All the while the nova became optically brighter. As soon as the visual brightness peaked, the ultraviolet emissions bottomed out and began to climb. The recovery comes from a second change in ionization. As the gas expands, its density drops. Then the iron group elements once again become ionized and hence transparent.
Radiation now flows from the interior, enhancing the ionization and in turn the transparency.
In effect, the iron curtain lifts, and ultraviolet light from the hot, deep layers penetrates through the outer layers. Within two months the ultraviolet brightness had climbed back up to its original value. At the same time as the ultraviolet brightness increased, the visual brightness of the nova declined. This constant bolometric luminosity phase, predicted by our 1972 simulations, was finally confirmed in detail by observations of V1974 Cygni. Anticipating that the radiation peak would continue toward shorter wavelengths, Ronald S. Polidan of the NASA Goddard Space Flight Center requested that Voyager 2, then flying beyond the orbit of Neptune, observe the spectra of V1974 Cygni.