Nucleosynthesis

P.Loos revised 1/8/06

   About 14 billion years ago, the universe began with a big bang.  As it expanded it cooled.  In 1965, Rice alumnus Robert Wilson, working at Bell Labs with Arno Penzias, discovered the microwave background radiation left over from the big bang, corresponding to black body radiation at a temperature of 2.73 degrees K.  This was one of the key scientific discoveries of the past century and earned them a Nobel prize (in 1978).

  About 300 seconds after the big bang, the temperature had dropped to a billion degrees K; and, through various nuclear reactions, matter in the universe was H 92%, He 8% and Li 10^-8% by number of atoms (ref.2).  Over billions of years gravity formed this matter into galaxies, stars and planets.

   Fusion processes in stars create most of the heavier elements.  Temperatures inside small stars like our sun do not exceed about 10⁸ K and are only capable of producing elements as heavy as O.  Inside more massive stars temperatures reach 2x10⁹ K and the resulting fusion processes produce nuclei as heavy as ⁵⁶Fe.

   Nuclei heavier than about ⁵⁶Fe (ref.4) are created during short-lived endothermic reactions in certain stellar supernova explosions.  These supernovae occur in the more massive stars once they have exhausted all their nuclear fuel.

   The end result of the various nucleosynthesis processes is shown by the following table, which lists the abundance of the elements relative to 10¹² hydrogen atoms (from ref.1):

element       abundance      element  abundance      element  abundance

   =======================      ==================      ==================

   H   1 1,000,000,000,000      Cu  29      19,000      Ce  58          43

   He  2    80,000,000,000      Zn  30      47,000      Pr  59           6

   Li  3             2,000      Ga  31       1,400      Nd  60          31

   Be  4                30      Ge  32       4,400      Pm  61          <1

   B   5               900      As  33         250      Sm  62          10

   C   6       450,000,000      Se  34       2,300      Eu  63           4

   N   7        92,000,000      Br  35         440      Gd  64          13

   O   8       740,000,000      Kr  36       1,700      Tb  65           2

   F   9            31,000      Rh  37         260      Dy  66          15

Ne 10       130,000,000      Sr  38         880      Ho  67           3

   Na 11         2,100,000      Y   39         250      Er  68           9

   Mg 12        40,000,000      Zr  40         400      Tm  69           2

   Al 13         3,100,000      Nb  41          26      Yb  70           8

Si 14        37,000,000      Mo  42          93      Lu  71           2

   P  15           380,000      Ru  44          68      Hf  72           6

   Su 16        19,000,000      Rh  45          13      Ta  73           1

Cl 17           190,000      Pd  46          51      W   74           5

Ar 18         3,800,000      Ag  47          20      Re  75           2

   K  19           140,000      Cd  48          63      Os  76          27

   Ca 20         2,200,000      In  49          75      Ir  77          24

   Sc 21             1,300      Sn  50         140      Pt  78          56

   Ti 22            89,000      Sb  51          13      Au  79           6

   V  23            10,000      Te  52         180      Hg  80          19

   Cr 24           510,000      I   53          33      Tl  81           8

Mn 25           350,000      Xe  54         160      Pb  82         120

   Fe 26        32,000,000      Ce  55          14      Bi  83           5

   Co 27            83,000      Ba  56         160      Th  90           1

   Ni 28         1,900,000      La  57          17      U   92           1

   Elements 43 and >83 are rather unstable and are found in negligible quantities; however Th (90) and U (92) have half lives greater than the age of the earth and are included in the table above.

   The irregular patterns of elemental abundance seen in the above table reflect the paths of nuclear fusion and decay.  Pb, for example, is relatively common for a heavy element partly because it is the stable decay product for many of the elements above it.  Notice the somewhat inconsistent pattern where atoms with even atomic numbers are usually more common than those with odd atomic numbers.  This is because the building blocks of many fusion reactions are helium nuclei of atomic number 2.

   Li, Be and B are less common because in stars, even at temperatures around 10⁶ K, their nuclei readily absorb protons and split into the more stable He.  Most Li and Be are apparently produced by cosmic ray collisions breaking up

some of the carbon produced in stars.

   In any case (per ref.3) after about 10¹⁵⁰⁰ years, through the phenomenon of quantum tunneling, all the matter in the universe will apparently be converted into its lowest energy state, ⁵⁶Fe. 

   Once this happens no one is going to get much work done.  So, in the meantime we’d better figure out what’s important and get after it.  And since there’s going to be plenty of iron around to work with, being a ferrous metallurgist isn’t all bad.

References:

1)Stellar Mechanics & Evolution by Greg Goebel  www.vectorsite.net/tastga4.html

2)Big Bang Nucleosynthesis by Edward Wright www.astro.ucla.edu/~wright/BBNS.html

3)The End of the Universe by John Baez      math.ucr.edu/home/baez/end.html

4)Hyperphysics, Nuclei   hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin2.html

See also:

 Astrophysical Concepts by Martin Harwit (Concepts, 1982)