Natalie Portman is captivating as a damaged electro-pop star known as Celeste in But she doesn’t appear until nearly an hour into the film, which begins with a prologue on her origins (she’s initially played by Raffey Cassidy, who doubles as the pop icon’s daughter in the second half). A violent high school tragedy sets the teen Celeste and her sister Eleanor (Stacy Martin) on a path to martyrish stardom after a song they compose about it quickly goes viral. Soon afterward, slick management appears in the form of Jude Law as an alternately paternal and predatory handler. Willem Dafoe provides the voice-over, narrating Celeste’s rise to fame as if in a “Behind the Music” special. But the film yields no insights into stardom’s perils that surpass, say, the recent Netflix documentary on Lady Gaga’s life. Like Gaga, Portman’s Celeste is at her most interesting onstage in full glam, giving it all to the fans — in her case, singing songs expertly written by Sia.