A prepositional phrase functions as an adjective when it modifies a noun or pronoun in the sentence.

A star is a huge nebula of gas.

The nearest star to our sun is Proxima Centauri.

Sirius is the brightest star in our sky.

A group of stars that appear close together is called a star cluster.

In the above sentences, the prepositional phrases of gas, to our sun, in our sky, and of stars modify the nouns nebula, star, and group.