Dr Karl: Yes, humans are stardust
Dr Karl Kruszelnicki
Dr Karl Kruszelnicki
It’s true that: about 86 per cent of our mass is “stardust”; almost certainly we each carry at least a few million atoms of gold inside us; and yes, the carbon atoms we carry are mostly many billions of years old. As of 2022, there are 118 different elements in the Periodic Table, and there are seven main ways these have been created.
Hydrogen and helium are not stardust; they formed first, before stars came into existence. These two elements were created just 380,000 years after the Big Bang of 13.8 billion years ago, once things had cooled down enough for atoms to condense out of pure super-hot energy.
After a few hundred million years, some of the hydrogen and helium had condensed into the first generation of stars. You can think of stars as giant machines that combine smaller atoms together to make heavier atoms and, at the same time, release a lot of energy. So some of the atoms in your body could date back more than 13 billion years.
All the other elements were made by six major pathways: inside low-mass stars in their death throes; inside exploding massive stars (oxygen, for example, was created in this way); by cosmic ray fission; as the debris
from colliding neutron stars; as white dwarf star explosions; or from human synthesis.
The human body is made of about 20 different elements. An 80kg human carries about 52kg of oxygen –
about 65 per cent of the mass of your body. Carbon makes up 14.4kg – about 18 per cent of your mass. About 8kg (10 per cent) of our body weight is hydrogen atoms that go back practically all the way to the Big Bang. The fourth-most abundant element in your body is nitrogen – about 2.4kg (3 per cent). Between them, those four elements (hydrogen, oxygen, carbon and nitrogen) account for more than 99 per cent of all your atoms, and 96 per cent of your mass.
So the song is partly true – although getting life advice from song lyrics is usually a bad move. But I still like the song.
