Complex sentence: Only around 15 events have been classified as a calcium-rich supernova (as of August 2017)-a combination of their intrinsic rarity and low luminosity make new discoveries and study difficult, meaning they are one of the most mysterious supernova subclasses currently known.
Split into simpler sentences: Only around 15 events have been classified as a calcium-rich supernova (as of August 2017)-a combination of their intrinsic rarity and low luminosity make new discoveries and their subsequent study difficult. This makes calcium-rich supernovae one of the most mysterious supernova subclasses currently known.