This illustration depicts a cataclysmic variable system, in which a compact white dwarf, the white star at right, collects material from its swollen companion, the red star at left. The infalling gas forms an accretion disk, which in turn pours matter onto the white dwarf's ultrahot surface. Two types of dramatic brightenings may result: a classical nova when gas on the white dwarf explodes, or a dwarf nova when gas in the accretion disk explodes. The two types aren't always easily distinguishable, as explained on AAS Nova and in an article to appear in the Astrophysical Journal. [NASA, CXC, M. Weiss]