A defensestar isn't really a star, but it is something shiny in the sky that can look like a star if it's bright enough. This is a satellite, and while it's not always defensive, satellites can have defensive properties. Think ion canons and video and things like that.
This is a jukugo word that uses the on'yomi readings of the kanji. You know them both already, so you should be able to read this word!