Pedestrian account of an exciting theory about the extinction of the dinosaurs. Was it because their bodies grew too big for their brains? Because wily mammals raided their food supplies? Forget these schoolboy theories, says physicist Muller: the big reptiles bit the dust when a death star--code-name Nemesis--hurled a massive comet or asteroid into the earth 65 million years ago. Moreover, Nemesis, which orbits our sun from about three light-years away, unleashes such disasters every 26 million years. The frightful conclusion, for those with an eye on the future: humans are next on the hit-list--in 13 million years or so. The best part of the story circles around Muller's mentor, Luis Alvarez, a Nobel-laureate physicist who first suggested that a five-mile-wide asteroid killed the dinosaurs. Too bad Muller didn't stick with his portrait of this grown-up Mr. Wizard, who counts himself an explorer in the tradition of Sir Richard Francis Burton and leaps lightly from one brilliant invention to another (his most unusual being a method akin to X-raying for examining the interior of Egyptian pyramids). Instead, we are treated to a workmanlike blow-by-blow account of how Muller evolved the Nemesis theory--a fairly tepid tale of scientific research and analysis warmed up now and then by outward events (such as the media firestorm following public disclosure of the theory). In other words, this is a sort of Double Helix for the slide-rule boys, but with none of James Watson's nastiness or punch. Too bad, too. that what's being debated sounds substantially less profound than the fundamental structure of the gene--and, as far as Muller's death-star notion goes, far less convincing; while evidence mounts for Alvarez's collision theory, Nemesis remains unaccepted by the bulk of the scientific community. Astronomy and dinosaur buffs will eat this up--the terrific title alone will grab a lot of readers--but others may find themselves nodding off.