Learn Spanish (Latin America): Aquellas son unas estrellas muy grandes.

Both words mean "those", but they indicate different distances. You use esas for things closer to the person you are talking to, and aquellas for things far away from both of you. Since stars are very far up in the sky, aquellas is the perfect choice!

Is aquellas an adjective or a pronoun in this sentence?

It is a pronoun here. It stands on its own as the subject of the sentence to mean "those ones" or "those things over there". If it were acting as an adjective, it would sit right next to the noun it describes, like in aquellas estrellas son muy grandes (those stars are very big).

Why do we need the word unas here? The English translation doesn't say "some stars".

In Spanish, when you use the verb ser (to be) followed by a modified noun (a noun plus an adjective, like estrellas muy grandes), you almost always need an indefinite article like unas. Even if we don't translate it to "some" in English, leaving it out would sound unnatural in Spanish.

Since estrellas is feminine and plural, why is the adjective grandes and not "grandas"?