June 17, 2002 AMHERST, Mass. – Two astronomers at the University of Massachusetts have observed the remains of exploded stars called supernovae in two "neighbor" galaxies of our own Milky Way. The report on their observations is being presented today by postdoctoral researcher Rosa Murphy Williams and faculty member Q. Daniel Wang, in collaboration with You-Hua Chu and John Dickel of the University of Illinois at Urbana-Champaign, at the meeting of the American Astronomical Society in Albuquerque, N.M. These results provide new insight into the distribution of gas shocked by the supernova blast wave, and the structure of material between stars, the researchers said. The findings expand our understanding of the long-term evolution of a star after it explodes, and of how the remnants of a star are ultimately swept back into the universe.
The authors are presenting a series of observations made with new satellite-based telescopes such as the Hubble Space Telescope, the Chandra X-Ray Observatory, and the X-Ray Multi-Mirror Mission, all of which reveal the supernova remnants in two of our neighboring galaxies in unprecedented detail, Williams said. The Large and Small Magellanic Clouds are small galaxies adjacent to our own Milky Way, about 160,000 and 190,000 light-years from Earth, respectively. The findings combine a review of previous research, along with newer findings and observations by the UMass team.
"When a massive star nears the end of its lifespan, its internal fusion reactions can no longer support the mass against gravity," explained Williams. "The star implodes and, within a fraction of a second, rebounds in a tremendous explosion in which much of the star's material is flung outward at speeds of about 20 million miles per hour." This violent outburst sweeps up interstellar dust and debris in its path like a natural snowplow, Williams said, forming an expanding sphere of gas and dust which can reach more than a hundred light-years in diameter. After approximately 100,000 years the expansion slows considerably, and the supernova remnants merge with their surroundings. The process by which supernova remnants interact with interstellar residue and eventually merge with it is the puzzle that Williams and Wang are probing.
These supernova remnants are the means by which energy and heavy elements produced by supernovae are distributed throughout the host galaxy. Supernovae are, in fact, the only known source of heavy elements such as iron and calcium, many of which are vital to human life, Wang noted. "Human bodies are made of heavy elements, such as iron and calcium, which were synthesized during supernova explosions," he said. "The evolution of supernova remnants further determines how the elements are redistributed into the interstellar space, from which new stars and planets are formed. The UMass team's research is filling a gap in our understanding of this chemical distribution and evolution in galaxies."
Probing the deaths of stars, page two
Although previous research has been conducted involving supernova remnants, much of that research has focused on remnants in our own Milky Way galaxy. There are some disadvantages to studying remnants so close to home, Williams said. Material in our own galaxy blocks our view of the remnants, as though they were being viewed through a fog. But recent advances in scientific instrumentation, including the Hubble Space Telescope and the Chandra X-Ray Observatory, have made it possible for scientists to see supernova remnants in neighboring galaxies as though they were much closer.
"This provides us with a much better idea of what happens long after the star explodes," Williams said. The UMass study has combined past images with newer, high-resolution ones, revealing a new level of detail in multi-wavelength pictures that rely on X-ray, optical, and radio data, she said.
Many objects in the two neighboring galaxies that the UMass astronomers studied – the Large Magellanic Cloud and the Small Magellanic Cloud – have been researched during the past 30 years, Williams notes, but have been the subject of increased scientific focus as instrumentation has improved. "Hubble and Chandra have given us, practically, a quantum leap forward in terms of what we can see," Williams said. "What seems to happen is that as the supernovae sweep up more material, that affects their 'aging' process," Williams said.
"Essentially, we do forensic science on dead stars, and on what happens to them physically and chemically as they 'decompose.' We're not looking for the cause of death, but rather for clues as to what happens when the body returns to the soil," Williams said. "We know what elements will remain, but we don't know how those elements get from the supernova's explosion to help form the next generation of planets and stars. That's what we're looking for, and we're just starting to see the beginning stages of that."
The supernova remnants that the team is exploring seem to be going through the aging process, cooling and collecting interstellar material, much more quickly than usual. This has given the UMass team a rare opportunity to watch the process before the star fades out entirely.
Related images are available at: http://xray.astro.umass.edu/~rosanina/press1.html
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