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CVs are the progenitors of classical novae, which derive their luminosity from explosive nuclear burning on the
surface of the white dwarf. For the observed frequency of classical nova outbursts and the above CV space
density, the recurrence time for a given system for a classical nova outburst is ~ 105 years. This
interval is long compared to human history, but short compared to the lifetime of a CV, so although any given CV
has been observed to experience only one classical nova outburst, CVs experience many classical nova outbursts
during their lifetime.
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The sequence of events leading up to a classical nova explosion are illustrated below (Figures and text are
adapted from the slideshow "Blobs in Space," created with support from NASA contract NAS5-26555 to the
Space Telescope Science Institute, operated by the Association of Universities for Research in Astronomy,
Inc., and reproduced with permission of AURA/STScI).
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The white dwarf captures matter lost through the inner Lagrange point of the secondary. To conserve angular
momentum, this material does not accrete directly onto the white dwarf, but forms an accretion disk around the
compact star.
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As it losses angular momentum, the material in the disk slowly drifts inward and accretes onto the surface of
the white dwarf.
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An envelope or "ocean" of hydrogen-rich material builds up on the white dwarf surface. The intense heat and
pressure at the base of this envelope eventually leads to a thermonuclear explosion as hydrogen is burned to
helium. The explosion blows off the outer layers of the envelope.
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The nova outbursts lasts for tens to hundreds of days. Eventually, as in this HST image of Nova Cygni
1992, the ejected envelope is visible as a limb-brightened shell expanding away from the binary (the central
point source in this image) at speeds of a few hundred to a few thousand km/s.
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