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Jan 15 2013

Element Formation in Stars

We are all made of star stuff, the left of detritus from stars that exploded long ago. Nearly all the elements in the solar system, Milky Way and beyond were created in the extreme conditions at the centre of stars. Ultimately you and I are made from this star stuff.

Were do the elements come from?

About 300,000 – 350,000 years after the Big Bang the simple elements hydrogen and helium formed from the aftermath of the start of the universe. Subsequently these elements started to clump together into larger and larger balls of gas. Eventually they formed what are called protostars which start to collapse and heat up under the increasing gravitational pull of the ball of gas. Once a critical point is reached the temperature and pressure at the centre of the protostar gets so high that the hydrogen atoms start to bump into each other with such force that they fuse together. This is called nuclear fusion and the process of making elements through nuclear fusion is called nucleosynthesis. The protostar now becomes a fully fledged main sequence star.

What elements are produced through nucleosynthesis?

The fusion process releases a tiny amount of energy each time two hydrogen atoms are fused together which is eventually radiated away from the surface of the star in the form of heat and light. The fusing together of two hydrogen atoms produces a helium atom. Technically this process is called the Proton – Proton (PP) Chain and it is the main form of helium production in main sequence stars. As the star runs out of hydrogen it collapses, increasing the temperature and pressure until it stars to fuse helium.

Fusing helium produces beryllium which, although unstable (it lasts just 7×10-17s), can fuse with another helium atom to produce stable carbon, the basis of all life on planet Earth. If another helium atom fuses carbon it forms stable oxygen.  This type of element formation can continue up the period table but depends on the stars size. Larger stars produce heavier and heavier elements.

So we have all come from stars?

Yes. The oxygen we breath, the water that we drink and which makes up 75% of our bodies, the soil we are standing on and the computer or iPad screen you are reading this on are all made up of elements formed at the centre of stars.

How did we get to here from the centre of the star?

Once the star has fused all it’s atoms into heavier and heavier elements it is unable to maintain the temperature and pressures required continue fusion. After iron (Fe) an input of energy is required to enable fusion of two iron atoms. Since this is impossible the star violently collapses resulting in a massive explosion called a supernova. A supernova explosion throws off massive amounts of material from the star into the surrounding interstellar void.

The material thrown off is called a Planetary Nebula and it contains all the elements that have been formed in the star. The Planetary Nebula is the how the elements formed during the stars lifetime or redistributed throughout the galaxy. Eventually the gas and dust in the planetary nebula will start to clump and, over a long enough time period,  might form another star and maybe a planet or two in orbit around the star.

This is what happened to Earth and how we eventually came to live on the planets surface.

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