V404 Cygni 
V404 Cygni has something of an identity crisis. The "V" in its name indicates that it's a variable star, so it gets brighter and fainter. It's also known as a nova, because at least three times in the 20th century it produced bright outbursts. And finally, it's known as a soft X-ray transient because it occasionally emits brief flashes of X-rays.
Together, these identities tell astronomers that V404 Cygni is a binary -- two objects locked in orbit around each other. One of them is an orange star that's a little smaller and cooler than the Sun. The other is a black hole.
They're so close together that the black hole is stealing gas from its companion. But the flow of gas is a bit lumpy, which is why the system sometimes emits bursts of X-rays.
The gas forms a bright, hot disk around the black hole. When enough gas builds up, it produces a dramatic outburst known as a nova. The outburst causes the system to shine hundreds of times brighter than normal.
The first eruption from V404 Cygni was seen in 1938, with the most recent in 1989. And it's likely building toward another eruption even now. The eruptions may occur about every 20 to 30 years. If so, then the next one could come at any time.
The system is in Cygnus, the swan, which is high in the eastern sky at nightfall. It lurks near the line that connects the swan's beak with the center of its body -- a faint star system that may be about ready to get a lot brighter.
Script by Damond Benningfield, Copyright 2011