Answer: mass energy of hydrogen fusing into helium
Step-by-step explanation:
To shine, the Sun transforms the hydrogen into helium by means of nuclear fusion.
In this sense, nuclear fusion is the union of two light atoms, releasing energy in the process, which is the cause of the great luminosity of a star like the Sun. Especifically, the hydrogen nuclei will begin to fuse, generating helium nuclei in the process and releasing huge amounts of energy.
In other words, the Sun generates its own energy, by nuclear fusion.
Hence:
The long-term source of energy that powers the Sun is mass energy of hydrogen fusing into helium.