What is a Type Ia Supernova?
A Type Ia supernova is thought to be the explosion of a white dwarf star (consisting of carbon and oxygen) that is accreting mass from a companion star and eventually becomes unstable to thermonuclear runaway when it reaches a critical mass of 1.4 solar masses (called the Chandrasekhar mass, after UChicago physicist Subramanian Chandrasekhar). Since all Type Ia supernovae have about the same mass, they all have about the same explosion energy and therefore similar peak luminosity . . . hence, they are nearly standard candles. Since the white dwarf contains [almost] no elements lighter than carbon, Type Ia SNe are identified by the lack of hydrogen and helium features in their spectra, and the presence of heavy elements such as silicon, nickel, and iron. The evolution of a star until explosion is nicely illustrated here. Why do we see a Type Ia SN ? With an explosion of such enormous energy, it may seem obvious that it would appear bright in the sky. However, the fact that SNe are visib
