Cygnus Loop Supernova Remnant
When a supernova explodes, its blast sweeps up surrounding gas with a powerful shock wave. A typical supernova shock has more energy in the motions of its material than the Sun will emit in its lifetime. As the shock wave moves outward, the shocked gas that is swept up forms a shell that glows as brightly as a million Suns. Virtually all of the elements essential to life were created in stars or in supernovae, not at time of the big bang; they were dispersed by these supernova remnants (SNRs) into space, where they became available to make new stars and planets. Astronomers are keenly interested in studying the glowing shells of SNRs to determine their chemical composition and properties.
A significant fraction of the elements in space collect in the form of dust grains. Silicon and carbon, especially, coagulate into silicate (i.e., sand) or graphite grains, and these grains in turn act to collect other chemicals on their surfaces. As the SNR shock wave moves outward, it heats and disrupts these dust grains, and ionizes some of the the ejected material. A team of three SAO astronomers, John Raymond, Terrance Gaetz, and Andrew Szentgyorgyi, together with three colleagues, has used the Far Ultraviolet Spectroscopic Explorer satellite to probe the light emitted by the hot gas in one famous SNR, the "Cygnus Loop" nebula. The scientists discovered a set of characteristic atomic features that were never before seen in a SNR. Arising from iron, silicon, and other elements that had been known from other emission features, the new features result from million-degree gas in the shock.
In a typical interstellar medium perhaps 40%-90% of the silicon is locked up in these dust grains. The astronomers calculate from their data that at least 50% of this silicon is liberated from the grains by the shock, thus making it available for other chemical reactions in the interstellar medium. The new research also tracked changes in the state of the ionized gas across the remnant material, and determined the oxygen abundance of the emitting gas. The results also refine our knowledge of many other details of SNRs, shock densities and structure for example, and hence our understanding of the key process in which chemical elements are dispersed into space, perhaps to find their way into future planets. Reference: "Far UV Spectroscopic Explorer spectroscopy of the XA region in the Cygnus Loop Supernova Remnant," R. Sankrit et al., Astronomical Journal, 133, 1383, 2007.