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Nucleosynthesis of The Elements

This page is concerned with where the chemical elements come from: how atomic nuclei are forged. It is a long, complicated story, largely deduced in the second half of the twentieth century, that romantically concludes: We Are Stardust.


The Start: Big Bang Nucleosynthesis

Current thinking is that the the universe erupted from the cauldron of the Big Bang some 13.8 billion years ago, as described on this Wikipedia timeline page.

And from NASA:



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 ~20 minutes from the beginning itself.

During this period:

proton    +    electron     ⇄     neutron

A graph, from astro.ucla.edu, shows the (log) time evolution of the abundances of the light elements:



Two linked 'how science works' points:

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 this epoch.

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.



Stellar Nucleosynthesis

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-4, 4He, and heavier nuclei.

There are several nuclear synthetic routes:CNO cycle, Triple α & pp-chain:

As a result a variety of atomic nuclei are formed, including:

8Be,   13N,   13C,   14N,   15O,   15N,   12C,   16O   &  17F

although many of 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, from here:


The temperature in the stellar interior increases and more nuclear synthetic pathways become available producing:

20Ne,   23Na,   23Mg,   24Mg,   28Si,   31P,   31S,   32S... and all the way up to   56Fe


Supernovae

The chemical elements are ejected into space by several processes, each involving a dying star:

proton   +   electron     →     neutron   +   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.

Whatever the mechanism, 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 formation.

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 geseous hydrogen, H2,  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 inorganic methane a comparatively rare substance today.



Hard Data

Generally greed upon data, as reported here:

Composition of the universe:


Graphical Timeline



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!


The nucleosynthesis reactions discussed on this page can be found in The Chemical Thesaurus.

The main reference for this page: Intro to Modern Astrophysics by Carroll & Ostlie, Addison-Wesley (1996) as well as the Wikipedia.


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Introduction to Chemogenesis Segrè Chart

© Mark R. Leach 1999-


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