Nucleosynthesis

Discussion

big bang nucleosynthesis

By the first millisecond, the universe had cooled to a few trillion kelvins (1012 K) and quarks finally had the opportunity to bind together into free protons and neutrons. Free neutrons are unstable with a half-life of about ten minutes (614.8 s) and formed in much smaller numbers. The abundance ratio was about seven protons for every neutron. Before one neutron half-life passed nearly every neutron had paired up with a proton, and nearly every one of these pairs had paired up to form helium. By this time the universe had cooled to a few billion kelvins (109 K) and the rate of nucleosynthesis had slowed down significantly. By the time the universe was three minutes old the process had basically stopped and the relative abundances of the elements was fixed at ratios that didn't change for very long time: 75% hydrogen, 25% helium, with trace amounts of deuterium (hydrogen-2), helium-3, and lithium-7. Big Bang nucleosynthesis produced no elements heavier than lithium. To do that you need stars, which means waiting around for at least 200 billion years.


[magnify]

we are all made of stars

More than ninety per cent of the universe is composed of hydrogen and helium. Both elements have been around since shortly after the beginning of the universe. Yet, hydrogen and helium together won't make anything as complex and as interesting as the earth, or a bacterium, or a refrigerator, or you and I. To do that we need carbon and oxygen and nitrogen and silicon and chlorine and every other naturally occurring element. Almost all the hydrogen and helium present in the universe today (and some of the lithium) were created in the first three minutes after the big bang. All of the other naturally occurring elements were created in stars.

Stars like the sun


[magnify]

Details were discussed in the section on Fusion. The basic parts of the reaction are …

2( 11H  +  11H  →   21H  +    0+1e  +  00ν)   0.4 MeV + 1.0 MeV
2( 11H  +  21H  →   32He  +    00γ ) 5.5 MeV
  32He  +  32He  →   42He  +  2 11H   12.9 MeV

Which overall yields …

4(11H) → 42He + 2(0+1e + 00γ + 00ν)   26.7 MeV              

Stars heavier than the sun use carbon-12 as a catalyst.


[magnify]

You need really massive stars for this — say 20 to 120 times the mass of the sun.

Really, really heavy stars do something different.

The Mass-5 and Mass-8 Bottlenecks. There are no stable isotopes (of any element) having atomic masses 5 or 8. But there is always a very small amount of beryllium-8 at any moment that is available to fuse with a third helium to produce carbon-12. This extremely improbable sequence is called the triple-alpha process because the net effect is to combine 3 alpha particles to form a carbon-12 nucleus. The triple-alpha process is not relevant in main sequence (normal) stars like the sun because their core temperatures are too low. However, in the red giant phase, after many stars have accumulated vast amounts of helium in their core, the central temperature can rise high enough (108 K) to initiate the triple-alpha process.

42He +  42 He  + 92 keV   →   84Be*
42He +  84Be*  + 67 keV   →   126C*
    126C*   →   126C + 00γ + 7.4 MeV

Overall

3(42He) → 126C + 00γ + 7.4 MeV


[magnify]

In order of increasing alpha number, the following forms of fusion take place …

Stable Isotopes Built from Helium Nuclei (Alpha Particles)
alpha
number		 mass
number		 element(s) comments
1 4 He helium formed in all stars
2 8     no stable isotopes with this mass number
3 12 C carbon triple alpha process
4 16 O oxygen  
5 20 Ne neon  
6 24 Mg magnesium  
7 28 Si silicon  
8 32 S sulfur most abundant isotope of sulfur
9
 		 36
 		 S
Ar		 sulfur
argon		 0.02% of all sulfur atoms
most abundant isotope of solar argon
10

 		 40

 		 Ar
K
Ca		 argon
potassium
calcium		 most abundant isotope of atmospheric argon
0.01% of all potassium atoms
most abundant isotope of calcium
11 44 Ca calcium 2.1% of all calcium atoms
12
 		 48
 		 Ca
T		 calcium
titanium		 0.19% of all calcium atoms
 
13 52 Cr chromium  
14 56 Fe iron nuclear "ash"

