Shu is known for pioneering theoretical work in a diverse set of fields of astrophysics, including the origin of meteorites, the birth and early evolution of stars and the structure of spiral galaxies. One of his most highly-cited works is a 1977 seminal paper Self-similar collapse of isothermal spheres and star formation describing the collapse of a dense giant molecular cloud core which forms a star. This model (commonly referred to as the “inside-out” collapse model or the “singular isothermal sphere” model) helped provide the basis for much later work on the formation of stars and planetary systems, although it has been criticized for its shortcomings. Shu has also performed calculations on the structure of planet-forming disks around very young stars, the jets and winds that these stars and their disks generate, and the production of chondrules, inclusions in meteorites. Much of this work has been done in collaboration with his postdocs and graduate students, many of whom have gone on to successful academic careers in their own right.
* – Academy-Springer Nature Chair Professor Public Lecture
⁺ – Colloqium
Astrophysics Of Molten Salt Breeder Reactors
Astron Solutions Corporation
Many experts believe that a solution for climate change, especially in nations that are still trying to develop their economies, is not possible without nuclear power. However, among the general public there is discomfort about the four S’s:
Under the leadership of its top nuclear scientists, India has a far-sighted program to address these concerns by switching from the current fuel cycle of light-water reactors burning uranium-235 to an alternative fuel cycle that would burn uranium-233 bred from thorium-232 (the only form of thorium found in the world, and for which India has rich deposits). From the perspective of theoretical astrophysics, we argue in this talk that the best reactor to achieve the four-S goal uses, not solid fuel elements, but liquid fuel elements in the form of molten salts. Knowledge of how specific isotopes are produced in the big bang, in advanced stages of stellar evolution, and in cosmic rays, help in the choice of the composition of the salts and the materials used in the construction of molten-salt breeder reactors (MSBRs). The exchange of the kinetic energies of neutrons and atomic nuclei in a spherical reactor is akin to similar interactions among low-mass and high-mass stars in a globular cluster, the basic equations for which were formulated by S. Chandrasekhar in the 1940s. Tailoring the nuclear reactions to favour the formation of certain isotopes versus competitors has an analogy with the s- and r-processes of neutron-capture transformations in pre-supernova stars. Achieving economic superiority relative to other sources of energies relies on a basic understanding of energy hierarchies in the natural world. Managing the decay heat in fuel salt dumped into metal tanks in severe emergencies benefits from knowing how white dwarf stars cool by stages to a final inert state as a solid. If the world adopts sustainable, secure, superior, and safe thorium-MSBRs as the primary clean replacement for coal and natural gas, halting climate change could be a solved problem by the end of the twenty-first century without destroying the aspirations of future generations for a better life.
Six decades of Spiral density – Wave Theory
University Professor Emeritus
UC Berkeley and UCSD
The theory of spiral density waves had its origin approximately six decades ago in an attempt to reconcile the winding dilemma of material spiral arms in flattened disk galaxies. Our review begins with the earliest calculations of linear and nonlinear spiral density waves in disk galaxies, in which the hypothesis of quasi-stationary spiral structure (QSSS) plays a central role. The earliest success was the prediction of the nonlinear compression of the interstellar medium and its embedded magnetic field; the earliest failure, seemingly, was not detecting colour gradients associated with the migration of OB stars whose formation is triggered downstream from the spiral shock front. The reasons for this apparent failure are understood with an update on the current status of the problem of OB star formation, including its relationship to the feathering substructure of galactic spiral arms. Infrared images can show two-armed, grand design spirals, even when the optical and UV images show flocculent structures. We suggest how the nonlinear response of the interstellar gas, coupled with overlapping subharmonic resonances, might introduce chaotic behaviour in the dynamics of the interstellar medium and Population I objects, even though the underlying forces to which they are subject are regular. We then move to a discussion of resonantly forced spiral density waves in a planetary ring and their relationship to the ideas of disk truncation, and the shepherding of narrow rings by satellites orbiting nearby. The back reaction of the rings on the satellites led to the prediction of planet migration in protoplanetary disks, which has had widespread application in the exploding data sets concerning hot Jupiters and extrasolar planetary systems. We then return to the issue of global normal modes in the stellar disk of spiral galaxies and its relationship to the QSSS hypothesis, where the central theoretical concepts involve waves with negative and positive surface densities of energy and angular momentum in the regions interior and exterior, respectively, to the corotation circle; the consequent transmission and over-reflection of propagating spiral density waves incident on the corotation circle; and the role of feedback from the central regions. Lastly, we discuss how the amplitude modulation predicted for the destructive interference of oppositely propagating waves that form standing wave patterns may have been observed in deep infrared images of nearby spiral galaxies. We also present without comment the tantalising ALMA image of spiral structure in the protoplanetary disk around the forming star Elias 2–27.
