Nucleosynthesis | Chemical Elements | Chemogenesis
of The Elements
This page is concerned with where the chemical elements
come from: how atomic nuclei are forged. It is a long story, largely deduced in the second half of the
twentieth century, that ultimately and rather romantically says: We Are
There is a Basque translation of this web page.
Current thinking is that the
the universe erupted from the cauldron of the Big Bang some 13.7 billion years
ago, as described on this Wikipedia
The period of baryionic matter formation: protons, neutrons and some of the lighter elements – the epoch of Big Bang Nucleosynthesis (BBN) – lasted from 10 seconds to about 20 minutes from the beginning itself.
During this period:
The quark soup had cooled to an ionised plasma
of photons, electrons, positrons, neutrinos, protons and neutrons.
Initially the temperature
was so high that the protons and electrons could combine into neutrons:
+ e n
Equilibrium meant that both protons and neutrons were present
in large numbers.
The universe expanded
and cooled to ~1010 Kelvin. At this temperature the nuclear chemistry
changed and no more neutrons were formed. Free neutrons have a half
life of 617 seconds and once they stopped being made their numbers,
relative to the stable protons, started to decline.
When the universe had cooled to ~109 Kelvin there were 164 neutrons to every 1000 protons. At this lower temperature
neutrons are able to combine/react with protons to form deuterium nuclei,
2H. In this bound state neutrons are stable to decay.
In nuclear chemistry
terms, deuterium nuclei,
2H, are very reactive. For several minutes the deuterium nuclei, 2H, reacted by a variety of nuclear reactions to give a mixture
of isotopes: 3He, 4He, 7Li, along with
the primordial 1H and 2H.
A graph, from here, shows the (log) time evolution of the abundances of the light elements:
The ratios of 1H,
2H, 3He, 4He and 7Li in
the early universe can be measured by astronomers – with considerable difficulty – and
the numbers obtained constrain the mass, temperature and density conditions at
The nuclear chemistry
described above is confirmed by high energy physics experiments at CERN,
the Stanford Linear Accelerator
and a few similar establishments that can reproduce the conditions fractions of a second
after the Big Bang, albeit on a small scale. This science is all part of the standard model of contemporary physics.
As far as chemists are concerned, little else happened for several hundred thousand years after this crucial epoch.
The universe expand outwards and cooled and nuclear chemistry ceased.
The expanding universe had been an optically opaque plasma of photons,
free electrons and 1H, 2H, 3He, 4He & 7Li nuclei. But when the temperature fell to about 3000°, the electrons were able to combine with the atomic nuclei to form neutral atoms, and the universe became optically clear.
This would be the end of the
story, except that the rapidly expanding universe had a built in brake – gravity, the great sculptor – which operated both globally and locally. The implications of
gravity for the entire universe are still the subject of debate, but local
effects are better understood. After about 100 million years gravity caused and still causes matter to collapse into bodies that become
hot and light up the dark sky as stars.
Stars are hot and
dense enough to burn hydrogen, 1H, to helium, 4He. There are
several nuclear synthetic routes and various nuclei are formed as by-products,
13C, 14N, 15O, 15N, 12C,
16O, 17F & 8Be
although these nuclei
are either radioactive or are quickly consumed in the stellar furnace.
Stars evolve so that they have onion-skin like shells
of thermonuclear combustion with differing nuclear chemistry. The exact structure depends on the mass of the star.
The temperature in the stellar
interior increases and more nuclear synthetic pathways become available
23Mg, 24Mg, 28Si, 31P, 31S, 32S, & all the way up to 56Fe
The chemical elements
are ejected into space by several processes, each involving a dying
sequence stars like our Sun burn out and become cold white dwarves.
But before they die, they go through a red giant stage where the outer
mantle layers, enriched in elements like oxygen, nitrogen and carbon often in the
form of nanometre size diamonds are quietly 'blown off' into the
Stars eight times heavier than our Sun explode as a super nova. Due to the thermochemistry of the various nuclear processes, each shell of nucleosynthesis proceeds on an accelerating time scale and Si burns to Fe in hours. Conditions in the core become so extreme that electron pressure is overcome and the protons are forced to react with electrons to give neutrons
p + e n + neutrino
and a neutron star is born in less than a second. The rebounding shock wave plus radiation pressure from the escaping neutrinos causes the outer layers star to explode outwards as a Type II supernova.
These conditions have a massive flux of free neutrons and the various nuclei are able absorb one or more of these neutrons, undergo beta decay, absorb another neutron or neutrons, another beta decay... a process which moves nuclei up (heavier) the periodic table towards and past uranium.
third process is the Type Ia supernova. This occurs when a white dwarf
is held in a tight binary association with a main sequence star. The
small, dense white dwarf pulls the surface layers from the companion
star until enough mass builds so that a runaway thermonuclear incineration
occurs on the surface of the white dwarf which explosively disassembles...
During a supernova, the process of neutron capture builds up elements as far as Z=100, Ferminium. The nucleus 257Fm has a half-life of a few months, but apparently the process of nucleosynthesis can go no further. [Some Nucleosynthesis Effects Associated with r-Process Jets, Astrophysical Journal 2003, 587: 327-340]. Thanks to the Marks Bros for the tip... their periodic table formulation uses this information.
It is now thought (2017) that merging neutron stars are a major source of heavy r-process elements. Among several options, such mergers can/may produce a black hole, but there will also be a great deal of material ejected, and this material will be subject to a high neutron flux.
In each case,
shells of debris consisting of a isotopic zoo of atomic nuclei are
ejected into the interstellar medium. Over
millions of years this material cools to a mildly radioactive clinker that collects together by gravity... where it participates in next generation of star
For reasons not yet fully understood,
the contracting cloud of hydrogen, helium and metals astronomers
regard all elements other then H and He as metals evolves to form a
disk of planets around a central star.
The inner planets of our solar
system are not massive enough to have sufficient gravity to hold onto
hydrogen and helium, and these gases escape into space leaving the Earth
depleted in these elements, but enriched in heavy elements with respect
to the universe as a whole. Our planet is large enough so that the residual
radioactivity, mainly from 40K, is able to heat
the mantle so that it remains hot and fluid.
Some hydrogen remains on Earth by chemically
combining with oxygen and trapped as water, a substance essential for
our planet's biology. When the Earth formed much of the hydrogen was combined with carbon as methane, CH4, however this is a comparatively
rare substance today.
In 2003 as some data was agreed
upon, as reported here:
universe 13.7 (+/ 0.2) billion years old
After about 500
million years the first stars and galaxies appeared
Composition of the
23% dark matter
4% matter: stars,
planets, interstellar dust
200 billion galaxies
200 billion stars
During the lifetime
of our local galaxy there have been about 100 million supernova explosions.
Their frequency and yield are consistent with the observed abundance
the iron on the Earth was produced in Type Ia supernovae events and
most of the oxygen in Type II supernovae explosions: We
Are Star Dust
Some Amazing Scale Images
When we look at the night sky
we see dots of light, but these come from a heterogeneous group of stellar
entities. All stars more than eight times more massive than the
sun are destined to explode, the big ones all go bang!