© Robert Kirshner.
The above plot shows light curves for five different supernovae. The brightness of each supernova (in the units of magnitude favored by astronomers) is plotted as a function of time. Note that the light from the brighter supernovae (purple and blue curves) decreases more slowly over time than does light from the dimmer supernovae (the black curve). This subtle difference allows astronomers to calibrate the brightness of the type Ia supernovae and use them as standard candles. (Unit: 11)