‘We are made of star stuff’ – Carl Sagan

The fact that the elements that make up our bodies, our animals and our ‘apple trees’ were created in the interior furnaces of stars and then catapulted across the universe in violent stellar explosions was said with great poetic beauty by Carl Sagan in 1973. In his book: “The cosmic connection: an extraterrestrial perspective” he said:-

‘Our sun is a second- or third-generation star. All of the rocky and metallic material we stand on, the iron in our blood, the calcium in our teeth, the carbon in our genes were produced billions of years ago in the interior of a red giant star. We are made of star-stuff’

The very first elements to be formed were Hydrogen and Helium, during the formation of the universe (around 14 billion years ago). As the cloud of cosmic dust and gasses cooled stars formed. These then grouped together to form some of the galaxies that we know today. The core of these stars acts as giant hydrogen and helium ‘factories’ (with some beryllium and lithium formed also), these reactions release the energy that drives life on Earth. As a result almost everywhere in the universe consists of 99% hydrogen and helium.

Might the other chemical elements have evolved from hydrogen and helium? To overcome the electrical repulsion, the nuclear matter would have to be brought very close which can only happen at temperatures of tens of millions of degrees.

In nature, such high temperatures are found only in the insides of stars. The core of ‘our’ sun converts some four hundred million tonnes of hydrogen into helium every second (four hydrogen nuclei combine to become one helium with the release of a gamma-ray photon).

However, when a star runs out of hydrogen it begins to die. As the fuel runs out hydrogen fusion will shut itself off in sequence from interior to exterior. The gravity of the sun will force a contraction upon its helium rich core, so much so that its interior temperatures and pressures increase, pushing helium nuclei together. This triggers the sun into a second round of elemental production and will generate oxygen and carbon. The helium fusion at the core combined with the last of the hydrogen fusion at the shell will cause the sun to expand and cool, thus becoming a giant red star. The shell is so far from the core that its gravity becomes weak and these formed elements begin to ‘leak’ into space.

At the end of the stars lifetime a supernova occurs, the largest explosion that takes place in space. As the star runs out of fuel its mass flows into its core forming a white dwarf star with mind-blowing densities (more than a tonne per spoonful). The core will eventually get so heavy that it cannot withstand its own gravitational force and the core collapses, resulting in the massive explosion of a supernova.

Between the giant red star and white dwarf star phases elements ranging from oxygen to iron are formed. But during a supernova the star releases so much energy that elements heavier than iron, such as gold and uranium, are formed. It is this giant explosion that propels these elements out to space.

The origin of life and the origin of the stars are linked in the most intimate way. The very matter we are made of was formed in the hearts of giant red stars. The elemental abundance on earth almost matches the relative abundance of elements created by stars. These dying stars are the crucible of creation.

We are literally made of star stuff.


Gif credit: http://www.nobelprize.org/educational/physics/energy/fusion_2.html


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Biologist. Archaeologist. Aspiring writer.

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