Star Stuff? Try “Big Bang Stuff!”

One of the (deservedly) frequently quoted observations by my hero Carl Sagan is that we are all star-stuff. The chemical elements in our bodies – and everything we see around us on Planet Earth – were forged in exploding stars billions of years ago. This is a profound realization. It seems to me that doesn’t go far enough, however.

I started thinking about the origins of the elements in our bodies, and made a connection I haven’t seen elaborated before. To explain myself, I have to explain the origin of the universe first.

The 98 elements that occur in nature are divided up by astronomers into two groups: hydrogen and helium, and “metals:” all the other stuff. Hydrogen and helium were the products of the evolution of matter following the big bang. The “metals” were subsequently produced in a process dubbed nucleosynthesis: nuclear fusion taking place within stars (the process was worked out over half a century ago; the landmark paper is E. M. Burbidge, G. R. Burbidge, W. A. Fowler, F. Hoyle. 1957. Synthesis of the Elements in Stars, Rev. Mod. Phys. 29: 547). The proportions of these things are considered very important. The ration of hydrogen to helium in the observable universe is one of the hallmark tests for cosmology and models of the origins of the universe. Different models predict different ratios, and only the natural ration of about 76% hydrogen to 24% helium gets to decide which models fly.

The other stuff is used to characterize stars, with a measure called metallicity – the proportion of the stuff of the star that is not hydrogen or helium. For example, the metallicity of the sun is approximately 1.8% by weight. Put the other way, the sun is 98.2% hydrogen+helium by weight. This quantity is very helpful to astronomers as it’s a measure of the age of stars, among other things. The older the star, the higher the expected metallicity, as the metals are added by the very process of fusion. Looked at one way, it’s stellar pollution.

This started me thinking about human metallicity. There’s a nice summary on Wikipedia on the elemental composition of the human body ( Here are the top ten elements and the percentage of the body, by weight and atomic proportion, that they represent:

  • Oxygen – 65% by weight but 24% by atomic proportion
  • Carbon – 18% by weight but 12% by atomic proportion
  • Hydrogen – 10% by weight but 63% by atomic proportion (!!)
  • Nitrogen – 3% by weight but 0.58% by atomic proportion
  • Calcium – 1.4% by weight but 0.24% by atomic proportion
  • Phosphorus – 0.78% by weight but 0.14% by atomic proportion
  • Potassium – 0.25% by weight but 0.033% by atomic proportion
  • Sulfur – 0.25% by weight but 0.038% by atomic proportion
  • Sodium – 0.15% by weight but 0.037% by atomic proportion
  • Chlorine – 0.15% by weight but 0.024% by atomic proportion

Ok, so what, I hear you say. Well, look at #3 in this list – hydrogen. Ten percent of our body mass is hydrogen, in chemical compounds like water, sugars, and all sorts of other things. However, two facts about hydrogen are important. First, it’s the lightest element there is, so 10% by weight is a big number by atoms. Second, hydrogen was not made by nucleosynthesis. It was made by the Big Bang itself – and sixty-three percent of the atoms in our bodies are hydrogen.

If we shift our attention away from overall proportions by mass and re-list things by number of atoms, we see a different picture of our own composition. Yes, we are star-stuff – but 63% of the atoms in our bodies have their origins in the Big Bang itself. These humble hydrogen atoms that are the majority population in our bodies – and are the most abundant stuff in the visible universe –  went through stars that exploded, but they came from the Big Bang. In a real sense, so did we.

© 2012, David Allan Galbraith