Constellations, groups of stars, provide astronomers with a roadmap of the galaxy.

Identify the appositive or appositive phrase.

groups of stars

Constellations

with a roadmap

astronomers

Group of stars.

An appositive or appositive phrase renames a noun or pronoun that is right beside, specifically after, it. It is usually used to provide additional information, like a definition or clarification. In this sentence, "group of stars" is defining "constellations" making it the appositive phrase.