1. Formation of Heavier Elements
An explanation of how elements heavier than Fe are produced in supernovae
AUTHORS:
Jasmeet K Dhaliwal, Scripps Institution of Oceanography, UCSD
Jason Moore, Mira Mesa High School, San Diego
SUMMARY: This lesson focuses on the heaveir elements produced in supernovae reactions. It is a short addendum to the previous lesson, but provides a context for understanding the periodic table (i.e. alpha processes are important, but other reactions like decay and s- and r- processes are necessary for the formation of many elements)
CONTEXT FOR USE: This lesson explains the final step of stellar nucleosynthesis, through the explosion of stars that have exhausted all their energy. This is an important concept because it demonstrates how energy-consuming (endothermic) reactions resulting in heavier elements require the energy of a supernova. This is the final lesson in this unit, and at this point, the students should be familiar with the entire periodic table.
MISCONCEPTIONS:
1) The periodic table is organized in the order of the discovery of the elements
2) All elements produced in the supernova are immediately stable

2. Periodic Table
The periodic table is used to organize the chemical elements by their atomic number
Atomic number: sum of protons.
Atomic Mass: sum of the protons and neutrons.
The origin of the elements is in stars and supernova, as explained in earlier lessons.
MAIN POINT: The periodic table and understanding its organization is the main motivation for developing the entire unit.
TEACHER NOTES:
The main point about the periodic table is that is constructed on the basis of atomic number, which is equivalent to the number of protons in an atomic nucleus. The number of neutrons in an atom can vary, which gives rise to isotopes (discussed in the next slide). The number of electrons (and electron shell configurations) are also related to an element’s place on the periodic table; however this is beyond the scope of this lesson.
A central point to linking stellar nucleosynthesis with the periodic table is to understand that all atoms up to iron (Z=26) can be formed in stars, through fusion processes (and decay, for odd elements). All atoms heavier than iron require an energy input, which is provided by a Type II supernova. During this process, atomic nuclei are bombarded with neutrons and form heavier nuclei. Out of these elements, the stable nuclei remain (these are stable isotopes; see Valley of Stability on slide 5) and the others decay rapidly.
REFERENCES:
Periodic Table:
PICTURE/GRAPHICS CREDITS: 2

3. Isotopes
Isotopes are atoms of the same atomic number, but different atomic mass.
This is because they have a different number of neutrons.
All isotopes of the same element have the same number of protons
(This is necessary, because an elements identity is based on the proton number)
MAIN POINT: Some elements have more than one isotope, which consist of a different number of neutrons and therefore a different mass.
TEACHER NOTES:
Isotopes are different forms of a particular element, with a different number of neutrons in the nucleus. This results in isotopes having different atomic masses (but the same atomic number, because the number of protons does not change). In the example above, three isotopes of hydrogen are shown; they all have one proton, but different number of neutrons (and therefore different masses).
Different isotopes of the same element are represented by the same place within the periodic table. Isotopes of a particular element have different abundances, and these typically depend on the mechanisms of formation (in the stellar environment), and nucleus stability.
For example, oxygen has three stable isotopes (O-16, O-17, O-18), and of these three, O-16 is by far the most abundant and this is related to the mechanisms of its formation. In this case, O-16 is made directly at the end of helium fusion (after the triple alpha process, with an addition of an alpha particle to a C-12 nucleus). Therefore, it could be produced by stars only consisting of hydrogen, making it a primordial isotope that has been present in the universe from very early on.
REFERENCES:
Isotopes:
O-16 :
PICTURE/GRAPHICS CREDITS:

