The Reactions

The Main Sequence – The P – P Chain

1H + 1H 2H + proton + neutrino

2H + 1H 3He + energy

3He + 3He 4H + 1H + 1H + energy

4 ( 1H ) 4He + energy + 2 neutrinos

The net result -

The Reactions

The Main Sequence – The CNO Cycle

M > 1.2 Mּס and T > 17 million K

12C + 1H 13N 13N 13C (unstable radioactive decay)

More massive stars burn hydrogen via a catalytic reaction called The CNO CYCLE. Because the initial step in the CNO Cycle requires a Carbon nucleus (6 p+) to react with a proton it requires higher temperatures and is much more temperature sensitive than the P-P Chain (The energy produced is proportional to T20 for the CNO cycle vs T4 for the P-P Chain). Stars of mass greater than about 1.2 M with core temperatures, Tcore > 17 million K, produce most of their energy by the CNO cycle.

13C + 1H 14N

14N + 1H 15O15O 15N (unstable radioactive decay)

15N + 1H 12C + 4 2He

The Reactions

Both the p – p chain and the CNO cycle produce Helium

The Reactions

The Triple Alpha Process T > 100 million K

3 ( 4He ) 12C

Advanced Nuclear Reaction Stages

12C + 4He 16O

Advanced Nuclear Reaction Stages

What’s next

* “Common” Element Fusion

* Helium Capture

Advanced Nuclear Reaction Stages

T > 500 million K

Carbon Fusion to Magnesium

12C + 12C 24Mg

Advanced Nuclear Reaction Stages

T > 1 billion K

Oxygen Fusion to Sulfur

16O + 16O 32S

Advanced Nuclear Reaction Stages

What’s next

* “Common” Element Fusion

* Helium Capture

Notice from Previous slides: “Common Element Fusion” requires VERY high

temperatures

Advanced Nuclear Reaction Stages

What’s next

* “Common” Element Fusion

* Helium Capture

Since “Common Element Fusion” requires VERY high temperatures, Helium capture is

much more probable in the core of a star

Advanced Nuclear Reaction Stages

Helium Capture to form Oxygen, Neon, Magnesium and Silicon

12C + 4He 16O

16O + 4He 20Ne20Ne + 4He 24Mg

24Mg + 4He 28Si

Advanced Nuclear Reaction Stages

Silicon can be broken apart by the high energy photons in the core (photodisintegration).

Photon + 28Si 7 (4He)

The Helium produced in the photodisintegration of Silicon drive

further reactions

Advanced Nuclear Reaction Stages

Helium Capture to form Sulfur, Argon, Calcium and Titanium

28Si + 4He 32S

32S + 4He 36Ar36Ar + 4He 40Ca

40Ca + 4He 44Ti

Advanced Nuclear Reaction Stages

Helium Capture to form Chromium, Iron and Nickel (unstable to and isotope of Cobalt and then to an isotope of Iron)

44Ti + 4He 48Cr

48Cr + 4He 52Fe52Fe + 4He 56Ni

56Ni → 56Co 56Fe

Advanced Nuclear Reaction Stages

T > 3 billion K

Each reaction produces a nucleus with two more protons.

As a result, elements with an even number of protons are produced.

However, there are enough free protons in the core that single proton capture can occur as well.

Although not as probable as the previous reactions, proton capture will produce elements with an odd

number of protons.