Download Notes: Stellar Nucleosynthesis

10/26/16
Stellar Nucleosynthesis
Objective: I will model the steps of nuclear
fusion in stars similar to our Sun.
PAGE 31
Hydrogen Burning (Proton-Proton Chain)
• 1H + 1H → 2H
1
1
1
1
• 2H + 1H → 3He
1
1
2
• 3He + 3He → 4He
2
2
2
This process is the longest
stage in a star’s life!
Proton-Proton Chain
• primary nuclear fusion process in
stars similar to our Sun
• this process is the longest stage
in a star’s life!
Step 1: Proton-Proton Fusion
• Start with two sets of 2 1H atoms
• 1H has only 1 proton, no neutron
• As the 1H atoms fuse:
• positron (creates energy) and neutrino
released
• one of the protons becomes a neutron
• Finish with one 2H (deuterium)
atom
• With a nucleus that has 1 proton and
1 neutron
Step 2: Deuterium Proton Fusion
• Fuse one 1H and one 2H
• 1H is 1 proton nucleus
• 2H is 1 proton, 1 neutron nucleus
• Gamma ray (energy) released
• Finish with one 3He atom
• 3He is 2 protons and 1 neutron
nucleus
Step 3: Helium-3 Fusion
• Start with two 3H atoms
• 3H is 2 protons and 1 neutron
nucleus
• Finished with a 4He atom
• 4H is 2 protons and 2 neutrons
• Two 1H atoms released
released
• Start the process all over again
Proton-Proton Chain
• 1H + 1H → 2H
1
1
1
1
• 2H + 1H → 3He
1
1
2
• 3He + 3He → 4He
2
2
2
This process is the longest
stage in a star’s life!
Summary of Proton-Proton Chain
• Begins when two 1H atoms fuse into 2H
• positron and neutrino released (energy!)
• 2H atom then fuses with 1H to form 3He
• gamma ray released (energy!)
• Two 3He atoms fuse into the final product, 4He
• At each step, a small amount of matter is lost and
converted into a large amount of energy!
Proton-Proton Chain
• 1H + 1H → 2H
1
1
1
1
• 2H + 1H → 3He
1
1
2
• 3He + 3He → 4He
2
2
2
This process is the longest
stage in a star’s life!
Fusion of Heavier Elements
• When all hydrogen is used up, heavier
elements begin to be fused
• ex. Carbon, Oxygen, Silicon
• Elements up to Iron are created
Fusion of Heavier Elements
• Iron is a “star killer”
• fusion of iron takes energy (endothermic)
instead of giving off energy (exothermic)
• therefore, stars begin to “die” once iron is
created.
Carbon-Nitrogen-Oxygen Cycle
(Large Stars)
• In stars much more massive than our
Sun:
• 1H and deuterium (2H) get used up very fast
• Therefore, nuclear fusion must occur
through the CNO Cycle in these stars
Hydrogen Burning (CNO Cycle)
Carbon Burning
16O
+ + +
+ 12C →
6
+ + +
+ + +
12C
+ 12C
→
+ + +
+ + +
+ + +
12C
+ 12C
12C
6
+ +
6
+ +
6
+ +
+
6
+
+
+ + +
6
+ + +
+ + +
+
8
4He
+
+
+ +
→
20Ne
10
+
+
+ ++++ +
+ ++
23Na
+ 4He
2
+
11
+
+
+ + +
+ + +
2
+
+
+
4He
+
2
+
2H
1
+
+
Neon Burning
→
20Ne
2
0
+
8
1
6 +
+
1
0
1
0 20
+
4He
2
4
+
2 +
→
4He
2
4
2
8
20Ne
10
16O
24Mg
1
2 24
12 +
+
Oxygen Burning
16O
+
+
16O
+
→
+ 16O
+ 16O
+
28Si
+
4He
+
→
31P
+
+
+
1H
+
Silicon Burning
A chain of alpha processes
28Si
+ 4He
→
+
+
32S
+ 4He
+
+
28S
+
→
36Ar
+
Silicon Burning (cont)
36Si
+
40Ca
+
+ 4He
→
+
+ 4He
+
40Ca
+
→
44Ti
+
Silicon Burning (cont)
44Ti
+ 4He
+
+
48Cr
+
→
+ 4He
+
48Cr
+
→
52Fe
+
Hydrogen Burning (Proton-Proton Chain)
1.
2.
3.