Flush With Stardom, Supernova May Be Gearing Up for Encore

The supernova that lit up the sky in the Southern Hemisphere last year may “re-explode” within the next decade, according to a leading expert.

Kenneth Brecher, a Boston University astrophysicist, told the American Astronomical Society meeting here that historical records reveal that several supernovas in past centuries seem to have followed a similar course: first lighting up the sky with the initial stellar explosion, then gradually dimming until they disappeared within a couple of years, and finally reappearing a decade or more later.

Brecher speculated that the historical reincarnation probably occurred when hot gases from the explosion eventually reached a “shell” of particles left over from an earlier epoch in the star’s history.

“This is a sprinkling of facts and a dash of guesswork,” Brecher told a news conference Thursday. But he cited several specific records dating back 1,000 years to support his theory.

Brecher’s prediction was one of several surprises that emerged here as scientists continue to puzzle over what many consider the most important astronomical event of the century--the explosion of a star last February in a nearby galaxy, the Large Magellanic Cloud. That explosion, the most brilliant supernova since 1604, has given scientists an extraordinary opportunity to check out some of their theories of stellar evolution.

Most critical of all, perhaps, has been the growing evidence from the supernova that supports the theory that all of the heavy elements that make up so much of the universe were formed in the thermonuclear furnaces of supernovas. That includes the atoms that form the Earth as well as the stardust of which even humans are made.

The evidence consists mainly of various forms of radiation that are now reaching the Earth, each bearing messages of specific events. A gamma ray observatory aboard a high-altitude balloon last month, for example, convinced Rice University astrophysicist Donald D. Clayton that he had proved a theory he had been trying to prove for two decades.

The instrument detected gamma rays from the supernova. The gamma rays were emitted when radioactive cobalt that spewed out of the supernova decayed into iron, thus proving that such heavy elements are formed in exploding stars. Other experiments, including NASA’s Solar Maximum Mission Satellite, also confirmed the theory.

More evidence of other phenomena is expected during the coming months as gas and debris from the explosion encounters remnants from the star’s past.

Stars pass through many stages in their evolutionary course, according to theory, and each stage leaves its own historical record.

The supernova had at one time been an ordinary star similar to the sun, called a “main sequence” star. It subsequently cooled and evolved into a “red giant,” and still later turned into a brilliant “blue supergiant.”

The fact that it exploded as a blue supergiant has been the biggest surprise of all. According to theory, supernovas were believed to be exploding red--not blue--stars.

During each stage of its evolution, the star created a shell of material, just as the sun is now creating a solar atmosphere of charged particles encompassing the solar system. According to theory, when the sun turns into a red giant, it will produce a different kind of shell, and the old one will be pushed farther out. The same should happen again when it becomes a blue supergiant.

A supernova releases radiation and debris that eventually reach the various shells, a process that takes several years because the distances are so great. As different forms of radiation and particles collide with different elements in each shell, other types of radiation are created--such as gamma rays--telling astronomers on Earth, 170,000 light-years away, what has happened.

Alice K. Harding of NASA’s Goddard Space Flight Center should be able to answer one of the most important questions of all later this year. If instruments she is monitoring detect a shower of subatomic particles in the Earth’s atmosphere coming from the direction of the supernova, it will mean that the atmosphere is being bombarded by gamma rays of extremely high energy.

Such gamma rays, Harding said, would mean the supernova is powered by a spinning neutron star, called a pulsar, that sends out bursts of radiation as it spins rapidly in the center of the supernova, somewhat like a celestial lighthouse.

The shower should begin within the next few months, she said. If it does, it will prove the existence of the pulsar, thus making a lot of astronomical theorists very happy. They have said for some time that pulsars are formed in supernovas, but so far no one has been able to prove it.

The shower that Harding is looking for will be detectable only with very sensitive instruments, but even amateur astronomers with small telescopes will be able to follow some of the coming events, provided they are in the Southern Hemisphere where they can see it.

Sometime within the next year or so, the star will have dimmed to the point that a “light echo” will be visible, according to Bradley Schaefer of the Goddard Space Flight Center.

Some light from the supernova travels directly to the Earth, but other light is scattered by dust particles and gases that make up the interstellar medium. Some of that scattered light will be reflected back toward the Earth, thus producing a halo around the star that will change and evolve as light strikes different areas of the interstellar medium.

The halo, Schaefer said, “will become a very easy target for amateur astronomers for centuries.”

The star itself--which has dimmed to the point that it is just bright enough to be seen with the unaided eye--will fade completely, but the halo will remain, he added.

“Like the Cheshire cat,” he said, “all you are left with is the smile.”

The cat, however, could return if Kenneth Brecher is right.

Brecher has combed through records of ancient sightings of supernovas and found that in many cases supernovas appeared in groups of two, separated by 10 to 50 years.

Perhaps the most brilliant supernova in human history occurred in 1006, Brecher said. It faded after a year or so, but other records report a similar event--though far less brilliant--in the same area of the sky in 1016.

That pattern was repeated in 1572-1612, and again in 1604-1664.

Since supernovas that are visible from the Earth are so rare, even the most scant historical records play an important role, Brecher said.

“Even if you’ve got crummy old records, they’re worth something because they’re the only thing you’ve got,” he said.