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Large-scale, three-dimensional numerical simulations of the deflagration stage of a thermonuclear supernova explosion show the formation and evolution of a highly convoluted turbulent flame in the gravitational field of an expanding carbon-oxygen white dwarf. The flame dynamics are dominated by the gravity-- induced Rayleigh-Taylor instability that controls the burning rate. The thermonuclear deflagration releases enough energy to produce a healthy explosion. The turbulent flame, however, leaves large amounts of unburned and partially burned material near the star center, whereas observations that imply these materials are present only in outer layers. This disagreement could be resolved if the deflagration triggers a detonation.
According to observations and models, many stars that steadily bum their nuclear fuel for millions or billions of years suddenly end their lives with a powerful explosion that produces a bright object called a supernova. A supernova explosion can be powered either by the gravitational energy released during the core collapse of a massive star or by the nuclear energy released by explosive thermonuclear burning of a star. Here, we focus on thermonuclear supernovae that belong to the type la (SN Ia) in the observation-based classification (1-3).
Thermonuclear supernovae are produced by explosions of white dwarfs (WDs), small dense stars composed of carbon and oxygen nuclei and detached degenerate electrons (1, 3-10). The term "degenerate" means that electrons occupy all possible quantum states below a certain energy. The hydrostatic equilibrium in a WD is supported for the most part by the degenerate electron pressure that does not depend on temperature. WDs form at the end of the evolution of stars whose original masses are less than 8 solar masses (M^sub *^). A star can lose a large fraction of its material by ejecting outer layers into space at the final stages of evolution. The mass of a remaining WD is always less than the Chandrasekhar limit, 1.4 M^sub *^, above which a hydrostatic equilibrium of degenerate matter is impossible. An isolated carbon-oxygen WD is stable and almost inert, because its temperature is not high enough to induce any substantial nuclear reactions. This isolated dead star can exist almost indefinitely, slowly cooling down as it radiates its energy into space. Observations show, however, that more than 50% of all stars are not isolated. They...