stellar association vs grouping

stellar association and grouping both are nouns.

Word	Noun	Adjective	Verb	Adverb
stellar association	Yes	No	No	No
grouping	Yes	No	No	No

As nouns, grouping is a hypernym of stellar association; that is, grouping is a word with a broader meaning than stellar association:

stellar association: a very large, loose grouping of stars that are of similar spectral type and relatively recent origin.
grouping: any number of entities (members) considered as a unit

Other hypernyms of stellar association include group.

stellar association (noun)	grouping (noun)
a very large, loose grouping of stars that are of similar spectral type and relatively recent origin.	any number of entities (members) considered as a unit
	the activity of putting things together in groups
	a system for classifying things into groups