7.1.2 – Research – Fusion in Stars
Stars are giant nuclear reactors. In the center of stars, atoms collide in a process called nuclear fusion, where two
atoms will collide and fuse into one larger atom (usually kicking out some waste product). Fusion always consists of
taking 2 smaller things and combining them to make 1 larger thing. We are going to start off discussing the fusion of
hydrogen into helium, however larger stars can combine larger elements to form elements like oxygen and carbon,
and even larger ones.
Chemistry Recap
Atoms are made of protons, neutrons, and electrons. Protons (positive charge) and
neutrons (neutral charge) are found in the nucleus, or center of the atom. Together, they
account for essentially all of the mass of the atom. Every single element, and atom, will
have an atomic number and an atomic mass. The image to the right shows you that the
atomic number is always the smaller of the two, and is essentially the number of protons
in an atom. This is important, because the number of protons dictates which element you have. 1 protons =
Hydrogen. 8 protons = Oxygen. 11 protons = Sodium. 17 protons = Chlorine. Each of those elements will have a
larger mass than that however, which is what the other number on the periodic table is indicating.
Each element has different isotopes, or versions of each other that have different amount of neutrons. Because they
have a different amount of neutrons, they also have a different mass. For example: Hydrogen has 3 different
versions: Hydrogen-1, Hydrogen-2, and Hydrogen-3. Each one has one proton in the nucleus. In H-1, that is the
only thing in the nucleus, thus the mass of 1. In H-2, there is a 1 proton and 1 neutron in the nucleus, giving a grand
total of 2 particles in the nucleus. H-3 has 1 proton and 2 neutrons, totaling 3
particles in the nucleus. Some isotopes are way more common and stable than
other isotopes. For instance, H-1 accounts for 99.98% of all hydrogen atoms. H-2
accounts for 0.058%, while H-3 accounts for 0% of all atoms because it is unstable
and breaks down into a simpler form very soon after formation.
From here on out, we
are going to write elements in
isotope notation, which
simply lists the element symbol,
with the mass number on the top left and the atomic
number on the top right. See the
image of lithium to the right for a guide. To find the
number of neutrons of an atom,
you must subtract the protons from the mass of an
element. For example, with that
lithium atom, we know that there are 3 protons in the
nucleus based on the atomic
number of 3. We also know there are 7 total things in the
nucleus, based on the mass
number of 7. If we wanted to find the number of
neutrons, we would simply
subtract those two numbers like this: ( 7 total nucleus particles – 3 protons in the nucleus = 4 neutrons in the
nucleus).
***Answer the chemistry questions before moving on to the next section.
Nuclear Fusion + Making New Elements
Stars are constantly undergoing nuclear fusion. On the scale that stars do it, nuclear fusion produces enough energy
to keep the star “alive” for billions of years. Nuclear fusion is the process of two smaller atoms slamming together to
form one larger atom. There is a problem with this however. The protons, being positive, don’t want to collide with
each other. They won’t do it unless the pressure and temperature are high enough that it essentially forces them to
collide together. Once that temperature is reached however, nuclear fusion that converts hydrogen atoms into
helium atoms will begin.
Essentially, stars (and our Sun) spend over 90% of their lives completing the Proton-Proton Chain reaction, which is
a 3 step process that turns hydrogen into helium.
As you can see to the right:
1. Slam two Hydrogen-1 atoms
together to form a Hydrogen-2 atom
(called deuterium)
2. Slam another Hydrogen-1 at the
Hydrogen-2 atom to get Helium-3.
3. Slam together two Helium-3 atoms
to get Helium-4 (and release two
Helium-1 atoms)
Go to this link and scroll 1/3 of the way down to the title How to Turn Hydrogen into Helium:
http://www.kcvs.ca/martin/astro/au/unit4/92/chp9_2.htm Notice how we aren’t too concerned with the waste
products of this reaction, except for that every time we complete this process a little bit of energy is released.
However, when you account for the trillions of times this happens every single second in the Sun, you start to
understand how much energy is being released by nuclear fusion: enough to power the Sun for billions of years.
Stars that are the size of our Sun and smaller spend their entire life cycle in the main sequence performing ProtonProton Chain Reactions.
Once a star’s core has been converted from hydrogen to helium, if the star is large enough and the temperature is
high enough, the helium may fuse to form carbon. This is what our Sun will do, because the size of our sun dictates
it will never endure enough pressure to fuse heavier elements. This is the second nuclear fusion reaction phase of a
star.
Once stars stray outside the main sequence, specifically giants and red
giants, they will fuse different, heavier elements in their core by using the
CNO, or Carbon-Nitrogen-Oxygen process. It is the same basic idea as
proton-proton chain reactions, just with larger starting atoms, and it is
much more efficient at producing energy. The temperature inside the star
governs the rate of nuclear fusion. Bigger stars will have more temperature
and pressure, meaning they can fuse even bigger atoms. The star’s element
composition is determined by how many fusion reaction phases it has gone
through, with each new element than can fuse being a different fusion level.
Elements such as hydrogen and helium will still undergo fusion on the
outside, but as pressure and temperature increase towards the core,
carbon, neon, oxygen, and silicon begin form. Each element is formed in a
different fusion stage of the star, with there being a maximum of 7 fusion
stages. The final stage creates Iron, which is the heaviest element to form
via fusion within massive incredibly high temperature and pressure stars.
This only occurs in incredibly massive stars, but once Iron forms, the core
generally implodes and the star will experience supernova.
When certain stars (those that are 10x to 30x bigger than the sun) explode in a massive explosion, known as a
supernova, elements heavier than iron are created and enrich the universe. The includes elements just heavier than
Iron, Cobalt with 27 protons, all the way up to the largest natural element created in supernova, Uranium with 92
protons.
Review Questions
Chemistry Questions
1. What is the atomic number?
2. What is the atomic number important?
3. What subatomic particles makeup the atomic mass?
4. What are the charge of protons and neutrons?
5. What is an isotope?
6. Give me a quick example.
7. Give me 3 examples of isotopes?
8. Based on the periodic table, write the isotope notation for:
b. Hydrogen
Helium
Oxygen
Chlorine
Iron
9. Based on the periodic table, tell me how many protons there are, neutrons there are, or what the mass is:
Isotope Name
Isotope Notation
Protons
Neutrons
(Element + Mass)
Helium – 4
Lithium – 7
Sodium - 23
Iron – 56
Hydrogen – 1
Hydrogen - 2
Fusion
10. What is nuclear fusion?
11. What are high temperatures essential for nuclear fusion to occur?
12. What are the 3 basic steps of the Proton-Proton Chain Reaction?
13. What allows helium atoms to start fusing?
14. If I were to say a star increased its number of fusion reactions, what do I mean?
15. What is the heaviest element that can be made inside a star because of fusion?
16. How many fusion stages are necessary to create this element via fusion inside the star?
17. What makes elements that are heavier than that heaviest element made inside a star because of fusion?