• Question: why do red giant stars fuse helium into carbon and oxygen? why doesn't it go in the order of the periodic table like they did when they were normal stars (hydrogen-helium), e.g. fusing helium into lithium, then beryllium? :)

    Asked by marybarry to Damien, Suzi, Tim on 23 Jun 2011.
    • Photo: Suzi Gage

      Suzi Gage answered on 23 Jun 2011:

      Hi @marybarry
      GREAT question!
      I wasn’t sure of the answer, so I looked it up. Apparently some red giants have an inner core of inactive helium, and an outer shell which is still fusing hydrogen in to helium.
      But there are also some red giants called carbon stars which turn helium to carbon, via something called the ‘triple alpha process’. As helium begins to accumulate in the core of the star, sometimes it fuses with hydrogen, and this creates unstable atoms (of lithium and beryllium, as you said) which quickly break in to smaller helium nuclei again. But when hydrogen and helium are fusing quick enough, in enough volume, some of the unstable larger beryllium nuclei created will fuse with another helium atom before they break down into helium again themselves, and this creates carbon 12 atoms, which are stable.

      This wiki page is a little complicated, but there’s a really nice diagram showing what happens on it
      So this is why it doesn’t simply go up in steps as you might expect.

      I’m afraid I can’t find anything about red giants creating oxygen though, sorry about that.

      But I hope the first part of my answer helps! I learned something new with this too, which is always cool, so thanks for the question!