Novae are caused by runaway thermonuclear burning in the hydrogen-rich envelopes of accreting white dwarfs, which results in the envelope to expand rapidly and to eject most of its mass. For more than 30 years, nova theory has predicted the existence of a "fireball" phase following directly the runaway fusion, which should be observable as a short, bright, and soft X-ray flash before the nova becomes visible in the optical. Here we present the unequivocal detection of an extremely bright and very soft X-ray flash of the classical Galactic nova YZ Reticuli 11 hours prior to its 9 mag optical brightening. No X-ray source was detected 4 hours before and after the event, constraining the duration of the flash to shorter than 8 hours. In agreement with theoretical predictions, the source's spectral shape is consistent with a black body of
K ( eV), or a white dwarf
atmosphere, radiating at the Eddington luminosity, with a photosphere that is
only slightly larger than a typical white dwarf. This detection of the
expanding white dwarf photosphere before the ejection of the envelope provides
the last link of the predicted photospheric lightcurve evolution and opens a
new window to measure the total nova energetics.