Astronomers are poised to learn how stars rotate at the surface and within and how that rotation affects mass loss and stellar evolution. They seek a better understanding of how magnetic fields are generated in stars across the mass spectrum and of how these fields power the chromospheres and coronae that produce observed magnetic activity. Finally, the origin of highly magnetized main sequence stars, in which surface fields approach 104 gauss, remains mysterious, and the investigation of these stars promises to shed light on the star-formation process that produced them as well as on the origin of even more highly magnetized compact objects.

The prospects for progress on this question in the next decade stem from the emergence of greatly improved tools for measuring magnetic fields from polarization, for resolving the atmospheres of some stars with interferometry, for probing the interiors of stars through their vibration spectra, and for extending observations into X-rays and gamma rays. When combined with more thorough understanding of the static and dynamic properties of magnetic atmospheres, astronomers will learn how stellar atmospheres really work and how rotation and magnetism affect the evolution of stars.

What Are the Progenitors of Type Ia Supernovae and How Do They Explode?

Many lines of evidence converge on the idea that Type Ia supernovae are thermonuclear explosions of white dwarfs in binary systems. Because of their high luminosity, and with effective empirical methods for determining their distances from light-curve shapes, Type Ia supernovae have acquired a central role not just in stellar astrophysics but also in tracing the history of cosmic expansion and in revealing the astonishing fact of cosmic acceleration. Because this result points to a profound lack in the understanding of gravitation, a problem right at the heart of modern physics, completing the astronomical story of Type Ia supernovae is a pressing priority for the coming decade.

First of all, the provenance of the exploding white dwarfs seen in other galaxies is not known with certainty. The prevailing picture is that Type Ia explosions arise in binary systems in which a white dwarf accretes matter until it approaches the Chandrasekhar limit, simmers, and then erupts in a thermonuclear flame. But it is not known how this picture is affected by chemical composition or age, two essential ingredients in making a precise comparison of distant events with those nearby. Events that are precipitated by the merger of two white dwarfs are not excluded. The Type Ia supernovae in star-forming galaxies and in ellipticals are at present treated in the same way, but this is the result of small samples, not of evidence that they should be analyzed together. More broadly, these gaps in knowledge illuminate the need for a better understanding of the evolution of interacting binary stars, which are responsible for a variety of crucial, yet poorly understood, phenomena.

It can be expected that both theory and improved samples will place this work on a firmer foundation. The complex, turbulent, unstable nuclear flame that



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