Massive Stars

Core Nuclear Reactions in Massive Stars
lifetime remaining core temperature core reaction
10,000,000 years   1H  ⇒  4He
1,000,000 years 170,000,000 K 4He  ⇒  12C, 16O
1,000 years   12C  ⇒  20Ne, 24Mg
10 years 1,500,000,000 K 20Ne  ⇒  16O, 24Mg
1 year 2,000,000,000 K 16O  ⇒  28Si, 32S
1 day 3,000,000,000 K 28Si, 32S  ⇒  56Fe, 56Ni
1 s explosive fusion
neutron capture		  ⇒ 
 ⇒ 		 light elements
heavy elements

Mix it all up and get everything from hydrogen to uranium (and maybe even up to californium).


[magnify]


[magnify]


[magnify]

Top 20 Elements in the Universe
rank element per million kg per million atoms
1 H hydrogen 750,000 930,000
2 He helium 230,000 72,000
3 O oxygen 10,000 800
4 C carbon 5,000 500
5 Ne neon 1,300 80
6 Fe iron 1,100 20
7 N nitrogen 1,000 90
8 Si silicon 700 30
9 Mg magnesium 600 30
10 S sulfur 500 20
11 Ar argon 200 6
12 Ca calcium 70 2
13 Ni nickel 60 1
14 Al aluminum 50 2
15 Na sodium 20 1
16 Cr chromium 15 0.4
17 Mn manganese 8 0.2
18 P phosphorus 7 0.3
19 Co cobalt 3 0.06
20 K potassium 3 0.1
… … everything else 6 0.2
Source: WebElements

how like a god

Rutherford was the first to transform one element into another.

147N + 42He → 178O → 11H

technetium

promethium

transuranic, cisuranic, superheavy

First Synthesis of Various Artificial Elements (some claims are subject to debate)
element year location process
43 Tc technetium 1936 Italy
61 Pm promethium 1944 ORNL
93 Np neptunium 1940 LBL
94 Pu plutonium 1940 LBL
95 Am americium 1944 Chicago
96 Cm curium 1944 Chicago
97 Bk berkelium 1949 LBL
98 Cf californium 1950 LBL
99 Es einsteinium 1952 Pacific Ocean found in radioactive fallout
100 Fm fermium 1952 Pacific Ocean found in radioactive fallout
101 Md mendelevium 1955 LBL
102 No nobelium 1958 LBL
103 Lw lawrencium 1961 LBL
104 Rf rutherfordium 1964 JINR
105 Db dubnium 1970 LBL
106 Sg seaborgium 1974 LBL
107 Bh bohrium 1976 JINR
108 Hs hassium 1983 GSI
109 Mt meitnerium 1982 GSI
110 Ds darmstadtium 1994 GSI
111 Rg roentgenium 1994 GSI
112 Cp copernicium 1996 GSI
113 [Uut] [ununtrium] 2003 JINR
114 [Uuq] [ununquadium] 1999 JINR
115 [Uup] [ununpentium] 2003 JINR
116 [Uuh] [ununhexium] 2000 JINR
117 [Uus] [ununseptium] 2010 JINR 4820Ca + 24997Bk → 293117Uus + 410
118 [Uuo] [ununoctium] 2002 JINR 4820Ca + 24998Cf → 294118Uuo + 310n
119 [Uue] [ununennium] – – not yet synthesized
120 [Ubn] [unbinilium] – – not yet synthesized
121 [Ubu] [unbiunium] – – not yet synthesized
122 [Ubb] [unbibium] – – not yet synthesized
Chicago   University of Chicago
GSI   Gesellschaft für Schwerionenforschung; Darmstadt, Germany
LBL   Lawrence Berkeley National Laboratory; Berkeley, California
ORNL   Oak Ridge National Laboratory; Oak Ridge, Tennessee
JINR   Joint Institute for Nuclear Research; Dubna, Russia
(Объединенный Институт Ядерных Исследований; Дубна, Россия)

The Physics Hypertextbook

  • No condition is permanent.