4. Supernova Explosion
When the star’s core consists of Fe, the star can no longer balance inward gravitational energy with outward thermal pressure.
This results in ‘core collapse’ in which the star beings to implode.
This then results in a rebound that creates an outward shock wave.
The energy released results in a supernova explosion.
MAIN POINT: A Type II Supernova explosion occurs through core collapse. This phenomenon results in the formation of elements heavier than Fe.
TEACHER NOTES:
There are two types of Supernova: Type I and Type II.
Type I supernova typically occur in white dwarfs. While the exact mechanism is unclear, one explanation is that the white-dwarf reignites carbon fusion, which leads to a rapid nuclear fusion chain reaction that releases energy, resulting in a supernova. Supernova are distinguished on the basis of spectral emission and Type I supernova characteristically have no hydrogen-emission line.
Type II supernovae occur in active stars that undergo core collapse and a subsequent rebound and explosion. When the core of this star consists entirely of Fe and the inward gravitational pressure is greater than the outward thermal pressure (from nuclear fusion), the star undergoes core collapse. In this, the star actually implodes on itself, pushing all the atoms closer together and resulting in electron degeneracy (where electrons are in the same energy levels). When this occurs and a star is smaller than 1.44 solar masses, the star then experiences rebound (so that the electrons can maintain different energy levels). This causes the entire star to rebound, sending out a shock wave, which then releases energy and results in a supernovae Type II explosion.
These Type II supernovae are important for stellar nucleosynthesis because elements heavier than Fe can only be produced in this manner. The reason for this is that the formation of elements heavier than Fe require an energy input (and therefore are not energy generating); this energy is provided by the explosion. The elements form through neutron (r: rapid and s: slow) and proton capture (p) processes. This large output of energy causes the formation of many different types of nuclei, but eventually only the stable nuclei remain. These are shown on the Valley of Stability (slide 5).
REFERENCES:
Supernova:
PICTURE/GRAPHICS CREDITS: 4

5. Valley of Stability
Atomic stability is determined by an ideal ratio of protons to neutrons (seen in the diagram).
For elements, there are multiple isotopes possible.
The stable isotopes are located along the black line.
The other regions in color consist of radioactive isotopes.
MAIN POINT: The Valley of Stability explains the configurations of atoms that are most stable. These are based on the ratio of protons to neutrons.
TEACHER NOTES:
The diagram shows the “Valley of Stability,” in which the black line indicates the stable nuclides (stable isotope species), which do not spontaneously undergo radioactive decay. Any nuclide that is not on the black line is unstable and subject to nuclear decay. The types of decay are summarized below.
Elements that have only one stable isotopic are monoisotopic. Many common elements have multiple stable isotopes, such as oxygen (O-16, O-17, O-18); however, they occur in different abundances and O-16 is by far the most common.
Provided a long enough half life, isotopes that are unstable can be used for dating. A half-life is the time it takes a nuclide to decay to half of its original amount. For any given decay system, the dating is valid for approximately 6 half lives (e.g. if C-14 has a half life of 5730 years, so dating using C-14 is valid for ~34,000 years).
Radioactive Decay:
Beta: This can be beta minus (emission of an electron, e-) or beta plus (emission of a positron, e+).
Beta minus: This type of decay results in the atomic number increasing by 1 (this is because a neutron decays and loses a negative charge, resulting in an extra proton).
Beta plus: This type of decay results in the atomic number decreasing by 1 (this is because a proton decays and loses its positive charge, resulting in an extra neutron).
In both instances, the mass remains the same because a neutron and proton have the same mass.
Alpha: An atomic nucleus emits an alpha particle (He-4 nucleus) and therefore decreases in atomic number by 2 and in atomic mass by 4.
Spontaneous Fission: A large unstable nucleus decays into two or more smaller nuclei or particles
Proton Emission: A proton is emitted from the nucleus of an atom. This decreases both the atomic number and atomic mass by 1.
Neutron: A neutron is emitted from the nucleus of an atom. This decreases the atomic mass by 1, but the atomic number remains the same (because no change in proton number).
REFERENCES:
Stable Nuclide:
Radioactive Decay:
PICTURE/GRAPHICS CREDITS: 5

6. Made of Stars
This image is titled “The Pillars of Creation” and was taken by the Hubble Telescope in 1995.
It is an image of a gas and dust in the Eagle Nebula.
It is an iconic image and is a reminder that the beginnings of the Solar System and life on Earth come from these primitive clouds of dust and gas.
MAIN POINT: The idea that “We are all made of stars” is true, because the formation of the elements occurs in stars and supernovae. These atoms then become the building blocks of planets and also the life that has come to evolve on Earth.
REFERENCES:
Pillars of Creation:
PICTURE/GRAPHICS CREDITS: 6

7. Summary
Elements heavier than iron (Fe) are formed during a Type II supernova explosion
The elements on the periodic table are organized according to their atomic number (Z, which is equivalent to the number of protons).
Isotopes are different forms of the same element, with different atomic masses. This is because isotopes of an element have a different number of neutrons (but the same number of protons).
The stability of a nucleus is determined by the ratio of neutrons to protons. Unstable nuclides are subject to radioactive decay 7