1. An introduction to the hydrogen burning and the alpha particle
Nuclear Fusion
An introduction to the hydrogen burning and the alpha particle
AUTHORS:
Jasmeet K Dhaliwal, Scripps Institution of Oceanography, UCSD
Jason Moore, Mira Mesa High School, San Diego
SUMMARY: This lesson focuses on presenting information about the hydrogen atom and the building blocks of atoms. It explains the main process of hydrogen burning (proton-proton fusion) in stars that generates helium atoms. The purpose is to provide the necessary background for understanding the formation and structure of alpha particles, as this is the basis for larger atoms.
CONTEXT FOR USE: This is a basic introduction to atoms and their building blocks (subatomic particles) in the context of hydrogen-burning in stars. This will provide background for understanding how heavier elements are formed from the fusion of helium atoms.
MISCONCEPTIONS:
Atoms can be seen in a microscope
Atoms can be divided
The identity of an atom depends on what it looks like
The atom is the smallest unit of matter

2. Subatomic Particles The subatomic particles combine to form an atom
Proton:
Positively charged, one mass unit, located in nucleus
Neutron
Neutral, one mass unit, located in nucleus
Electron
Negatively charged, no mass, located in electron cloud (surrounding nucleus)
MAIN POINT: Review of subatomic particles
TEACHER NOTES: Present and review basics of atomic structure and the properties of protons, electrons, and neutrons.
Subatomic particles are smaller than atoms and are the building blocks of atoms. For a simple model of an atom, these consist of protons, neutrons and electrons. The protons and neutrons both have a mass of one, while the mass of an electron is considered negligible. The protons and neutrons are located within the nucleus of an atom, while the electrons spin freely in an ‘electron cloud’ that surrounds the nucleus.
The number of protons is equal to the atomic number and defines the identity of an atom. The number of neutrons can vary for a given element; these are called isotopes and will be discussed later in Lesson 5.
REFERENCES:
Proton:
Neutron:
Electron:
PICTURE/GRAPHICS CREDITS: 2

3. Basics of an Atomic Nucleus
An atom is defined by the number of protons it has in its nucleus
Atomic Number:
Number of protons
Atomic Mass:
Sum of neutrons and protons
Hydrogen Nucleus
Atomic Number: 1
Atomic Mass: 1
Atomic Mass (A) 1 H 1 +
MAIN POINT: The basics of an atom: atomic number and atomic mass
TEACHER NOTES:
The subatomic particles consist of electrons, protons and neutrons. Electrons have a negative charge and negligible mass. Protons are positively charged with a mass of 1 atomic mass unit (amu). Neutrons also have a mass of 1 amu, but are charge-less particles.
The atomic nucleus is found at the center of an atom. It is a very dense region and is made up of protons and neutrons. Around the nucleus is the electron cloud, in which electrons orbit the nucleus. The mass of an atom is concentrated in its nucleus, and the contribution from electrons is almost negligible. Since neutrons and protons both have a mass of 1 amu, the mass number (A) of an atom is equal to the sum of protons and neutrons within its nucleus.
The atomic number, or proton number, is equivalent to the number of protons in the nucleus. The atomic number of the nucleus is typically represented by Z. The atomic number is synonymous with the chemical identity of an element. While the number of neutrons and electrons can change for a given element, if the proton number changes, this results in an entirely different element. This is a key point—that the atomic number (i.e. the proton number) defines the identity of an element.
In addition, for a neutrally charged atom, the number of protons is equivalent to the number of electrons (so positive and negative charges are balanced). The number of neutrons is not necessarily equivalent to the number of protons; this is related to concepts of stability and will be re-visited in lesson 5.
REFERENCES:
Atomic Nucleus:
Atomic Mass:
PICTURE/GRAPHICS CREDITS:
Authors (Dhaliwal, Moore)
Atomic number (Z) 3

