Stellar nucleosynthesis is the process by which stars forge elements inside their cores.
The only element not formed in this way is hydrogen, the most abundant and lightest element in the universe: it was formed in the initial aftermath of the Big Bang.
The cores of stars have crushing pressures and temperature.
For example, the sun’s core temperature is about 15 million degrees C.
In these harsh conditions, the nuclei of atoms undergo nuclear fusion.
The hydrogen nucleus is just one proton.
Inside the core, these nuclei come together to form helium nuclei (two protons and two neutrons).
This is the p-p (short for proton-proton) process.
In more massive stars, however, stellar nucleosynthesis treads a different path — one dictated by the availability of energy.
More massive stars have a higher core temperature.
There, in the so-called carbon-nitrogen-oxygen (CNO) cycle, the nuclei of these elements come together in different ways to form elements from helium onwards.
When a star runs out of nuclei to fuse, its core contracts.
This in return increases its temperature, triggering nuclear fusion yet again.
This process goes back and forth until the star starts to produce iron in its core.
Iron is the lightest element for which fusion consumes more energy than it releases
Elements heavier than iron can only be synthesised outside a star when it goes supernova.
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