Transcript: When a massive star dies the supernova that results can rival an entire galaxy in brightness, so it can be seen to a very large distance, a billion lightyears or more. When a single massive star dies it does not do so in a well regulated way, but the supernovae that result from a binary system are standard bombs that can be used as distance indicators. Basically the mass from the companion spoons slowly on to a white dwarf until it pushes it over the Chandrasekhar limit, 1.4 solar masses. At that point explosive nucleosynthesis occurs, carbon and oxygen are fused up to silicon and then nickel, and then nickel 56 decays to cobalt 56 and then to stable iron 56. This type of supernova explosion called a Type Ia is well regulated enough to have a variation of only ten percent from one explosion to the next and so is the best distance indicator to use in cosmology.