Researchers studying images of the distant star N6946-BH1 were shocked to find that the otherwise extremely bright star had outright disappeared. The star was still visible in images taken by the Hubble Space Telescope as recently as 2009, but in images taken in 2015, it was barely a faint glow when viewed in infrared, and not visible at all in the visible spectrum.
Residing in NGC 6946, otherwise known as the Fireworks Galaxy, N6946-BH1 was an impressive star 20 million light years from earth, estimated to be 25 time the mass of our own sun and around 100,000 times brighter. In early 2009 it was recorded to have brightened to several million times brighter than Sol, but has almost completely disappeared in the years since. Even more puzzling was the absence of a supernova that typically would have been associated with the collapse of such a massive star, prompting researchers to hypothesize that a new phenomenon had been observed, called a "failed supernova".
If N6946-BH1’s mass had indeed collapsed onto itself to form a black hole, the collapse might have converted a portion of its mass into a massive neutrino burst, with the resulting shock wave causing the brightness observed prior to the star’s dimming. The N6946-BH1’s powerful gravitational field kept its mass from exploding outward altogether, with that remaining mass instead collapsing into a black hole, with a residual accretion disk surrounding it, providing the faint infrared glow. The implication made by this hypothesis is that the gravitational collapse of a massive star that forms a black hole doesn’t necessarily have to result in a spectacular supernova as well, ending its life "with a whimper rather than a bang", as NASA’s Astronomy Picture of the Day Archive describes it.