"Hydrogen nuclei" refers to the core or center part of hydrogen atoms. It consists of a single positively charged particle called a proton. Full definition
This means that it generates its energy by nuclear fusion of hydrogen nuclei into helium and is in a state of hydrostatic balance — neither contracting nor expanding over time. (newworldencyclopedia.org)
The team looked at the alignment of the spins of carbon and hydrogen nuclei in molecules of methyl iodide. (newscientist.com)
This theory assumes that the heavier elements have properties which can only actually result from a very rare process in the universe which makes several hydrogen nuclei fuse into heavier ones. (religion-online.org)