Number 175 (Story #3), April 25, 1994 by Phillip F. Schewe and Ben Stein|
THE INVENTORY OF ELEMENTS IN SUPERNOVA REMNANTS , and the structure of their progenitor stars, has been studied with ASCA by recording x-ray images that correspond to emission lines from specific hot, multi-ionized shells of matter surrounding the supernova blast. Textbook diagrams of heavy stars on the eve of a supernova explosion show concentric layers of successively heavier fusion products: carbon, oxygen, neon, silicon, sulphur, and iron piled up at the heart of a star as it runs out of nuclear fuel. The force of the subsequent supernova explosion flings the matter in these layers out into space, where the layering of the elements is somewhat retained. At the APS meeting, Robert Petre of NASA/Goddard presented separate Si, S, Fe (etc.) ASCA pictures of remnants. These illustrate that fact that some supernovas are symmetric, while others are asymmetric. For example, pictures of the Cassiopeia-A remnant reveal the material to be more ringlike than spherical, suggesting that the progenitor star had been rapidly rotating. This is borne out by separate doppler maps of different parts of the Cas-A remnant. The ASCA pictures of Cas A also show, for the first time, two different x-ray processes at work in a single supernova: x rays from specific elements (silicon and sulphur, say) in the north and east and, in the southwestern corner of the remnant, "continuum" radiation coming from the collisions between electrons and all sorts of ions.