Sky & Telescope May 1986 [antikvár]

The Iron SunJames R. Graham, Lawrence Berkeley Laboratory, CaliforniaIRON is the dead-end elettient. Its atomic nucleus is the most tightly bound, unable to yield energy by either fission or fusion reactions. It is the ultimate ash of nuclear fires.The formation of iron sounds the death knell of massive stars. But it also provides certain other stars with one last tremendous burst of glory, according to a theory of supernovae recently confirmed by myself and colleagues based in Great Britain, Australia, and Hawaii.Theorists have long suspected that the gradual decline in brightness of certain supernovae indicates they are powered by radioactive decay. Until recently, this idea was little more than a guess. But in June, 1983, a supernova was seen to erupt in the galaxy Messier 83. Almost a year later my colleagues and I observed its expanding debris and discovered, at the limit of detectability, the spectral signature of iron. Had this supernova not been the brightest of its type seen this decade, our discovery could not have been made.Astronomers have learned to recognize two varieties of supernovae Type I and Type II. The former are exploding white dwarfs; the latter occur in massive giant stars. When a giant star has exhausted its supply of hydrogen fuel, its core shrinks and can become hot enough to "burn" helium, then carbon, oxygen, and so on, ultimately building up a massive core of iron thermonuclear ash.This inert core cannot supply any more nuclear energy, so the star becomes cannibalistic. It raids its supply of gravitational energy by shrinking a strategy that leads, quite literally, to the star's downfall. The collapse, gradual at first, soon becomes uncontrolled, releasing energy so rapidly that it drives a titanic blast wave that rips the star apart. For a few days the fireball shines with the brilliance of more than a billion Suns.The explosions of such Type II supernovae can be identified because their spectra are rich in hydrogen from the star's ejected outer layers. This same hydrogen shrouds our view of the highly processed material from the star's interior.Type I supernovae, such as the one observed in M83, are quite different. Their spectra, for example, show no sign of hydrogen. Type 1 explosions seem to occurin old stars not much more massive than our Sun. Stars like these spend most of their lives burning hydrogen to helium; then, for a comparatively short spell fuse helium to carbon and oxygen. However, their central temperatures are insufficient for further nuclear reactions. Instead, these stars end their lives by gently blowing off their outer layers (forming a planetary nebula), leaving behing a dense, inert, core of carbon and oxygen about the size of the Earth a white dwarf.The fires of a supernova constituted the factory where iron was first created.Solitary white dwarfs radiate their energy reserves and cool down. However, if the star is a member of a close binary system it may suffer a more spectacular fate. In such cases, the dwarf star may steal matter from its companion and grow.A white dwarf is only stable if its mass is less than about 1.4 Suns. If the star's mass grows beyond this "Chandrasekhar limit," it will start to collapse. The temperature and pressure begin to rise, rekindling the nuclear fires. However, this time the burning does not take miUions of years, but only a few seconds. In this event the whole white dwarf is burned, not to iron, but to a radioactive isotope with the same atomic weight: nickel-56.If the explosion did not produce this radioactive nickel-56, a Type I supernova would be virtually invisible. Because white dwarfs are so dense, their interiors are very opaque. The exploding white dwarf has to expand by a factor of 100,000 before light can escape. However, in expanding so much the star pays a price; it cools just as a refrigerator is chilled by the expansion of its coolant gas. In a Type I supernova, the expansion is carried to such a degree that the gas should be too cold to radiate at all, once it is sufficiently rarefied for muchlight to escape.If this is so, why do we see Type I supernovae? The key is the radioactive nickel. If the nuclear burning were gentle rather than explosive, this nickel would only be an intermediate stage in the process of forming iron. But nickel-56 decays slowly, with a half-life of 6.1 days, so the reaction cannot keep pace with the explosion. Thus, the nuclear cooking is interrupted before it is complete.The decay of the nickel to radioactive cobalt-56 releases gamma rays that heat the expanding debris, counteracting the cooling. Further decay to stable, everyday iron (a reaction with a half-life of 77 days) releases positrons and more gamma rays. The net effect is to produce a spectacularly luminous display. This model explains the characteristic shape of the supernova's light curve. The light declines at the same rate as the nickel decays.The complicated optica! spectra of Type I events make it very difficult to identify the signatures of nickel, cobalt, or iron despite the fact that the supernova must be composed almost entirely of these elements. A breakthrough came with the realization that the infrared spectrum was the place to search for evidence of iron.In 1984, an infrared spectrum taken with the 3.9-meter Anglo-Australian Telescope (AAT) revealed the first tentative evidence for the telltale iron line at 16,000 angstroms. Subsequent observations with the AAT and the 3.8-meter United Kingdom Infrared Telescope in Hawaii placed the discovery beyond doubt.In a paper in the January 1st issue of Monthly Notices of the Royal Astronomical Society, my colleagues and 1 show that the supernova, designated I983n, contained about 'A of a solar mass of iron. The formation of this much