Nucleosynthesis

Nucleosynthesis of The Elements

Chemistry is concerned with the periodic table, but where do the chemical elements come from? It is a long story, largely deduced in the second half of the twentieth century, that ultimately and rather romantically says: we are stardust.

Introduction

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 timeline page. The crucial period of baryionic matter formation, The Epoch of Big Bang Nucleosynthesis, started at about one second and lasted for about 300 seconds:

One second after the initial explosion, the quark soup cooled to an ionised plasma of photons, electrons, positrons, neutrinos, protons and neutrons.
Initially the temperature was so high that the protons and electrons combined into neutrons: p + e -> n. Equilibrium meant that protons and neutrons were both present in large numbers.
The universe expanded and cooled to a temperature of ~ 10¹⁰ Kelvin, 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.
At about 100 seconds after the Big Bang the universe had cooled to 10⁹ Kelvin and 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 are very reactive and able to react with each other – by a variety of nuclear reactions – to give a mixture of isotopes: ³He, ⁴He, ⁷Li, along with the primordial ¹H and ²H.
The ratios of ¹H, ²H, ³He, ⁴He and ⁷Li in the early universe can be measured (with considerable difficulty) and the numbers 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 seconds after the Big Bang, albeit on a small scale.

Stellar Nucleosynthesis

For several thousand years the expanding and cooling universe was an optically opaque plasma of photons, free electrons, ¹H, ²H, ³He, ⁴He and ⁷Li nuclei. But when the temperature fell to 3000 Kelvin, the electrons were able to combine with the atomic nuclei to form neutral atoms and the universe became optically clear. Wikipedia.

This would be the end of the story, except that the rapidly expanding universe had a built in brake, gravity, 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, including:

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

although these nuclei are either radioactive or are quickly consumed in the stellar furnace.

Depending upon their mass, stars evolve to the stage where they have onionskin like shells of thermonuclear combustion with differing nuclear chemistry.

For large stars, moving inwards:

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

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

Supernovae

The newly formed chemical elements are liberated as ejecta by at least three processes, each involving a dying star:

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. The conditions cause a massive flux of free neutrons and 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 the periodic table towards and past uranium.

Main 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 such as carbon – often in the form of nanometre size diamonds – are quietly blown off into the interstellar medium.

The 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.

In each cases, shells of debris – consisting of a zoo of atomic nuclei – are ejected into the interstellar medium. Over millions of years this material cools to mildly radioactive clinker that is collected by gravity, where 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 – form a disk that evolves into a system 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 is enriched in heavy elements with respect to the universe as a whole. Our planet is large enough so that the residual radioactivity, mainly from potassium-40 (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 being trapped as water, a substance essential for our planet's biology. When the Earth formed much of the hydrogen was combined with carbon and trapped as methane, CH4, however this is a comparatively rare substance today.

Hard Data

In 2003 as some data was agreed upon, as reported here:

The universe 13.7 (+/– 0.2) billion years old

After about 100-200 million years the first stars and galaxies appeared

Composition of the universe:

73% dark energy

23% dark matter

4% matter: stars, planets, interstellar dust

200 billion galaxies in total

200 billion stars per galaxy

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 of elements.

Most of 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

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.