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We are made of the stars - where atoms come from


The charismatic astrophysicist Carl Sagan once said:

“The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.”

What does he mean exactly? Or was he just a scientist with poetic flair? 


To understand this, we go back to a table which turns many of us off. The Periodic Table*. I know I know, it's a really ugly table filled with random letters and numbers. A lot of them. The audacity of the universe to produce so many different types. Why can't there just be oxygen and hydrogen?


Don't worry. There are just a few broad was in which atoms have formed, which are very easy to remember:

  1.   The Big Bang produced just 3 elements - a lot of Hydrogen, a little bit of Helium, and a very tiny amount of Lithium. It is no coincidence that these are the first 3 elements on the periodic table, the 3 lightest.

  2.   The stars that formed after the Big Bang, like our sun (our sun is a relatively tiny star), produce the next batch of elements.  The heat from the star causes the initial 3 elements to fuse together, becoming bigger and heavier elements. (Btw, this is also how atomic bombs are designed, from nuclear fusion of elements). Nuclear fusion is responsible for many gases which make up a lot of our bodies, such as Oxygen and Nitrogen. This fusion of lighter elements into heavier ones can create elements all the way up to Iron (Fe on the table, atomic number 26). Iron is too heavy and dense to be created by nuclear fusion. It has to formed in a more explosive setting. 

  3.   Supernovas occur in the last moments of huge stars, where the star burns out leading to a massive supernova explosion. Supernovas are so powerful that for a short period of time, the amount of light produced from that one star is equal to the light of an entire galaxy. This explosion creates sufficient energy to produce heavier elements, Iron​, Copper, Zinc, which are our bodies also contain. We've touched on Iron as being this sort of inflexion point, where nuclear fusion no longer works. All our red blood cells contain a protein called haemoglobin; it is only with iron that haemoglobin is able to bind onto oxygen, which is needed by all our body parts. No iron, we simply cannot use the oxygen we breathe in. 

  4.   Neutron star collisions are what creates the heaviest of our elements. Neutron stars are the collapsed core of giant stars. At the end of its time, these giant stars explode in a supernova (as we covered above), and the star undergoes gravitational collapse, where the gravity of the star compresses all its matter into a super-dense core (equivalent to compressing an entire Mt Everest into one squared cm). Because of how dense these stars are, 2 neutron stars colliding creates even greater energy, that forges all these tightly packed neutrons and protons into heavier elements like Gold and Platinum. Astrophysicists often remark that there is plenty of gold in space - indeed so. In 2019, the collision of 2 neutron stars created 10,000 times the mass of Earth in gold. Most of us also have some traces of gold in us. 


So Carl Sagan wasn't making a metaphor or writing poetry. He literally means what he says that we are made of stardust. Must of our bodies come from stars, in different stages of a cosmic cycle of immense size and time. Whether it is from its burning or from its eventual death, we are made of stars, you and I.


Even though we are this tiny organism on a tiny planet, located in one of 100 billion solar systems in our Milky Way galaxy, itself one of hundreds of billions of galaxies in the universe, the atoms in us might already have been to far-flung corners we would never be to reach, having existed for millions of years, and having experienced being in the cores of some of the biggest stars that no longer exist. How remarkable.

Related links:

We share the breath of every person who has ever lived

Just how big is the universe?

(quick note: in case you're wondering - what's the difference between an atom and an element? Atoms are the smallest form of matter containing protons, neutrons, and electrons; as can be seen in the chart above, each atom has a different atomic number starting from 1 (i.e. 1 proton) and going up to 118. elements contain only 1 type of atom. Read more here

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