4. Main Sequence Stars Main sequence stars are like our sun
They have an balance between outward thermal pressure and inward gravitational pressure.
This balance is called hydrostatic equilibrium.
The outward push from thermal pressure
The inward push from gravity
The pressure is generated from the thermal energy from nuclear fusion in the core
MAIN POINT: Hydrogen Burning Occurs in the Cores of Main Sequence Stars
TEACHER NOTES:
All main sequence stars are stabilized by hydrostatic equilibrium, in which outward thermal pressure is balanced with inward gravitational pressure. This outward thermal pressure is produced by the hot core, and the fusion within it. The inward gravitational pressure is a result of the mass of the star. The rate of energy generation is important in maintaining this balance, as heat is carried away from the core by radiation or convection.
Main sequence stars are represented by the main band on the previously introduced H-R diagram. Its location along the main sequence is largely based on its mass, but other factors include its chemical composition, which also affect its temperature and luminosity. The thermal energy of a main sequence star is produced by hydrogen burning (or proton-proton fusion) within its dense core region.
When a star no longer has sufficient thermal energy (generated by nuclear fusion) to balance the gravitational energy, it can undergo core-collapse. This is an important first-step in the generation of Type II Supernova, and will be discussed in Lesson 5.
REFERENCES:
Main Sequence Stars:
PICTURE/GRAPHICS CREDITS: 4

5. Hydrogen Burning
The process of hydrogen burning occurs in the core of stars
It is also called ‘proton-proton fusion.”
It consists of 3 steps.
It requires temperature of 10 million K or hotter
This process is the longest stage in a star’s life
MAIN POINT: Hydrogen Burning is a Key Process in Stars
TEACHER NOTES:
Hydrogen burning describes the process in which the fusion of protons ultimately leads to the formation of a Helium-4 nucleus (also known as an alpha particle). This occurs in the cores of main sequence, Sun-sized stars and takes place through a proton-proton chain reaction. The hydrogen burning process is important because it both provides energy for stars, and produces alpha particles, which help form larger, heavier elements.
The proton-proton chain reaction consists of three steps, each of which is described in greater detail in Slides 6-9.
In the first step, two protons fuse at very high temperatures to create a Deuterium nucleus (in this step, one of the protons actually becomes a neutron, through beta-plus radioactive decay; see slide 6). Deterium has an atomic number of 1 and an atomic mass of 2 and therefore is a heavy isotope of hydrogen.
In the second step, the deuterium nucleus fuses with a proton to form a Helium-3 nucleus, which consists of 2 protons and 1 neutron.
In the third step, two Helium-3 nuclei fuse together. This is an energetic reaction that results in the release of 2 protons. The final product is a Helium-4 nucleus, with 2 protons and 2 neutrons; this is also referred to as the alpha particle.
REFERENCE:
Hydrogen Burning:
PICTURE/GRAPHICS CREDITS: 5

6. Proton – Proton Fusion Step 1
Fusion: the combining of 2 or more atoms to create a new, larger atom
Hydrogen burning, or proton-proton fusion, occurs in main sequence stars.
Forms a Deteurium atom (“heavy Hydrogen atom”)
Atomic Number: 1
Atomic Mass: 2 1 H + + 2 1 H n 1 + H
MAIN POINT: Hydrogen Burning (Step 1): Two proton colliding
TEACHER NOTES:
The proton-proton chain reaction, or hydrogen burning, is one of the main reactions that convert hydrogen to helium and is important in generating energy for main sequence stars. This reaction can only begin if the kinetic energy (i.e. temperature) of the protons is high enough to overcome their repulsive forces (since they are both positively charged particles). This occurs in the core of a star with a minimum temperature of 3 million Kelvin degrees.
The first step in this chain reactions consists of two protons that fuse together to produce a deuterium nucleus (1 proton and 1 neutron). In fact, the two protons fuse together to form a “diproton” (or Helium-2). This species then decays to a deuterium nucleus (or Hydrogen-2) through beta-plus decay. This occurs because of one of the protons decays to a neutron. In beta-plus decay, a proton decays and loses its positive charge (positron emission), resulting in an extra neutron. This type of decay results in the atomic number (A) decreasing by 1 (but the mass number remains the same).
For the purposes of this lesson, the exact type of radioactive decay and the particles released need not be focused on. The point to make is that under high pressures and temperatures, two protons can overcome repulsive forces for fusion, but that for the species created to be stable, one of these protons must decay to a neutron so that the repulsive forces are dissipated.
In this section, have the students pair up or form groups of three. Distribute the supplies to each group (see supplemental information). After the overview of hydrogen burning and its purpose in stars, explain this first step in detail. Before moving on to the next step, have the students model the first step in the proton-proton chain reaction by gluing pom poms on the paper so that it looks like the diagram in the slide.
REFERENCES:
Proton-proton chain reaction:
PICTURE/GRAPHICS CREDITS: Authors (Dhaliwal, Moore)

