This Chandra image of the Tycho supernova remnant contains new evidence for what triggered the original supernova explosion. Tycho was formed by a Type Ia supernova, a category of stellar explosion used in measuring astronomical distances because of their reliable brightness. In the lower left region of Tycho is a blue arc of X-ray emission. Several lines of evidence support the conclusion that this arc is due to a shock wave created when a white dwarf exploded and blew material off the surface of a nearby companion star. This supports one popular scenario for the trigger of a Type Ia supernova. Understanding the origin of Type Ia supernovas is important because they have been used to determine that the Universe is expanding at an accelerating rate.