Alex Chilton had the classic slow-build pop-career arc in reverse: having scored a massive hit (“The Letter”) at the beginning of his career as the Box Tops’ teenage singer, he spent the remaining four decades of his life supported by a small cult of fans making increasingly eccentric records. His artistic peak, though, came with Big Star, the early-’70s band that exemplified power pop’s ideal of stringing together crystalline guitar hooks and creamy harmonies in the context of profound emotional darkness. Commercial flops in their day, Big Star’s records became hugely influential for rock bands a generation later. “September Gurls” is the group’s high point, an oblique but thrilling evocation of romantic ache. What are “September girls” and “December boys”? Chilton shows us the answer, rather than telling us — he articulates every spiky, tingling guitar note he plays and pushes his voice so high, it trembles.