7. Proton-Proton Fusion Step 2
Deuterium nucleus combines with a regular Hydrogen nucleus
Forms a Helium-3 nucleus:
Atomic number: 2
Atomic mass: 3 2 + H 3 1 n He + 2 + n 1 H + 1
MAIN POINT: Introduce the steps of proton-proton fusion
TEACHER NOTES:
In the second step of the proton-proton chain reaction, the deuterium nucleus produced in the earlier step can now fuse with a proton to produce a Helium-3 nucleus. In this step, the fusion is a simple combination (without any change in particle-type) and results in a nucleus with 2 protons and 1 neutron (where the deuterium provides 1 proton and 1 neutron, along with the single proton). This is a Helium-3 nucleus. This energy also produces energy and a gamma particle.
As in the previous slide, explain the second step of proton-proton fusion and have the students model it with their materials before moving onto the last and final step.
REFERENCES:
Proton-proton chain reaction:
PICTURE/GRAPHICS CREDITS: Authors (Dhaliwal, Moore) 7

8. Proton-Proton Fusion Step 3
Two Helium-3 nuclei combine to form a Helium-4 nucleus
The Helium-4 nucleus:
Atomic number: 2
Atomic mass: 4
This is also known as an alpha particle + 3 He + 4 2 + He 2 n + n 3 + He + n 2 +
MAIN POINT: Introduce the steps of proton-proton fusion
TEACHER NOTES:
In the final step of the proton-proton chain reaction, two Helium-3 nuclei (2 protons and 1 neutron each) fuse to produce a single Helium-4 nucleus (also called an alpha particle). In this reaction, the final nucleus has 2 protons and 2 neutrons, meaning that two protons are released during the reaction (proton emission). This reaction is most efficient between 10 and 14 million Kelvin degrees, and produces energy for the star.
As in the previous slide, explain the second step of proton-proton fusion and have the students model it with their materials before moving onto the last and final step.
REFERENCES:
Proton-proton chain reaction:
PICTURE/GRAPHICS CREDITS: Authors (Dhaliwal, Moore) n + 8

9. Alpha Particle
The 4He atom is known as an alpha particle
It is one of the building blocks of larger elements 4 He + 2 n + n
MAIN POINT: Introduce the students to the alpha particle
TEACHER NOTES:
The alpha particle, or Helium-4 nucleus, consists of 2 protons and 2 neutrons. It has an atomic number of 2 and an atomic mass of 4 (sum or protons and neutrons). In stars, it is produced during the third step of hydrogen burning (or proton-proton chain reaction). It is an important particle, because it is not only the end-point of hydrogen burning, but can produce larger, heavier nuclei during the alpha process. The alpha process (or alpha fusion) is a method by which stars convert helium nuclei (alpha particles) into heavier elements.
Because of the number of protons and neutrons in an alpha particle, the heavier elements produced by the alpha process have an even number of protons and neutrons. Elements with odd atomic numbers can subsequently produced by radioactive decay or from other reactions, such as during a supernova. The basic method by which alpha particles are used to create the nuclei of heavier elements will be the focus of the next lesson.
OBJECTIVES:
Alpha Particle:
PICTURE/GRAPHICS CREDITS:
Authors (Dhaliwal, Moore) 9

10. Summary Atoms are made of subatomic particles.
These consist of protons, neutrons and electrons.
An atom is described by its atomic number and atomic mass.
The gravitational pressure in stars is balanced by an outward thermal pressure.
The energy is generated by nuclear fusion in stars.
Hydrogen burning is a nuclear fusion process which results in the formation of alpha particles (Helium-4 nuclei).
This occurs through proton-proton fusion, which is a 3-step process. 10