`Close-up look' at a science superstar. Supernovas - breeding ground for Earth's chemical elements
With a burst of subatomic particles, an exploding star high over the southern hemisphere has given the first glimpse into the workings of a supernova. In the process, it has transformed the study of exploding stars - or supernovas - from ``a computer game into a science,'' says John Bahcall, an astrophysicist at the Institute for Advanced Study in Princeton, N.J.
Scientists in Japan and the United States have independently confirmed a report that the supernova emitted bursts of small subatomic particles called neutrinos. These play a fundamental role in current theories about the inner workings of stars and their destruction.
The confirmation gives scientists the first ``direct test of the internal dynamics of supernovas and of the formation of neutron stars,'' says Adam Burrows of the University of Arizona.
Supernovas are of keen interest to astrophysicists because they are the breeding ground for the heavier chemical elements that make up Earth. In addition, supernovas are thought to trigger the formation of stars from clouds of gas and dust.
A supernova occurs when a massive star runs out of nuclear fuel and collapses toward its core. At each stage of collapse, fusion reactions form successively heavier elements ranging from carbon to iron. Gravity compresses the core to what physicist Hans Bethe of Cornell University calls the point of ``maximum scrunch.'' Like the release of a squeezed rubber ball, the core rebounds, setting up a shock wave that blasts away the outer layers of the star. What remains is a neutron star with a dense iron core where, by one estimate, one cubic inch would weigh 10 billion tons.
Some of supernova's energy released is in the form of light and other electromagnetic radiation. But by far the greatest amount of energy is carried away by neutrinos.
Some eight hours before the supernova's light hit Earth, a team of scientists in Europe detected the passage of 5 particles over a 7-second period. They reported their findings on Feb. 28. Last Saturday, the Japanese began to circulate their observation: detection of 11 neutrinos in a 13-second period about 4 hours before the first light hit a ground-based telescope. Researchers in the US spotted the passage of 8 neutrinos in 6 seconds at precisely the same time as the Japanese.
These results ``are astoundingly close to prior expectation. I'm very surprised that theoretical calculations have come so close'' to what actually happened, says Dr. Bahcall.
Neutrinos have a minuscule mass and are electrically neutral. They pass through most forms of matter without reacting with it. When a star explodes, the neutrinos speed from the core largely unimpeded; the light isn't released until the shock wave bursts through the star's surface.
This difference in timing allows scientists to gain insights into the mechanics of supernovas. For example, using the delay between the arrival of neutrinos and visible light, scientists can estimate ``how long it takes for the shock wave to go from the center of the star to the surface,'' says Dr. Bethe.
Kenneth Lande, chairman of the astronomy department at the University of Pennsylvania, says neutrino data could have a bearing on theories about the future of the universe.
Astronomers are trying to explain the apparent fact that all of the material they can detect in the universe accounts for only about 10 percent of universe's mass. The nature of the other 90 percent could determine whether the universe continues to expand, stabilizes, or collapses.
One candidate for this ``missing mass,'' says Dr. Lande, is the neutrino. Assuming that the supernova's neutrinos were generated at the same instant, the fact that they arrived at different times suggests that they have slightly different masses. Using this information, he says, may allow theorists to decide whether to keep neutrinos in the running as ``dark matter'' candidates.