The image on the left is a zoomed in view of M101's western spiral arm taken after supernova 2011fe was discovered. It is the blue star indicated by tick marks. The image on the right is the same field of view taken in 2017 after the supenova had faded into obscurity. Supernova 2011fe was a type Ia supernova that occurs when a white dwarf in a binary system siphons enough material off its companion star to achieve 1.4 solar masses. Accordingly, all type Ia supernovae have the same luminosity and serve as standard candles for measuring cosmic distances. This one helped pin down the distance to M101 at 21 million light-years.
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