Great amounts of energy are released
in the core of a star as lighter elements
combine and form heavier elements
during the process of
(1) compaction
(2) condensation
(3) radioactive decay
(4) nuclear fusion

Great amounts of energy are released in the core of a star as lighter elements combine and form heavier elements during the process of nuclear fusion.

Explanation:

Nuclear fusion is the defined as the fusion or combination of many lighter elements to form heavier element. Example: Two hydrogen nuclei combine or fuse to form a helium. The greatest amount of energy in star is its hydrogen which gets exhausted consequently resulting in burning of helium and forming heavier elements.