field, sphere (of work), area (of study) / the area of one person's focus of work or study

(physics) nucleon - a general term of the proton and the neutron

important, significant

importance, consequence, significance

a child / (figuratively) a girl, especially a dear or desired one (compare use of English baby, babe) / a smaller or younger version of a bigger object

atomic nucleus / (algebra) kernel / (astronomy) core, nucleus / cell nucleus

to research, to study

a piece of research, a study, investigation

science / a field of study

nuclear / atomic / nuclear power / core / kernel