Triumphs of 20th-Century Astrophysics II: We Master the Stars

By Virginia Trimble

As in session I on observatories and telescopes, three experts were asked to address the past, present, and future of stellar astrophysics. All were present at this second session.

Matthew Stanley (Michigan State University, but soon to be at New York University) presented the intriguing tale of how we learned that stars run on nuclear energy, particularly the p-p and CNO (carbon-nitrogen-oxygen) cycles. He divided it into three pieces. First came establishing that there was a problem, after Joule demonstrated conservation of energy as a general principle, with ideas from Kelvin and Helmholtz about the in-fall of meteoric material or gravitational contraction as a solution, in good accord with the Kant-Laplace model of Solar System formation. Then came the discrepancy between the time scale of a few times 107 years permitted by gravitational processes according to Kelvin (who found a similar time for the cooling of the Earth) and the 108 to 109 years required by Darwin and others for geological processes and biological evolution to occur. Giant stars, especially Cepheids, also seemed to require a longer-lasting energy source than just contraction, as did some stellar dynamical processes. Resolution of this discrepancy required the concept of mass-energy equivalence; consideration of either transmutation of elements or proton-electron annihilation (by Jeans and Eddington); accurate measurements of nuclear masses (F. W. Aston and others); analysis of how an energy source could be distributed through a star and remain stable (by T. G. Cowling, for instance); demonstration that stars had a large hydrogen content (C. Payne); the concept of barrier penetration (R. Atkinson and F. Houtermans, G. Gamow); and detailed nuclear-reaction sequences devised by von Weizsäcker and Bethe. Stanley ended by noting that Rutherford had spoken in St. Louis at the 1904 World’s Fair.

Stirling Colgate (Los Alamos National Laboratory) had been asked to address current issues in stellar astrophysics, especially high-energy ones like supernovae and gamma ray bursters. In the session he gave a typical Colgate talk, which is to say that I do not feel I understood it very well but nevertheless came away suspecting that he was probably right about many issues. He began by focusing on the formation of supermassive black holes in the early Universe (temporally and logically prior to galaxies and stars, in his view, and not assembled hierarchically). The resulting free energy is in excess of 1060 ergs, at least as much as will come from supernova kinetic energy over the rest of the life of a typical galaxy, and he drew an analogy with the enormous shock wave of the 11 February 1954 BRAVO hydrogen bomb test. Forming these enormous black holes from an initial perturbation spectrum is awesomely complicated—transcended by the even greater complexity of transforming this energy into the magnetic fields, jets, radio lobes, and the extragalactic cosmic rays we observe. Angular-momentum transport and large-scale coherent dynamos are essential processes that probably get missed by computer simulations of structure formation. He concluded that emission of 100 MeV photons and 1030 eV or more cosmic rays should be possible.

Mark McCaughrean (University of Exeter) was the Kenneth Greisen Lecturer. Greisen is the G of the GZK cutoff (the others being Georgiy Zatsepin and Vadim Kuzmin), and the lecture was sponsored by his astrophysicist son Eric and by former students Irwin Shapiro, Alan Bunner, and Donald Gilman. McCaughrean addressed what is surely the single most important unsolved problem in stellar physics—how they form—and he entitled the talk “Standing on the Shoulders of Giants,” meaning the giant telescopes that, in the next decade or two, may enable us to answer some of the major outstanding questions about the birth and early evolution of stars, brown dwarfs, circumstellar disks, outflows, and planetary systems. The contributors will include X-ray and millimeter telescopes, and the James Webb Space Telescope (mostly infrared)—as well as the large ground-based optical facilities discussed by Betsy Barton in session I. Three of the major questions are the importance of feedback (for instance, once a massive star has formed in a region, do you get more or fewer stars there?); the processes responsible for the initial-mass function (N(M) of stars when they form) and whether it is universal; and the formation, evolution and significance of disks around young stellar objects. In common with the objects and processes advertised by Colgate, angular-momentum transport and magnetic fields must also be important here. Resolved imaging of disks in the infrared will be particularly important to understanding how they change as young stellar objects age and if, how, and when they can form planets.

Note Added: This article represents the views of the author, which are not necessarily those of the FHP or APS.