  Stellar explosions such as novae and supernovae produce most of the heavy
elements in the Universe. Although the onset of novae from runaway
thermonuclear fusion reactions on the surface of a white dwarf in a binary star
system is understood[1], the structure, dynamics, and mass of the ejecta are
not well known. In rare cases, the white dwarf is embedded in the wind nebula
of a red-giant companion; the explosion products plow through the nebula and
produce X-ray emission. Early this year, an eruption of the recurrent nova RS
Ophiuchi[2,3] provided the first opportunity to perform comprehensive X-ray
observations of such an event and diagnose conditions within the ejecta. Here
we show that the hard X-ray emission from RS Ophiuchi early in the eruption
emanates from behind a blast wave, or outward-moving shock wave, that expanded
freely for less than 2 days and then decelerated due to interaction with the
nebula. The X-rays faded rapidly, suggesting that the blast wave deviates from
the standard spherical shell structure[4-6]. The early onset of deceleration
indicates that the ejected shell had a low mass, the white dwarf has a high
mass[7], and that RS Ophiuchi is a progenitor of the type of supernova integral
to studies of the expansion of the universe.
