The legend of Big Star was almost over at the starting gate: when co- founding singer-guitarist Chris Bell quit the Memphis pop band, right after its 1972 debut, #1 Record, came out and flopped. The LP — a white-soul Abbey Road with a Lennon-McCartney-like tension between the grainy ennui of ex-Box Tops singer Alex Chilton and Bell's desperate romanticism — became an object of cult love. But Chilton, bassist Andy Hummel and drummer Jody Stephens regrouped for the atomic jangle of 1974's Radio City. Even after Hummel split that year, Chilton and Stephens hung on for a last weird victory, the pop-stripped-bare shock of 3rd (a.k.a. Sister Lovers), a record so emotionally wrenching that no label would touch it for four years. (It was finally issued in 1978.) Barring the discovery of more golden eggs, the four CDs of Keep an Eye on the Sky are the last word on Big Star's first, ultimately glorious lifetime: the albums plus outtakes, related curios and live tracks from a 1973 Memphis club show. The original sequences of #1 Record and Radio City are disrupted with alternate mixes of pure-pop pillars like "In the Street" and "Back of a Car." More interesting are rattling garage-pop demos for Radio City and Chilton's harrowing solo sketches for 3rd. But what looms largest here is what might have been. Bell, who died in 1978, is out of earshot before Disc One is over. (Rhino has reissued a posthumous 1992 solo set, I Am the Cosmos, in a deluxe edition.) As Bell's idea of a perfect pop band, Big Star ended when he left. Everything that follows is stubborn, brilliant defiance.