Cosmic Conflagrations

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.

Other Cases

Kirshner now thinks that other double supernovae are in existence. He says that since supernovae "probably do come from massive stars that form in clumps," it is "very likely" that double supernovae are "not so rare."

Kirshner says there may be other cases of a double supernova even in our own galaxy, but it would be a "50,000-year-old shell that we would hardly notice."

"Here, if we're right, there is a second event that is [relatively] young, and we can see change," he says. "It would be very interesting to see the effect of an explosion inside an explosion."

By studying supernovae, scientists can learn more about the creation of stars and other astronomical phenomena. "They can illuminate cosmic mysteries, like the size and shape of the universe."

Kirshner says one important reason to analyze these phenomena is because they are theorized to be the source of all heavy elements in the universe. When the stellar explosion takes place, it fuses all of the star's abudant light elements, like hydrogen and helium, into heavy elements, like iron. "There is a real sense that supernova elements are actually the physical origin of [the heavy] chemicals in our body," Kirshner says. The elements these supernovae create are in everything, he says, including "life, earth, iron and gold."

In a multiple explosion, though, it could be a "quite different physical situation," Kirshner says. If the explosions happen "together in space and time, the net effect could be larger," he says.

Although supernovae are said to "explode," in reality they start with a collapse, Kirshner points out. The inside of the star collapses to make what is also called a "neutron star" or "black hole" and this heats and eventually blows off the star's outer shells, thus spreading newly created substances throughout the universe.

Kirshner says another use for supernovae data is as a "yardstick for measuring the geometry of space." As they explode, supernovae are the brightest stars in the universe. By measuring its chemical and physical characteristics from its emitted light spectra, one can check to see if the universe is still expanding and at what rate, he says.

Kirshner also says places where supernovae have probably formed "might be the key to understanding what strong gravity really does," since explosion remnants are some of the most massive bodies in the universe.

Recent advances in technology have made observation for astronomers like Kirshner more effective. To take pictures of the light spectra of a supernova "it used to be photographic plates, but now we're using CCD's [Charge Couple Devise] that are the same that are in a home video camera." He says the CCD's are extremely sensitive to light and so are of immense use in detecting distant remnants.

Here at the CFA, where Kirshner has his office, researchers are mostly concerned with analyzing data picked up from tools like the CCD and the telescopes used in Chile, Kirshner says.

Future Studies

Early next month, Puppis A will be studied more extensively at Cerro Tololo. "We'll see if it stands up to closer scrutiny," Kirshner says, though he will not be in Chile for the continuing observation of his hypothesized double supernova, he says. Next semester he will be teaching the Core course, Science A-35, "Matter in the Universe," and says he needs to stay in town "for sectioning and stuff like that."

A former Quincy House resident, Kirshner received his doctorate in astronomy from Cal Tech and performed his post-doctorate work at Kitt Peak National Observatory in Tucson. Kirshner then worked at the University of Michigan as a professor of astronomy from 1976-1985, until he "received the call" to work at Harvard, he says.

Kirshner says he has worked with supernovae for the past 17 years, ever since he started his graduate work in California. But even as a junior concentrator in Harvard's undergraduate Astronomy Department, Kirshner decided to focus on his present specialty, when he took a tutorial on the Crab Nebula, a supernova remnant.

"I remember when I showed up at Cal Teach the first day, they asked me what I wanted to work on. I said, `I did some work on Crab Nebula, do you have any work on supernovae?' Some of these things are sort of thrust upon you."