White dwarf stars like our sun do not have enough mass for a core to collapse and create a supernova. However many of these white dwarfs are circling other stars and when the companion star expels it’s outer layers as a red giant the white dwarf gets extra mass and when it has enough mass it briefly resumes the fusion process and eventually creates a white dwarf supernova. All of these supernovae are exactly the same due to the fact that they all occur when the white dwarf reaches the Chandrasekhar limit of EXACTLY 1.4 times the mass of the sun. This is very important because since we know exactly how much energy is being released when these are spotted in the universe we can determine the distance from the earth. We can tell the difference between a large supernova and a white dwarf supernova by looking at it spectrum. The spectrum will show the outer layers of the star and a large supernova will still have hydrogen as its outer layer but a white dwarf used fusion to create carbon and expelled the rest of its outer layer so we will only see mostly carbon in the spectrum.
(The Great Courses – The Life and Death of Stars)