 The large binocular telescope, LBT for short, in Arizona, was scanning the fireworks galaxy 22 million light years away looking for supernova candidates. They examined the star named N6946-BH1, a star 25 times more massive than our sun. Stars this size usually end in a supernova explosion, leaving behind a neutron star or a black hole. Here's a graph of the star's luminosity, invisible and infrared light over time. In 2007 Hubble took this picture of the star. In 2009 the star shot up in brightness to become over 1 million times more luminous than our sun for several months. The expectation was it was about to supernova, but it didn't. The star just seemed to vanish, as seen in this image from 2015. After the LBT turned up the star astronomers aimed the Hubble for visible light and Spitzer for infrared light to see if the star was still there. All tests came up negative. The star was no longer there. The researchers eventually concluded that the star must have become a black hole without a supernova.