A Brief History of Astronomy
Final End States of Stars
A white dwarf in a binary system may find itself in a circumstance where it gains mass from its companion star. When that companion evolves to become a red giant, it may become large enough that portions of its outer atmosphere are attracted by the gravity of the orbiting white dwarf, which begins to capture this hydrogen rich material onto its surface. This is not a stable circumstance. When a critical point is reached in the accumulation of this new matter, thermonuclear reactions at the surface can be set off that explosively convert the hydrogen to helium. Observationally, such an event, a nova, would be seen as a rapid rise (less than a day) in brightness of the star to 10,000–100,000 solar luminosities, followed by a decline in brightness over the next few months. While some mass (usually much less than 0.01 solar mass) might be shed at high velocity up to 2,000 km/s, neither the basic structure of the white dwarf nor its orbit relative to its companion are affected in this event. Mass accretion would resume until perhaps 50 to 100 years later; the accumulated material again would undergo rapid thermonuclear reactions in another nova. A number of such stars have been observed to repeat and are termed recurrent novae.
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