mass loss, rotation, and magnetic fields in stellar evolution. Prospects are bright for the coming decade. All three phenomena can be assessed through high-dispersion spectroscopy. Rotational studies are possible with detailed long-term photometric monitoring. It is now becoming possible to study the structure and strength of magnetic fields on the surfaces of nearby stars, and changes in the magnetic fields can be diagnosed with X rays. At the same time, the major advance provided by the Advanced Technology Solar Telescope (ATST) will be an improved ability to observe and understand the rich array of magnetic activity exhibited by our nearest star, the Sun. Solar radio emission will be observed at high time and wavelength resolution on a continuous basis, providing unique data to combine with that of ATST.

Indeed, following the successful launch and commissioning of the Solar Dynamics Observatory (SDO; Figure 2.9), we are poised to understand the origin of the 11-year solar cycle, which underlies “space climate,” by relating the surface behavior of the Sun to its interior properties, in particular at the tachocline located at 70 percent of the solar radius where the hot gas begins to undergo convective motion. In addition, the high-resolution, all-disk imaging combined with the ability to map the surface magnetic field in three dimensions as it erupts into the solar chromosphere and corona is providing unprecedented understanding of how magnetic fields behave above the solar surface both in the “quiet” Sun and during massive flares associated with active regions. This understanding is of major importance for astrophysics beyond the solar system because the Sun is the best large-scale magnetic field laboratory we have. Meanwhile, ATST will come on line in 2017 and will provide complementary diagnostics for similar science goals to space observatories, specifically high-resolution imaging—it will have the capability of seeing down to 30-kilometer scales—and detailed spectroscopy. It will be able to see the strange ways that magnetic field lines twist and braid themselves as well as how they mediate the flow of energy. Understanding these physical processes is a key step toward explaining how the solar wind—the outflow of gas that blows past Earth and has such a large effect on our atmosphere—is powered.

Stellar seismology is maturing rapidly. Analogous to Earth-based seismology, this technique enables astronomers to probe the deep interior regions of stars using the complex oscillations observed at the star’s surface, much as the tone of a musical instrument reveals its internal construction. In the next decade, the rapidly increasing power of computers will allow us to take the known physical laws that are at play and synthesize them into detailed three-dimensional movies of the life and death of stars.

The life stories of stars can be strikingly changed if the star has a companion star orbiting in close proximity. One of the most dramatic examples of this occurs in a system containing a white dwarf star, which is the burnt-out core of a star like the Sun, with about as much mass as the Sun compressed into an object the



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