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Supernova Duds and Toothpaste

by John G. Cramer

Alternate View Column AV-31
Keywords: supernova, neutrinos, fluorine, nucleosynthesis
Published in the February-1989 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 7/23/88 and is copyrighted © 1988, John G. Cramer. All rights reserved.
No part may be reproduced in any form without the explicit permission of the author.


    The biggest astrophysical event of the century occurred in February of 1987, when Supernova 1987A flashed into the skies of the southern hemisphere. This rare event, the first supernova in our galactic neighborhood since the invention of the telescope, gave astrophysicists a relatively close-up look at a supernova. A class B3 blue supergiant star only 160,000 light years distant had run out of nuclear fuel, gone into gravitational collapse, and exploded. The event was carefully studied in the infrared, visible, and ultraviolet regions of the electromagnetic spectrum. Neutrinos from the supernova's core collapse were detected by large underground detectors in Ohio and Japan. [See my AV column "SN1987A - Supernova Astrophysics Grows Up" in the December-1987 Analog.]

    SN1987A also brought into focus the central problem of supernova astrophysics: that massive stars explode far more readily in the real universe than they do in the simulated world within large computers. Computer models are used by astrophysicists to test their understanding of how a supernova works. Within such models supernovas either refuse to explode or do so with great reluctance.

    A computer model is a program into which is coded all the physics that we know that is relevant to supernovas: how the matter in stars behaves under the extremes of temperature and pressure that occur during a supernova, how shock waves form and propagate, how energy is transferred from one form to another, how nuclear reactions work in stars, etc. While the computer clock ticks, the explosion is initiated and proceeds, step by step.

    The star in its deep interior furnace where there is enormous pressure and temperature has been producing energy by fusing hydrogen into helium, helium into carbon, carbon into oxygen, oxygen into neon, and so on, until it has accumulated a dominant "ash" of the nucleus iron-56. Iron-56 lies at the very peak of the curve of binding energy, and is therefore the most stable of nuclei. A star cannot burn iron in any nuclear fusion reaction to extract more energy. Iron is the bottom of the energy barrel.

    When the fusion reactor at the star's core becomes saturated with iron, it has a "flameout". The nuclear reactions grind to a halt. Up to this point a delicate equilibrium has been maintained between the force of gravity which pulls the stellar matter inward, and nuclear heating which pushes the stellar gasses outward. When the nuclear furnace switches off, gravity takes over. The matter of the star falls inward toward the core in near free fall, and the star collapses.

    There is an enormous quantity of energy liberated in this infall, the gravitational potential energy that had been stored in the outer "uphill" parts of the star. The star matter falls inward faster and faster, and the density of atoms in a given volume rises higher and higher. This goes on until the interior of the star hits a sort of brick wall. When the matter reaches a density about that of a nucleus, the strong nuclear force begins to act to resist further compression. Like a tennis ball hitting a solid wall the infalling matter literally bounces. An outgoing shock wave forms and begins propagating outward, carrying a fraction of the gravitational energy from the infall back to the surface of the star where it can blow off the outer layers and make the supernova explosion.

    All of these processes are modeled in the computer, and up to this point, every thing looks very supernova-like. But then a problem occurs. As the shock wave propagates outward, it encounters the infalling matter streaming through it in the opposite direction. When the shock wave reaches a certain radius, it stalls.

    Its outward movement is halted because the speed of the infall has matched the outward speed of the shock wave. The shock wave then "runs in place", moving through new matter not because it is moving outward, but because the matter is moving inward through it. The shock wave gives its energy to the matter moving through it, and finally dissipates. And so, in the computer, the explosion never happens. The violent supernova explosion has, in the computer simulation, become a quiet self-contained burp. Nature can make supernovas, but in the computers for a while we were making only duds.

    This problem, the non-explosion of supernovas in computer models, caused a careful re-examination of all the physical processes that are included in the models. It was soon realized that processes involving neutrinos can help solve the problem. When the central part of the collapsing star reaches nuclear density, it becomes energetically favorable for a proton to eat a nearby electron and spit out a neutrino. The proton then becomes electrically neutral and the electrical repulsion which had been pushing it to a higher energy state goes away. The exploding star develops a neutron-star at its core, and a flood of neutrinos, one for each new neutron, stream outward. A very large number of neutrinos are produced in this way. SN1987A probably produced 1058 of them. When they reach the stalled shock wave they can boost it with a flood of new energy from the core collapse.