Challenges Of Traveling To And Living On Mars
Founder, Astron Solutions Corporation
As the challenges of living sustainably on Earth grow ever more dire because of environmental constraints on the unlimited growth of population and demand for energy and material resources, many visionaries promote the idea of a new start for a subset of humans by colonising Mars. But is getting to Mars and living there as feasible as portrayed by certain captains of industry and directors of science-fiction movies? Earth and Mars differ in their surface gravities and in their distance from the Sun, which results in many disadvantages for Mars settlers:
Many of these disadvantages can be overcome by having access to nuclear power on Mars. Indeed, travelling to and surviving on Mars for many months at a time, without ready access to fresh water or air, is a problem similar to surviving in a sea-faring submarine that stays submerged under the ocean for months at a time. Unlike a nuclear submarine, the optimum nuclear reactor uses molten salt rather than water as a coolant and is a breeder that runs on the thorium fuel cycle rather than uranium-235 or plutonium-239. It is carried on board the spacecraft as a fission-fragment rocket engine, rather than as a driver of a turbine that can propel a payload from low-Earth orbit to Mars orbit that more resembles the spacious International Space Station than a cramped automobile. We outline how access to ample nuclear heat and electricity can help terraform Mars to be more like Earth, and how the spin-off application of such a development for space can help save civilisation on Earth from the ravages of climate change.
Formation Of Sunlike Stars And Planetary Systems
University Professor Emeritus
University of California at Berkeley and Sandiego
We discuss the main events in the birth and evolution of a sunlike star from the collapse of a cloud of molecules, to the accretion of a magnetized disk of material from which planets form, to the appearance of jets and bipolar outflows from young stellar objects. We relate how this sequence of events corresponds to the classification scheme from infrared and radio astronomy of class 0, I, II, and III objects. A recurring theme is the outward transport of angular momentum and the inward transport of mass that produce configurations where most of the mass of the system ends up as a ball at the center, and most of the angular momentum ends up in a flattened protoplanetary disk that revolves about the central ball that is the host star. From the disk, the dust separates from the gas to aggregate into planetesimals that grow to become protolanets and planets. Radio astronomical observations in spectral lines and the continuum will be crucial to deciphering how this separation of gas and solids occurs, and the likely crucial role played by water ice in forming icy giant planets relative to rocky terrestrial planets.
Reversing Climate Change With Molten Salt Technologies
Founder, Astron Solutions Corporation
Rational people agree that climate change poses an existential threat to civilisation. How do we mitigate its effects so that the global surface temperature does not rise by more than two degrees Celsius from pre-industrial levels to the end of the twenty-first century? Decarbonization is the buzz-word, but many experts believe that the two-degree goal is currently unachievable without technologies that can deliver negative carbon emissions. We present two molten salt approaches, especially suitable to the situation of India, which have considerable potential to halt and to begin to reverse the effects of climate change within this century. The first technology is supertorrefaction: the immersion of waste biomass under hot molten salt for a minute or so to sequester its carbon in a porous form for improving agricultural yields as well as for water filtration and the removal of industrial toxins. The second technology is two-fluid molten-salt breeder-reactors that can transform the nuclear power industry in a fundamental fashion by addressing societal concerns about its sustainability (against the depletion of uranium-235), security (against weapons proliferation), safety (against accidental release of massive amounts of radioactivity to the local environment), and superiority (against the economics of burning coal or natural gas for dispatchable electricity generation). With these two technologies, nations that bear little historical responsibility for the buildup of greenhouse gases in the atmosphere can continue to develop their economies while bequeathing to future generations a healthier and cleaner environment.
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