Transcript The Nuclear Reactions

The Reactions
The Main Sequence – The P – P Chain
1H
+ 1H  2H + proton + neutrino
2H
+ 1H  3He + energy
3He
+ 3He  4H + 1H + 1H + energy
The net result -
4 ( 1H )  4He + energy + 2 neutrinos
The Reactions
The Main Sequence – The CNO Cycle
M > 1.2 M and T > 17 million K
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.
12C
15N
15O
+ 1H  13N
+ 1H  12C + 4 2He
 15N
(unstable radioactive decay)
13N
 13C
(unstable radioactive decay)
13C
+ 1H  14N
14N
+ 1H  15O
The Reactions
The P – P Chain vs The CNO Cycle
Factors Controlling the Fusion Rates
* There must be C, N, or O present for the CNO cycle to occur, so it can only happen for stars where
this is true. However, only a very small amount is required, so this condition is often fulfilled.
* The reactions have very different temperature dependences, as illustrated in the figure.
The Reactions
The P – P Chain vs The CNO Cycle
At lower temperatures the PP chain dominates, but with rising temperatures there is a sudden
transition to dominance by the CNO cycle, which has an energy production rate that varies strongly
with temperature. This is why the CNO cycle is more important for heavier stars: their interior
temperatures are higher, thus favoring the CNO cycle.
The Reactions
The Triple Alpha Process T > 100 million K
3 ( 4He )  12C
Advanced Nuclear Reaction Stages
Following the Triple-alpha process there are a variety of reactions which
may occur depending on the mass of the star. Three general principles
influence the roles that these nuclear burning stages may play:
* Successive nuclear burning stages, involving more massive nuclei with
higher charges, will require increasingly high temperatures to
overcome the increased electrical repulsion.
* The amount of energy released by each successive reaction stage
decreases so that later nuclear burning stages become shorter and
shorter.
* Once fusion reactions have produced an iron core, further fusion
reactions no longer produce energy, but absorb energy from the
stellar core. As we shall see this may have a catastrophic effect on the
star as it nears the end of its life.
Advanced Nuclear Reaction Stages
12C
+ 4He  16O
16O
+ 4He  20Ne
Advanced Nuclear Reaction Stages
T > 500 million K
12C
+ 12C  24Mg
Advanced Nuclear Reaction Stages
T > 1 billion K
16O
+ 16O  32S
Advanced Nuclear Reaction Stages
T > 3 billion K
Silicon burning occurs through a series of reactions
that produce nuclei near the "iron-peak", that is near
56Fe on the Periodic Table, the element with the most
strongly bound nucleus.