A huge variety of chemical compounds make up the world around us, from the minerals in rocks to the DNA in our cells. But all of these compounds are made up of different combinations of the same basic elements, of which a mere 92 are found in Earth’s crust and biosphere. This rarely occurs in nature, which means Earth is essentially stuck with the same proportions of different elements that it started out with. But where did the elements come from in the first place? They’re the result of a process called nucleosynthesis, which produced the atomic nuclei that are an element’s basic building blocks. Some nucleosynthesis occurred in the very first moments of the universe, immediately after the Big Bang, when isolated protons formed the nuclei of hydrogen atoms. This is the lightest of all elements, followed by helium with two protons and lithium with three. And to make things a little more complicated, these heavier nuclei contain neutrons as well as protons, but this only affects their weight and not their chemical properties.
Although the primordial phase of nucleosynthesis following the Big Bang filled the universe with matter, this was all in the form of very lightweight elements – around three-quarters hydrogen, one-quarter helium and trace amounts of other elements. There was no hint at that stage of some of the most important elements from our own point of view, such as carbon, which has six protons, and in its most common form six neutrons.
This is where stars come to the rescue. They produce their heat and light through a process called nuclear fusion, which means exactly what it says – joining the nuclei of lighter elements together to make heavier ones. Stars can fuse hydrogen to make helium, which we already had from the Big Bang, but then carry on to heavier elements, including ones like carbon and oxygen that are so important to us. In the course of time, these new elements found their way into the interstellar medium – the extremely tenuous gas and dust pervading the space between stars. This is constantly replenished by the stellar winds that blow outwards from stars all the time, and from their final disintegrations – or dramatic explosions in the case of very massive stars – when they run out of material for nuclear fusion and die.
However, the death of one star isn’t the end of the story; it’s just a step in an ongoing process of cosmic recycling. Some of that chemically rich material in the interstellar medium will eventually become dense enough to collapse down into a nebula, and from this
