Using Supernovae in Cosmology
You may have heard of Einstein's equation, E = mc2. Here E stands for energy, m stands for mass, and c is the speed of light. This is true for core-collapse supernovae (types II, Ib, and Ic). The energy of the explosion increases with the mass of the star. The more massive the star, the brighter the supernova.
Type Ia supernovae are different. They explode when the mass of a white dwarf star reaches about the same mass limit - 1.44 times the mass of the Sun. So we can assume that because they have similar mass, all type Ia supernovae shine at a similar level of brightness when they explode. This means that their distance from Earth controls how bright they appear to be in the sky, not the star's mass.
This knowledge lets us use type Ia supernovae as 'standard candles' to work out distances in space. Because these explosions are so energetic they are also very bright. This means we can detect supernovae even in very distant galaxies, which makes them especially useful standard candles. In 2011, research that used type Ia supernovae in this way was given the Nobel Prize in Physics. The work found that the expansion of the Universe was speeding up.