Last year, Professor of Astronomy Robert P. Kirshner '70 thought he was a lucky man to observe during his lifetime one of the brightest supernova explosions our region of the universe has seen in centuries. But this year, he thinks his luck may have doubled.

Kirshner, a Harvard colleague, and two other scientists believe they have discovered the remnants of the first double supernova star ever observed, and evidence tells them that it may not be the only one.

Supernovae, in which Kirshner has specialized since his graduate school years at the California Institute of Technology, are stars which have ended their lives as energy producing bodies with a tremendous explosion. If the phenomenon occurs close enough to the Earth, it is observable in the sky as a bright star, Kirshner says.

Unfortunately for Kirshner and his fellow astronomers, who hope to find answers to a variety of astronomical questions from studying these stellar explosions, observable supernovae do not occur very often. In fact, 1987 was the first time since 1604 that Earth dwellers witnessed an actual explosion close enough to the Earth to be seen with the naked eye, Kirshner said, thus making the event the first young supernova to be carefully analyzed with modern scientific equipment. Astronomers, including Kirshner, are still collecting data from the phenomenon, called 1987A.

Evidence leading to the hypothesis of a double supernova, however, was collected by Kirshner and his co-workers while examining the remnant of an ancient explosion, which took place more than 4000 years ago. Astronomical observers uncover about 20 of such stellar fossils, which are less useful than phenomena occuring more recently and closer to the Earth.

Working at the Cerro Tololo Inter-American Observatory in Chile last February, Kirshner says his team was taking pictures of the light emitted from supernova Puppis A, a remnant discovered in the 1950s, which has been studied "off and on" ever since. Astronomers usually study the light spectra from all sorts of astronomical bodies to learn about their physical and chemical characteristics, Kirshner says.

When examining the spectrum of light energy emitted from Puppis A, however, "We found a funny little thing" that hinted to Kirshner that it might not be an average explosion remnant.

In a "very funny place inside the remnant," Kirshner says he noticed that chemicals released during the star's explosion which should have mixed together over the last four millenia were in fact still separated in space--as if they didn't have the time to mix yet. They discoved this by using several different filters when photographing the supernova, which bring out the separate bands of the elements, Kirshner says. "It still bears the imprint of" a more recent explosion than most data from Puppis A reveals.

Kirshner says that further detailed measurements of Puppis A's light wavelength indicated that part of the supernova was only 800 years old, when the rest of it had been previously estimated at 4000 years of age. On a universal time scale, however, the 3200-year difference is practically simultaneous, Kishner says.

The average lifespan of a star is 10 million years, Kirshner says. So what seems to be a huge difference, "is only one three thousandth of a star's life." He says it is comparable to "two events happening in the same second."

Puzzling Clues

Another puzzling thing about the supernova was there were no radio waves or X-rays emitted from it. "This would not be so odd, though, if there was something inside the other remnant," Kirshner says. "There's a good circumstantial case for a second supernova inside the other supernova. If it's really true it's the only case we know about."

Kirshner and his three co-workers, John P. Hughes of the Harvard-Smithsonian Center for Astrophysics (CFA), Stephen R. Heathcote from the Cerro Tololo Observatory and Frank Winkler from Middlebury College, published these findings in the January 5 issue of Nature magazine. The cover of the famous periodical shows a picture created by Kirshner portraying his theory of the double supernova explosions in Puppis A.

Kirshner says it took a year to publish the findings because of the time necessary to reduce the data and write the article. He says some editors thought the possible double supernova was interesting but not worthy to write about at first.

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