Goal: To understand how stars generate their energy

1. Goal: To understand how stars generate their energy Objectives: To learn about the Proton – Proton Chain To learn about the Carbon – Nitrogen – Oxygen Cycle

2. (Horsehead Nebula - in Orion -Image from APOD)

3. What would happen to the core of the sun if the sun stopped producing energy via fusion?

4. To prevent collapse • Remember when we looked at the core of the sun that we saw that the sun held itself up with a combination of gas pressure and radiation pressure (light has energy) • This was called “Hydrostatic Equilibrium”

5. Proton – Proton Chain • Short answer: method by which a star converts protons (Hydrogen nuclei) to Helium nuclei (the electrons in the core of a star fly around on their own).

6. Proton – Proton Chain • However it is a lot more complicated that I have made it seem. • After all, how do we take 4 protons and make a helium atom when a helium atom has 2 protons and 2 neutrons?

7. Why don’t the atoms in this room fuse together?

8. Repulsion • In the cores of stars all the nuclei have + charges. • + charges repel other + charges. • So, they won’t attract and fuse by accident. • So, what do we need to be able to do it?

9. Energy • It takes energy to overcome this repulsive force. • Much like it takes energy to get up the stairs. • So, how do we give a proton energy?

10. Temperature • Temperature is just a measure of how fast the particles of a gas, liquid, or plasma move. • Higher temperature = faster movement. • So, this means that the higher the temperature, the more energy the particles have. • Huzaah, the core of the sun is 100 million degrees!

11. Step one • We take 2 protons in the core to the sun and try to slam them together. • They get closer and closer. • Here come the fireworks! • And!

12. Step one • We take 2 protons in the core to the sun and try to slam them together. • They get closer and closer. • Here come the fireworks! • Nothing happens….

13. What happened? • Even at 100 million degrees, protons STILL don’t have enough energy to overcome the repulsive barrier. • So, how do we get fusion?

14. Quantum Mechanics! • No, I will not do a lecture on Quantum. • Just 1 basic principal: there is uncertainty in the position of each proton. • In laymen’s terms that means that a proton is not just in a specific position, but has a small probability at being in a nearby position.

15. So, • When 2 protons start to get close, there is a small probability they will actually be in the same spot. • This is called quantum tunneling – basically tunneling through the repulsive barrier. • This allows us to have fusion!

16. However, • The probability of this tunneling is very small, and it depends very highly on how close they get. • This means that how rapidly you fuse protons depends very highly on the temperature (and also on the density squared). • Fusion in the proton – proton chain (sometimes call p-p chain) relies on temperature to the FORTH power!

17. Most of the time though • Most of the time the protons will repel. • In our sun it takes an average of a billion years for a proton to fuse with another proton. • This is why the sun can last for 10 billion years (takes 10 billion years to use up its fuel).

18. Step 1 concluded • So, eventually we get 2 protons to collide. • What do we get? • No, we don’t get a Helium atom with 2 protons and no neutrons. Those don’t exist. • Another difficulty in the fusion process is that you turn 2 protons into deuterium (which is hydrogen with a neutron in it) + stuff. • So, that means a proton has to convert to a neutron. That is hard to do.

19. Step 2 • It would be easy to say 2 deuterium go to 1 helium. • It would give you 2 protons and 2 neutrons. • But, sadly, it does not work that way. • Reason, there just is not enough deuterium.

20. Instead • Deuterium fuses with what is the most common thing around, a proton. • This creates Helium 3 (Helium which has a weight of 3; 2 from the 2 protons and the last from 1 neutron).

21. Step 3a • 3a occurs 69% of the time in our sun. • In time you will get some amount of Helium 3. • If 2 of these fuse, then you get a Helium 4 and 2 protons.

22. Step 3b • 31% step 3b occurs instead. • In this case a Helium 3 fuses with a Helium 4 creating Beryllium 7. • The Beryllium 7 combines with an electron (converts a proton into a neutron) to create Lithium 7. • The Lithium 7 fuses with a proton to create 2 Helium 4 atoms.

23. Carbon – Nitrogen – Oxygen Cycle • While the sun utilizes the p-p chain. Other stars use this (called hereafter the CNO cycle). • Instead of fusing protons and protons we now fuse protons to carbon. • Will this be easier or harder?

24. Charges • Protons have 1 atomic charge. • Carbon has 6 (6 protons). • So, it is much harder. • Therefore, it takes more energy, which means higher temperatures. • This method depends on temperature to the 20TH power!!!

25. So, we need more heat • A star with a hotter core will be able to generate more energy, and will use the CNO cycle far more. • A start with a less hot core will use the p-p chain more and will generate less energy. • What kind of stars are these? Stay tuned. We will examine this next hour.

26. Step 1 • Okay so now for the steps. • Carbon 12 + a proton goes to Nitrogen 13 • A proton in the Nitrogen 13 converts to a neutron so that the atom converts to Carbon 13.

27. Steps 2 and 3 • Carbon 13 + a proton goes to Nitrogen 14. • Step 3 adds another proton to get to Oxygen 15. • The Oxygen 15 has a proton decay to a neutron and it goes to Nitrogen 15.

28. Step 4 • Add 1 more proton (the 4th total) and you get Carbon + a Helium atom. • Note that we started with Carbon and end with Carbon. • That makes Carbon a catalyst for this reaction. • A catalyst is a substance that aids a reaction but remains unchanged by it.

29. Conclusion • Stars generate energy in their cores via fusion. • This energy keeps them from collapsing. • Fusion is only possible because of quantum tunneling, therefore it highly depends on the temperature of the core. • The temperature of the core is very important. • There are 2 methods to convert protons to Helium. • To think about during the break - Why does fusion create energy?