V404 Cygni

A star system in the constellation Cygnus, the swan, just can’t seem to settle down. Every few decades, it flares to thousands of times its normal brightness — possible feeding frenzies by a black hole.

V404 Cygni is about 8,000 light-years away. It was discovered during an outburst in 1989 by X-ray telescopes in space. A little digging through old photographs showed that it had also popped off twice before. And it erupted again in 2015.

These outbursts probably are caused by interactions between the system’s two components: a black hole about a dozen times the mass of the Sun, and a “normal” star that’s less massive than the Sun.

The black hole pulls gas off the surface of the normal star. As the gas funnels toward the black hole, it forms a wide but thin disk.

The gas may build up in the outer regions of the disk, with only a trickle pouring into the black hole. Over a couple of decades or more, though, so much gas builds up that it suddenly floods toward the black hole. It’s heated to millions of degrees, so it shines brightly in X-rays. That high temperature creates “winds” of gas that blow away from the black hole.

The process isn’t steady, though. During the outburst in 2015, V404 Cygni’s brightness varied dramatically over just hours or even minutes. One study says that’s because the rush of gas isn’t even. Because of the size of the disk, there are gaps between waves of gas — creating an uneven feeding frenzy for a black hole.

Script by Damond Benningfield