Overview

The Big Bang created lots of hydrogen and helium, and trace amounts of lithium. But in today’s universe, we have over one hundred different elements. Where did they all come from?

Science Question

What are the different nuclear processes which create light and heavy atoms, and where do we find them in everyday life?

Physics

Stars shine in the sky not because they are on fire, but because they are powered by nuclear reactions which create light. The inside of a star is an extremely hot (millions of degrees) and extremely dense (about 20 times denser than iron) place where atoms can get so close together that they smash into one another and create new elements.

There are three types of nuclear reactions that we are going to learn about today: The proton-proton chain, the alpha ladder, and neutron capture. The first one creates helium. The second one creates elements up to iron. The last one creates everything heavier than iron. The hydrogen that we start with was created in the Big Bang.

The most common reaction inside of a star is called the “proton-proton chain”. This is the process that creates energy and starlight by turning four hydrogen atoms (four protons) into one helium atom. “But wait”, you say. “Doesn’t helium have only two protons?” That’s correct! In this process, protons will be converted into neutrons. Neutrons can also convert themselves into protons, as it turns out. It’s a good thing this is a feature of how the universe works—otherwise we would not have any elements heavier than lithium!

The proton-proton chain happens in all kinds of stars. It is the easiest process to get going because it requires the lowest temperatures and densities. The products of the proton-proton chain are helium and light.

The “alpha ladder” is responsible for creating all of the elements above helium and below iron on the periodic table. It is called the “alpha ladder” because helium is sometimes known as an alpha particle. In this process, helium atoms get smashed together to form heavier and heavier atoms. It goes something like this:

He + He + He = C

C + He = O

O + He = Ne

Ne + He = Mg

Mg + He = Si

…

Cr + He = Fe

The alpha ladder takes more energy and higher density because the atoms are heavier and have more positively-charged protons. Think of two pairs of magnets. Weak magnets make it easy to push North and North together. Strong magnets have more “magnetic charge” and are harder to push together. Hydrogen, which only has one proton, is like a weak magnet. Iron, which has 26 protons, is like a strong magnet. So it takes more energy to push a helium atom into an iron atom that it takes to push two hydrogen atoms together.

The alpha ladder has to happen in more massive stars, or those which are usually past their prime and have burned up all their hydrogen fuel. The products of it are all the elements between He and Fe (in the middle of all that +2 action there are often cases of atoms losing or gaining just one proton which fills in the gap between even elements), and light.

Neutron capture is the final nuclear physics process we will learn about. Like its name, neutron capture just means that a heavy atom has captured a neutron and turned it into a proton. When an atom gains a proton, it means that it goes up one number on the periodic table.

It actually takes a very high temperature and a very high density for neutron capture to occur. This is why it only happens when a star explodes (supernova), or when two dead stars crash into each other (neutron star merger). These are some of the most extreme conditions in the universe and they do not happen very often. Nonetheless, the Milky Way is still full of heavy elements like gold, platinum, and uranium which are created in the neutron capture process.

Scavenger Hunt

There are many different elements which can be easily spotted around the building, many in several different places.

Rules

1. THE HUNT BEGINS 15 MINUTES AFTER CLASS BEGINS AND YOU WILL HAVE ONE HOUR TO COMPLETE. THE TEAM WITH THE MOST POINTS WINS.
2. YOU CAN USE YOUR NOTES AND THE PROVIDED PERIODIC TABLE. YOU CANNOT USE GOOGLE TO FIND OUT IF SOMETHING HAS AN ELEMENT IN IT. YOU CAN ASK THE PROFESSOR THREE QUESTIONS MAXIMUM. YOU CAN TAKE PICTURES OF THINGS YOU FIND OUTSIDE THIS CLASSROOM.
3. YOU CAN USE THE SAME ITEM MULTIPLE TIMES BUT NOT FOR THE SAME ELEMENT (i.e. a substance with multiple elements in it)
4. YOU CAN FIND ELEMENTS IN THIS CLASSROOM AND IN THE DISPLAYS ALONG THE HALLWAYS ON THE FIRST, SECOND, AND THIRD FLOORS. DO NOT GO INTO ANY OTHER CLASSROOMS OR LAB SPACES OR OUTSIDE OF THE BUILDING.
5. EVERY ITEM ON THE LIST IS WORTH ONE POINT
6. IF YOU FIND AN ITEM WITH THAT ELEMENT IN IT, BUT IT’S NOT ON THE LIST, YOU WILL EARN ONE EXTRA POINT
7. IF YOU FIND AN ELEMENT THAT IS NOT ON THE LIST, THAT IS THREE POINTS

QUESTIONS

1. Element 6 sometimes has 6 neutrons, but other times has 7 neutrons. When this happens, the two different types of atoms are referred to as “isotopes”. In the universe, there are more 12-nucleon isotopes of Element 6 than there are 12-nucleon isotopes of Element 6. Why do you think this could be?
2. Elements low on the periodic table often have the same number of protons and neutrons, but elements high up on the periodic table usually have more neutrons than protons. Why do you think this is?
3. The sun is a small star halfway through its life. Do you think it will one day undergo the alpha ladder? What about neutron capture?
4. Jupiter is what we call a “failed star” because it is massive but does not undergo nuclear fusion. Why do you think this is?
5. If stars are powered by nuclear fusion, why don’t they just keep expanding outwards? What balances the star so that it stays the same size?
6. The star Eta Carinae B is 30 times more massive than the sun. Which nuclear processes do you think it will undergo during its life and death?