YY Herculis is the third of the symbiotics we're showing during our three-week series on this interesting class of stars. YY Her shows symbiotic outbursts of more than two magnitudes lasting for hundreds of days, and recurring sporadically on timescales of thousands of days. These outbursts -- and their physical cause -- focus our attention on what symbiotic stars really are and how they behave. The currently favored theory is that symbiotics are binaries composed of a white dwarf star that accretes mass and a red giant star that donates mass. This is similar to what we see in other cataclysmic variables like dwarf novae, but in symbiotics, the white dwarf is typically hot enough -- and accreting enough matter quickly enough -- that they can undergo thermonuclear burning of the accreted material, becoming somewhat like an exposed core of a red giant. The hot white dwarf puts out a lot of energy which provides the power source for all of the variability in the system. If changes occur in the rate of accretion, or in the stability of the accretion disk around the white dwarf, then it's possible that nuclear burning on the surface of the white dwarf could accelerate, and that's likely what occurs in symbiotic star outburst. That's probably what we see here in the large outburst of YY Her.
Is that the whole story? Probably not. For one, it's possible that in some cases, the accreting white dwarf might undergo extremely rapid nuclear burning, more like a classical nova outburst than a symbiotic star outburst. As an example, RS Ophiuchi is a recurrent nova and a symbiotic star. Another possible behavior for lower-intensity outbursts is the disk instability seen in the dwarf novae, which cause the accretion disk itself (rather than the white dwarf) to brighten. Sokoloski et al (2006; http://adsabs.harvard.edu/abs/2006ApJ...636.1002S ) suggest that the symbiotic star prototype Z Andromedae shows such events in its history. And as the paper linked below suggests, perhaps there are even more sources of variability in symbiotic systems -- like rotation of the donor star -- that make the light curve even more interesting.