• Question: How were atoms made?

    Asked by quickscope to Sara on 17 Jun 2011.
    • Photo: Sara Imari Walker

      Sara Imari Walker answered on 17 Jun 2011:

      Ooooo quickscope!! Great question!! This is cosmology at its finest =)

      When the universe was first made there were no atoms. Before atoms there was just a really really hot plasma of protons and electrons. But as the universe expanded and cooled the protons and electrons started to slow down. Electrons started to get captured by protons to make Hydrogen. Some Helium and even lithium was also made. But the temperature cooled too quickly to make anything else. So the abundance of elements at the start was about 75% Hydrogen, 24% Helium, and less than 1% Lithium. The buzz word for this process is “primordial nucleosynthesis” – primordial because its very ancient – happening only about 4 minutes after the big bang (wow!!! that’s just over 13.7 billion years ago) and nucleosynthesis because it is the making of atoms (nuclei). How do we know this is true, if it happened so long ago? Well we are bathed in radiation from the Big Bang (don’t worry its not harmful but it does cause static on our tvs!) called the cosmic microwave background. This radiation is a relic of nucleosynthesis, once the universe cooled to form atoms, light could travel without interacting (i.e. without getting stuck on electrons or protons – atoms have neutral charge). So the universe went from being an opaque plasma fog to being transparent. The light photons that were around then could suddenly travel unhindered!! These light photons are still around us everywhere today. They have a temperature set by the cosmic expansion, which is how we extrapolate back in time to calculate the when the temperature of the universe was just right for nucleosynthesis. The elemental abundances quoted above match extremely well with present observations!

      MOST of the universe today is still made of hydrogen and helium!! For example, stars are almost entirely hydrogen with some helium. So how do the rest of the elements get produced? Well in the stars that’s how!! For most of a stars lifetime it burns hydrogen in its core -> fusing hydrogen atoms together to make Helium. When a star runs out of Hydrogen to burn its main sequence lifetime is over (that’s the longest phase of its life when it burns H – our Sun is presently a main sequence star). Stars then start to burn Helium fusing it to Carbon. Helium eventually runs out so the star switches to Carbon burning to produce Oxygen and on and on all the way up to Iron in some cases. How do we get these elements out of the stars? Well they explode, in brillant supernova!! We are literally made of star dust =)

Comments

  • Photo: Sara

    Sara commented on :

    Hey again quickscope! For some reason there is some missing text in my answer above that I can’t edit in. So here is the full answer that was supposed to be posted:

    Ooooo quickscope!! Great question!! This is cosmology at its finest =)

    When the universe was first made there were no atoms. Before atoms there was just a really really hot plasma of protons and electrons. But as the universe expanded and cooled the protons and electrons started to slow down. Electrons started to get captured by protons to make Hydrogen. Some Helium and even lithium was also made. But the temperature cooled too quickly to make anything else. So the abundance of elements at the start was about 75% Hydrogen, 24% Helium, and fusing hydrogen atoms together to make Helium. When a star runs out of Hydrogen to burn its main sequence lifetime is over (that’s the longest phase of its life when it burns H – our Sun is presently a main sequence star). Stars then start to burn Helium fusing it to Carbon. Helium eventually runs out so the star switches to Carbon burning to produce Oxygen and on and on all the way up to Iron in some cases. How do we get these elements out of the stars? Well they explode, in brillant supernova!! We are literally made of star dust =)