature. Larger Main Sequence stars have a higher mass and a higher surface 
temperature. This means that larger stars contain more atoms that can be used to carry out nuclear fusion more frequently and for a longer period of time. Additionally, larger main sequence stars will also follow a different 
evolutionary path than medium and small size main sequence stars. All of these factors influence the type of nuclear fusion that is carried out by a star during each of its evolutionary stages. As a star moves through its lifespan, the type of atoms within its core change. In its early stage of life a star 
contains mainly hydrogen and helium atoms and therefore carry out nuclear fusion that involves hydrogen atoms. These atoms will fuse together to form larger helium atoms, as seen below in figure 4. As the type of atoms within a star changes, it has fewer atoms that can carry out the current type of nuclear fusion. Eventually,the star will evolve into a different type of star and the type of nuclear fusion that occurs will change. 

Nuclear Fusion and the Sun
Like all stars within our Universe, the Sun carries out nuclear fusion that results in a tremendous amount of energy being released out into the Solar System. This energy radiates outwards from the Sun in all directions providing energy to carry out a variety of processes throughout the Universe. Essentially, there would be no life on Earth is if were not for the Sun’s energy. 
The Sun is located in the center of our Solar System and is classified as a Yellow Main Sequence Star that is approximately 4.7 billion years old. This means that the Sun is in its earliest evolutionary stage and is carryout out Proton-Proton Nuclear fusion. Based on current data,      
scientists have determined that the Sun still have roughly 4.6 billion years left in its current evolutionary stage before it becomes a Giant Star.
The process of proton-proton nuclear fusion that occurs within our Sun is essential for all life on Earth. While it is nearly 93 million miles away, the light and heat that is produced because of this reaction travels through Space towards Earth, providing the energy needed to carry out many, if not all, processes on Earth. If the Sun did not exist, then life on Earth will not be possible. In fact, the Sun is the only known star in the Universe that is able to produce energy that sustains life on a nearby planet.