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Astronomers make 'movie' of supernova using radio images

Astronomers using an international network of radio telescopes have produced a "movie" showing details of the expansion of debris from an exploding star. Their sequence of images constitutes the best determination yet made of the details of a new supernova remnant, and already has raised new questions about such events.

The scientists used radio telescopes in Europe and the US--including a telescope at MIT's Haystack Observatory--to make very high-resolution images of Supernova 1993J, which is some 11 million light-years distant in the constellation Ursa Major. Their results are reported in the December 1 issue of Science.

MIT's involvement in the work, which was done in collaboration with 12 other organizations, included collection of some of the data and processing of the cumulative data received from other telescopes in the network. "We are one of the very few places in the world capable of processing this data, which is done with powerful machines called correlators," said Alan R. Whitney, associate director of Haystack and principal investigator for the work at MIT.

The capability to make such high-quality images with widely dispersed radio telescopes has existed for only a few years. Supernova 1993J was the first one that was both close enough and had radio emission strong enough to enable scientists to make such detailed images.

While the circular images show that the explosion debris is expanding symmetrically, the radio emission is stronger on one side of the shell. The explanation for this is unclear. Some astronomers have suggested that the stronger emission could result from the debris interacting with a companion star orbiting the one that exploded. The researchers believe that their sequence of images, with the stronger emission persisting for months after the explosion, makes the companion-star hypothesis unlikely.

In addition, the new images show that Supernova 1993J's debris shell has shown no signs yet of slowing because of interaction with material surrounding it. The material from the star's explosion is moving at nearly 10,000 miles per second, according to the researchers. At that speed, the material would travel the distance from Earth to Saturn in one day.

When the angular expansion rate of the supernova debris measured by the radio observatories is combined with the expansion speed of the same debris, measured by optical astronomers, it is possible to obtain an accurate value of the distance to M81 (the galaxy where 1993J resides). The value determined-11 million light-years-is similar to that obtained by other independent means. This is important, as astronomers continue to seek more accurate distances to celestial objects in order to gauge the actual size of the universe more closely.

The work was conducted by researchers from the following organizations: MIT; the University of Valencia, Spain; the Special Laboratory for Astrophysics and Fundamental Physics of Madrid, Spain; the Institute of Astrophysics at Andalucia, Spain; the National Radio Astronomy Observatory in Socorro, NM; the Harvard-Smithsonian Center for Astrophysics; the Jet Propulsion Laboratory; the Max-Planck Institute for Radioastronomy in Bonn, Germany; the Institute of Radioastronomy in Bologna, Italy; the Special Laboratory for Astrophysics and Fundamental Physics of Madrid, Spain; the Center for Advanced Studies at Blanes, Spain; the Joint Institute for VLBI in Europe and Leiden Observatory in the Netherlands, and the Institute of Radioastronomy in Noto, Italy.

A version of this article appeared in MIT Tech Talk on December 6, 1995.

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