Illustration of how a failed supernova can become a black hole. (P. Jeffries/STScI/NASA/ESA) In 2009 a giant star 25 times more massive than the Sun simply…vanished. Okay, it wasn't quite that simple. It underwent a period of brightening, increasing in luminosity to a million Suns, just as if it was ready to explode into a supernova. But then it faded rather than exploding. And when astronomers tried to see the star, using the Large Binocular Telescope (LBT), Hubble, and the Spitzer space telescope, they couldn't see anything. The star, known as N6946-BH1, is now considered a failed supernova. The BH1...