This makes the gas more opaque, and radiation temporarily becomes captured in the gas. This heats the gas further, leading it to expand once again. Thus a cycle of expansion and compression (swelling and shrinking) is maintained.

In many cases, a predictive mathematical model can be constructed of the variable behavior. Typically an assumption is made of a constant period of variability. The model can then be used to construct an O-C diagram, which is a plot of the observed (O) behavior minus the computed