On a deserted road at night, a streetlight flickers rarely but at full power. That's how WZ Sge-type stars behave: they smolder barely noticeably for years, then suddenly flare hundreds of times brighter. Astronomers decided to check what happens in their gas disks during quiescence, and found almost emptiness.
The disk around such a star resembles a very thin cloud: it is transparent to light, like a window pane. Matter falls into it in tiny portions — over a year, the accumulated mass is comparable to a small asteroid. According to previous calculations, before the flare the disk should have gradually accumulated gas and become denser. But observations showed that the disk's brightness does not increase; it remains rarefied right up to the moment of the explosion.
This discovery breaks the familiar scheme. If the disk does not accumulate matter before the flare, then something else triggers it. Perhaps the disk itself sharply changes its properties due to internal processes, not because of gas inflow from outside. The exact mechanism is still unknown, but one thing is clear: the old explanations no longer work.
Now astronomers must search for a new trigger for these rare flares. The quiet invisible disks keep their secret.
The star WZ Sge flares so rarely that about thirty years pass between its two superoutbursts — enough time for an entire generation of astronomers to come and go.