The image at left is a close up of the western spiral arm of galaxy M101 taken in 2011 after supernova 2011PTFkly appeared. The image is the same field of view taken in 2017, long after the supernova has faded into obscurity. This was a type I supernova, which occurs when a white dwarf in a binary system siphons enough gas off its companion star to trigger a supernova within its core. This always occurs when the white dwarf reaches 1.4 solar masses. Hence all type I supernovae are of equal brightness and they serve as excellent standard candles to help determine the distance to the host galaxy. This is in contrast to type II supernovae that are blue giant stars rapidly depleting their fuel for nuclear fusion. They tend to occur in HII regions of galaxies and may be of different brightness depending upon the mass of the exploding star.

Left image: L:R:G:B=275:80:75:75 minutes=8 hours 25 minutes total exposure at f/8.3. Right image: L:R:G:B=320:190:190:180 minutes=14hours 30 minutes total exposure at f/7.2.

This image was published in the March, 2020 issue of Astronomy magazine.