Stellar Alchemy - how the chemical elements were synthesised -

The Cosmos is within us, we're made of star-stuff - Carl Sagan (Astronomer, 1934-1996)

Have you ever considered that every atom in your body came from stars? Everything living and non-living thing and every part of you and me were forged in the grand life of stars. The carbon atoms in your right hand probably came from a different star than your left hand. We wouldn’t be here if stars hadn’t lived, died and exploded, and as they did so, fusing together atomic nuclei and building the elements - carbon, nitrogen, oxygen, sulphur, iron, all the things that matter for life to get started on earth. Every atom in our bodies was once part of something else. Everything is made of the same basic ingredients called the chemical elements, the building blocks of the universe. Written in every atom is the history of the universe.

Start with a Bang

Hydrogen and helium were generated in the first few seconds of the Big Bang. Hydrogen being the most basic of the 92 naturally occurring chemical elements as it consists of only one proton and one electron. The coalescing of four hydrogen atoms at enormous pressures and temperatures within stars generates a helium atom which consists of 2 protons and 2 neutrons. In the synthesis of helium from hydrogen, there is a loss in mass which is converted to energy (heat and light) and is described by Einstein’s eloquent equation, E=mc2. In a stellar fusion reaction, the nuclei of two atoms join to form a single atom of a different element. Helium is therefore the source material for the creation of heavier and heavier atomic nuclei. The sequence of building heavier elements from a helium nucleus proceeds in this sequence:

1. 2 protons + 2 neutrons = 1 helium nuclei.
2. Then the combination of 2 helium nuclei forms the element beryllium.
3. Following this, 2 beryllium nuclei + 1 helium nuclei gives carbon.
4. Then, 4 helium nuclei would give oxygen, 5 helium nuclei would give nitrogen, 6 helium nuclei would give magnesium, 7 would give silicon and 8 would give sulphur, and so on.
However, this only works for elements as heavy as iron or less. This is because creation of elements heavier than iron requires additional energy rather than the production of energy. Therefore there needs to be another mechanism to explain the generation of nuclei heavier than iron in stars.

Stellar Alchemy

When a star runs out of hydrogen it begins to die and collapses in on itself creating pressures and temperatures high enough to overcome nuclear forces which allow helium nuclei to fuse together to make atoms of heavier nuclei. Collapsing stars do not go out with a whimper, but explode in one of the most energetic events in the Universe, to produce a cloud rich in atoms of heavier atomic nuclei. These events are known as supernovae and are the final phase of element creation. The energy flux is so great during a supernova event, that all of the naturally occurring elements above iron (cobalt to uranium) are synthesised by the relentless pelting of atomic nuclei with neutrons. This highly energetic environment promotes the synthesis of elements heavier than iron, only now, nuclei are not fused together as in a fusion reaction, but the heavier nuclei undergo nuclear fission. Iron is converted to cobalt, which in turn is converted to cadmium, then, indium, tin, antimony and so on, including gold.

The stars died so that you could be here today - Lawrence Krauss (Physicist 1954 - )


The stuff we are made of

Hydrogen and helium were produced in the Big Bang, and heavier elements were created later by stars and scattered into space by stellar supernovae. There, in the spaces between the stars, these elements mixed with interstellar gas and became incorporated into subsequent generations of stars. The remnants of the stellar explosions coalesced to form the rocky and giant gas planets in our solar system. All 92 naturally occurring elements were incorporated into our planet during its accretion some 4.5 billion years ago. Water, essential to biology, is made of one atom of oxygen and two atoms of hydrogen. Next time you drink a glass of water, consider that you are imbibing hydrogen, which is 13.7 billion years old and also the most abundant element in the Universe. We inhale oxygen and iron-bearing haemoglobin carries this oxygen through the bloodstream. Chains of carbon, nitrogen, oxygen and phosphorus form the support structure for proteins, fats, and carbohydrates in our cells. Calcium strengthens our bones, while potassium and sodium ions are a conduit for impulses through the nervous system. The essential elements for biology on the early earth made it possible for life to start and for the eventual propagation of living species through Darwinian natural selection.


Our Cosmic Connection

The stars seem far detached from our everyday lives, but they are connected to us in the most profound way possible - none of us would be here if stars hadn’t been born, lived their epic lives and died in a dramatic way.



Figure 1: The Periodic Table of the Elements. Elements heavier than hydrogen are synthesised in the hearts of stars by the fusion of lighter atomic nuclei. Elements heavier than iron are made by nuclear fission in supernova eruptions. Image taken from: http://www.elementsdatabase.com/


Figure 2: The hydrogen making up the water in this glass would have been created in first few nanoseconds of the Big Bang and is therefore as old as the universe itself, 13.7 billion years.




Figure 3: The element iron is the last in the sequence of elements formed in stars. Massive deposits of iron oxide at Vergenoeg open pit mine. Picture: Bruce Cairncross


Figure 4: Every atom on Earth would have been made in the Universe's
infancy and in supernovae events. Picture: Allan Fraser


Figure 5: Gold is one of the heavier elements produced by nuclear fission in highly energetic supernovae events. This specimen of gold is 10.5 cm (9.5 oz.) and from the Vogelstruisbult mine, Witwatersrand goldfield. Picture: Bruce Cairncross.

Bibliography

1. Delsemme, A., (1994), “Our Cosmic Origins – from the Big Bang to the Emergence of life and intelligence”. Cambridge University Press.
2. McSween. H.Y., (1997) “Fanfare for Earth – the origin of our planet and life”.