    Neutrinos are very inert as particles go. Electrically neutral, weakly interacting, and with a rest mass at or near zero, a typical supernova neutrino could pass through a light year of lead without scattering from even one atom of lead. But the out-streaming neutrinos from a supernova must travel through an amount of matter equivalent to a few thousand light years of lead at normal density.

    Neutrinos are thought to lose about 1% of this energy by scatterings on their way out of the star. The neutrinos from the core collapse carry 100 times more energy than goes into the blast shock wave, and the shock wave carries 100 times more energy than ever emerges as visible light. The light from the supernova, which can outshine whole galaxies, is a minor side effect of the explosion.

    When the neutral weak current processes that allow neutrinos to scatter were first discovered, it was thought that the secret of the supernova explosion had been found, that the elastic scattering of the neutrinos would provide enough push to restart the shock wave and allow the supernova to explode within the computer as well as in the heavens. As it turned out, this extra push is not quite enough. When the effect of neutrino scattering was included in the computer programs the shock waves could, by making extreme assumptions, be restarted and explosions generated.

    But the results were too close to the edge of the parameter ranges to be completely plausible, and the explosions generated by the simulations did not bear enough resemblance to observed supernovas in distant galaxies. The consensus among astrophysicists was that a piece of physics was still missing from the computer models, that the last piece of the puzzle had not yet been found.

    The missing puzzle piece may have just been discovered. All nuclei have an excitation mode, a way of containing internal energy by vibration, that is called a "giant resonance". In a giant resonance vibration the neutrons slosh in one direction in the nucleus, while the protons slosh the other way, moving the net electric charge of the nucleus back and forth while leaving the center of mass of the nucleus undisturbed. It was recently realized that neutrinos scattering from the nucleus can trigger this vibration, leaving a sizable fraction of their energy in the nucleus as they do so.

    This new piece of physics had not been included in the computer models. It was discovered recently by Profs. Wick Haxton of the University of Washington and Stan Woosley of the University of California at Santa Cruz. Calculations including this mechanism have not yet been performed, but when they are included there is optimism that this process will boost the energy and momentum transferred from the neutrinos to the shock wave and help to produce more realistic supernova explosions in the simulations. And the neutrino excitations of giant resonances also has another interesting consequence.

    The high abundance of element fluorine has been an astrophysical mystery for some time. Astrophysicists can now account for the formation of most of the elements in the periodic table. The lightest elements, hydrogen through lithium, were synthesized in the early stages the Big Bang. The elements from beryllium through iron were "slow-cooked" in stars during the nuclear burning mentioned earlier. The heaviest elements, from cobalt to uranium, were "flash-fried" during supernovas of the early first generation stars, with elements repeatedly capturing neutrons streaming away from the center of the explosion, until the force of the explosion finally blasted the new-made heavy elements into space to be recaptured into metal-rich second generation stars like our sun. The chemical elements of our bodies were cooked in the violence of the Big Bang, in the super-hot furnaces of large early stars, and in the blasts of supernovas.

    This scenario of element formation works well, and the present abundances of most of the elements in the periodic table can be accounted for with detailed calculations based on this picture. But the only stable isotope of fluorine, fluorine-19, doesn't fit. It cannot be directly made by fusing several heliums together. It should be very rare, but it is only 1000 times less abundant than the element neon, which is easily produced by fusing helium and oxygen. It is also far more abundant than the lighter elements lithium, beryllium and boron. No theory of element formation has ever been able to account for the fluorine-to-neon ratio that we find in the universe.

    The Haxton-Woosley mechanism may solve this problem too. The abundant neon-20 isotope in the outer parts of the exploding star has a high probability of being kicked into a giant-resonance vibration by the out-streaming neutrinos. When this happens, the neon nucleus is very likely to spit out a neutron or proton as a way of carrying away its excess energy. If a proton is thrown off, the stable isotope fluorine-19 is left behind. If a neutron is evaporated, the isotope neon-19 results. But neon-19 is radioactive and will rapidly decay into fluorine-19. Thus, both processes contribute to the formation of fluorine-19 during a supernova. Calculations by Haxton and Woosley predict just about the observed the 1/1000 fluorine-to-neon ratio.

    What are the consequences of this? One science reporter has suggested that this might be nature's way of fighting tooth decay, by arranging for fluorine to form in supernovas so that it could be used in toothpaste. That's an interesting hypothesis, but I'm afraid it seems a bit far-fetched, even for a science fiction magazine, doesn't it?


W. H. Haxton and S. A. Woosley, Nature, July 7, (1988).

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