fusion. Within fractions of a second, this energy crisis triggers a collapse of the innermost solar mass of material to densities so high that the nuclei of atoms are literally “touching.” The rest of the star subsequently collapses onto the newly born neutron star, resulting in ejection of most of the star into the interstellar medium, spreading the products of millions of years of fusion reactions. Sometimes the collapsing material overwhelms the young neutron star, leading to a further collapse to a black hole.

Wide-field sky surveys during the next decade should reveal tens of thousands of these core-collapse supernovae per year and thus a diversity of stellar remnant outcomes much richer than currently known. Remarkable discoveries could occur if we are lucky enough to have a galactic supernova, as the overwhelming number of neutrinos from the young neutron star would provide an exciting probe of the competition between collapse and explosion going on in the inner 20 kilometers of these explosions. Even more remarkable would be to find direct evidence for gravitational wave emission from such a nearby explosion, a possibility for Advanced LIGO. Progress will also occur via continued theoretical and computational efforts, especially as three-dimensional simulations become computationally plausible. Finally, exploding stars leave remnants hypothesized to be the galactic particle accelerators that produce ubiquitous high-energy charged-particle cosmic rays, including those that crash into Earth’s atmosphere, producing telltale radioactive isotopes. X-ray, gamma-ray, and radio observations of these stellar remnants will test this hypothesis and reveal the accelerator dynamics of the stellar ghosts (Figure 2.10).

Planetary Systems

Our Sun is just one of the several hundred billion stars in the Milky Way, and its well-ordered configuration of eight planets just one of the many diverse planetary systems. Although we have studied our solar system with telescopes for 400 years, we have only, in the past two decades, been able to detect planets orbiting other stars and begun to appreciate their astonishing diversity. We have uncovered surprises ranging from Earth-size planets orbiting the compact corpses of burned-out stars to planets termed “hot Jupiters” that are more than 100 times the mass of Earth but that are so close to their stars that they orbit them in just a few days. Models of the formation of planetary systems predict that planets this massive should form at much greater distances; these bodies have forced us to consider processes of “migration” that bring large planets closer to their stars early in their histories.

The details of how planets form within disks are still being revealed by current astronomical techniques, including imaging from Hubble, Spitzer, and the largest ground-based telescopes, plus theoretical studies including computer modeling. Disks start out being dominated by gas—the hydrogen and helium of the pri-



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