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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. While we have studied our Solar System with telescopes for four hundred years, we have only, in the last two decades, been able to detect planets orbiting other stars and begun to appreciate their astonishing diversity. We have uncovered surprises ranging from Earth-sized planets orbiting the compact corpses of burned-out stars to planets termed “hot Jupiters” that are more than one hundred times the mass of Earth but which 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 primordial cosmos salted with the heavy elements out of which planets and life are composed—and evolve with time into dusty disks or rings between the newborn planets themselves. While most if not all stars like our Sun may possess disks early in their histories, what fraction of these turn into planetary systems is not known. FIGURE 2‐10 Image of the supernova remnant RX J1713.7‐3946 observed in the highest energy (TeV) gamma rays. Recent observations using atmospheric Čerenkov telescopes have demonstrated that cosmic rays are accelerated to energies in excess of 100 TeV and that magnetic field is amplified to high strength. Contours are x‐ray emission. Supernova explosions like those that left behind this source created and dispersed heavy elements as well as accelerated cosmic rays. Source: “A detailed spectral and morphological study of the gamma‐ray supernova remnant RX J1713.7‐3946 with H.E.S.S,” The HESS Collaboration: F. Aharonian et al. PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION 2-24
Life We have only the most rudimentary ideas for what conditions are necessary and conducive to the formation of life. Even here modern astronomy has a key role to play, by finding and characterizing planets with the features that allow for life around stars other than the Sun. It will require study of individual planets by directly sensing their light to find the molecular signposts of habitability in the atmospheres and surfaces of these distant bodies. This last task, possible now for nearby giant planets, is exceedingly difficult for Earth-sized bodies, with disks 100 times smaller in area than Jupiter’s. The signature of water, together with a suitable orbit around a parent star, would tell us that the medium for life as we know it is likely present as a surface liquid; methane indicates that organic molecules (the structural building block of life) are present; oxygen with methane would indicate a state of extreme chemical “disequilibrium” that could likely not be maintained in the absence of life. The most promising signatures of life on planets around other stars are features in the atmospheric spectra of planets around other stars, such as the “red edge” arising from photosynthesis. Less definitive is molecular oxygen, which is locked up in oxidized surface minerals unless continually replenished either by life (as on Earth) or catastrophic loss of surface water followed by photolysis of H 2 O in the atmosphere (as on early Venus). The presence of both water and methane in a planetary atmosphere is a more reliable biosignature of water-based organic life than that of one or the other alone. A different approach is to look for signals produced by technologically advanced entities elsewhere in our galaxy. FRONTIERS OF KNOWLEDGE New fundamental physics, chemistry, and biology can be revealed by astronomical measurements, experiments, or theory and hence push the frontiers of human knowledge. Science frontier questions in this category are: • Why is the universe accelerating • What is dark matter • What are the properties of the neutrinos • What controls the masses, spins and radii of compact stellar remnants One of the key insights of the past few centuries was the recognition that the same scientific laws that govern the behavior of matter and energy on Earth also govern the behavior of the cosmos: planets, stars, galaxies, and the entire universe. Newton inferred that the same physical forces causing apples to fall to Earth also govern the motions of the Moon around Earth and the planets around the Sun. One hundred and fifty years later it was discovered that chemical elements introduced into laboratory flames produced a unique set of spectral lines, and since many of these lines also appeared in the solar spectrum, it was concluded that the Sun was made of the same chemical elements as found on Earth, or as in the case of helium, a new one waiting to be discovered. Astronomers feel confident in using the universe as a laboratory to explore natural phenomena that are inaccessible to Earth-based labs. The study of how the universe and its constituent objects and phenomena work continues to yield unique insight into fundamental science. PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION 2-25
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THE NATIONAL ACADEMIES PRESS 500 Fi
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COMMITTEE FOR A DECADAL SURVEY OF A
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Panel on Stars and Stellar Evolutio
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DOUGLAS FINKBEINER, Harvard Univers
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SPACE STUDIES BOARD CHARLES F. KENN
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activities 2 that have emerged from
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In addition to the 27 panel meeting
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Acknowledgment of Reviewers This re
Page 22 and 23: The Accelerating Universe 2-26 The
Page 24 and 25: APPENDIXES A Summary of Science Fro
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Page 28 and 29: RECOMMENDED PROGRAM Maintaining a b
Page 30 and 31: TABLE ES.4 Space: Recommended Activ
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Page 34 and 35: this imprint would both probe funda
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Page 44 and 45: RECOMMENDATION: U.S. investors in a
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Page 48 and 49: dominate spending; (2) private-publ
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Page 52 and 53: BOX 2-1 Other Worlds Around Other S
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Page 56 and 57: FIGURE 2‐3 Numerical simulation o
Page 58 and 59: FIGURE 2‐4 Left panel: Image of t
Page 60 and 61: FIGURE 2‐6 Top: Schematic of the
Page 62 and 63: Most stars with masses smaller than
Page 64 and 65: BOX 2-2 The Origin of Planets After
Page 66 and 67: send material raining into the cent
Page 68 and 69: BOX 2-4 Lifecycles in Galaxies One
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Page 78 and 79: An important clue to the nature of
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fluctuating level of funding for as
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THEORY Emerging Trends in Theoretic
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Central to the Galaxies across Cosm
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TABLE 5‐2 Support for astrophysic
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would normally be for a five-year p
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FIGURE 5‐7 Highly cited HST publi
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y optical and radio astronomers are
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FIGURE 5‐8 Number of PhDs per yea
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ated by the Program Prioritization
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nulling interferometry. The appropr
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FIGURE 5‐9 The richness of the su
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RECOMMENDATION: NASA and NSF suppor
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FIGURE 6‐1. All sky map as observ
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FIGURE 6‐3 NASA mission cost over
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FIGURE 6‐5 Gemini North with Sout
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TOWARD FUTURE PROJECTS, MISSIONS, A
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3. Investment in new and upgraded i
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etween several competing elements i
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7 Realizing the Opportunities The p
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FIGURE 7.1 Survey time line showing
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SCIENCE OBJECTIVES FOR THE DECADE T
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Box 7.1 Implementing a Cosmic Dawn
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JWST, with its superb mid-infrared
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optical imaging of brighter galaxie
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Box 7.3 Implementing the Physics of
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FIGURE 7.2 Multiwavelength images o
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Recommendations for New Space Activ
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FIGURE 7.4 Science accomplishments
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significance of mergers in the buil
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independent advice committee review
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committee review. It could range be
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Laboratory Astrophysics As describe
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Recommendations for New Ground-Base
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Program for instrumentation and fac
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excel at high spectral and spatial
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The committee reviewed a technical
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FIGURE 7.10 ACTA would be, like the
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Advanced Technologies and Instrumen
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LISA as soon as possible subject to
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FIGURE 7.14 DOE recommended program
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A Summary of Science Frontiers Pane
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PLANETARY SYSTEMS AND STAR FORMATIO
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A preliminary recommended program w
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certified as independent work perfo
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penalty based on an assessment of s
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The results show excellent correlat
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$12 MILLION TO $40 MILLION RANGE CM
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decadal research strategy with reco
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CFP CHIPSat CIP CGRO CMB CMU CoBRA
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NRC NSB NSF NSF-AGS NSF-ARC NSF-AST
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