Then a scientist called Arthur Eddington suggested that nuclear fusion might be the source of the Sun's apparently long-lived brightness.
Fusion is when two atomic nuclei join close enough to become one atomic nuclei. A surprising amount of energy is released when this happens. TL;DR - this energy is what makes a star shine.
The Sun fuses hydrogen into helium, thus releasing vast quantities of energy in the form of gamma rays.
In smaller stars like our Sun, the fusing of hydrogen into helium takes place through a proton-proton chain reaction. Looks a little bit like this:
In bigger stars, they have fun fusing hydrogen into helium through a Carbon-Nitrogen-Oxygen cycle. It looks more like this:
Wanna have some fun? Look up stellar nucleosynthesis, which explains the output of fusion, and find out what nucleosynthesis makes.
What happens when a star starts fusing Mg into Fe?
Her Grace wonders if alchemists knew about fusion, if they'd still try to find methods of turning lead into gold?