What are Type Ia supernovae? White dwarfs can also produce what astronomers call a Type Ia supernova. In the most established theory behind Type Ia supernovae, the white dwarf grabs matter from its companion–until it reaches a critical mass. Previously, there has been no clear, visual evidence of a white dwarf undergoing a double detonation. While using Type Ia supernovae as a cosmic measuring tape, astronomers discovered the accelerating expansion of the universe.