Answer:
lighter atoms especially hydrogen
Explanation:
At the heart of the stars, nuclear fusion merges lighter atoms especially hydrogen together in the nucleus of atoms to create new elements.
During nuclear fusion small atomic nuclei combines to form larger ones with the release of a large amount of energy.
The energy released provides the needed temperature for another set of hydrogen atoms to fuse. This process is in turn yields another set of helium atom which releases a lot of energy.
A chain reaction progresses which leads to the formation of new elements.