Briefly explain why white-dwarf supernovae are useful for measuring cosmic distances.

 They all come from explosions of white dwarfs that reach the white-dwarf limit of 1.4 solar masses. As all white dwarf progenitors have the same mass, we expect them all to have the same luminosity. Observations of white-dwarf supernovae for which we can measure distance independently confirm that they all have the same luminosity. Because we know their luminosities, we can use their apparent brightnesses to determine their distances from the luminosity-distance formula. Another advantage is that white dwarf supernova can be as bright as entire galaxies so that they can be detected in very distant galaxies.