These are determinatives, written in a word to give an idea of the meaning, not the pronunciation. They are ignored when Egyptologists transcribe or pronounce hieroglyphs and the Egyptians wouldn't have pronounced them either.
The sky symbol can be used in words referring to the sky or in the word Hry heri "on top of" The star symbol appears in words about stars, planets, or worship.
Meaning what? As I said, they should be used with phonetic symbols. Unfortunately, I only have a small dictionary and grammar at home, and I can't anything to match that.