A pair of unusual stars known as DI Herculis has confounded astronomers for three decades. For most binaries, the theoretical and observed precession rates are in agreement. DI Herculis, however, has resisted explanation for 30 years. The observed precession rate is a factor of four slower than the theoretical rate -- a disagreement that once was interpreted as evidence for a failure of general relativity. Now, new observations by MIT researchers and their colleagues have provided data that solves the mystery once and for all. And the explanation knocks down another possible challenge to Einstein's theory of relativity.