For a quarter of a century, the star system V404 Cygni had been pretty quiet. It produced some occasional “flickers” of X-rays, but otherwise was unremarkable. But just three months ago, it woke up — the result of a black hole suddenly taking a “big gulp” of hot gas. The black hole is about a dozen times the mass of the Sun. It has a companion star that’s a bit smaller than the Sun. The two are so close together that the black hole pulls gas off the companion’s surface. The gas forms a hot, glowing disk around the black hole. The gas builds up until it reaches a tipping point. The gas then plunges into the black hole, making the system shine thousands of times brighter than normal. V404 Cygni first jumped to prominence during an eruption in 1938. It produced another eruption in 1989, which was discovered by an orbiting X-ray telescope. And on June 15th, it erupted once again. Over several days, X-ray telescopes in space recorded scores of powerful outbursts. Other telescopes in space a
