A super star goes out in style. The aftermath of a supernova blast reveals that nature is a successful alchemist
Boston — THIRTEEN months after it first burst on the scene, Supernova 1987A remains a subject of intense interest among astronomers and astrophysicists worldwide. For the first few months, looking at the exploded star ``was like looking at the bud of a flower,'' says Richard McCray, an astronomy professor at the University of Colorado at Boulder. ``Now it's blossoming.'' As it blossoms, scientists are getting their most detailed look yet at the processes responsible for seasoning the universe with nearly all the chemical elements heavier than helium. Based on their observations and detailed calculations, astrophysicists are piecing together the story of how the supernova's progenitor, Sanduleak -69 202, lost its battle with gravity.
About 10 million years ago, a growing, churning ball made largely of hydrogen passes a milestone: The relentless pull of its gravity raises the pressure and temperature of the gas at its center to a level high enough to fuse hydrogen nuclei into helium, releasing vast amounts of energy. The nameless star, situated in a nearby galaxy, ignites, burning blue-hot, its core reaching a temperature of 40 million degrees Celsius. The pressure of the energy radiating from the core counteracts gravity's tendency to collapse the star. The star's size stabilizes at about 50 times the size of the sun, or 43.5 million miles across, and has 20 times the sun's mass. A cubic inch of its core weighs about 3.3 ounces. After 170,000 years, its light first reaches Earth. Ten million years later, humans call the star a blue supergiant, Sanduleak -69 202.
Part of Supernova 1987A's significance is that it is the first supernova close enough to be seen by the naked eye in 384 years. It lies in the Large Magellanic Cloud, one of two companion galaxies, 170,000 light-years away from the Milky Way. Up to now, the most detailed records of supernovae in other galaxies have ``come from supernovae more than 5 million light-years away,'' Dr. McCray says.
In addition, this is the first time that scientists had seen the star before it blew up, says Robert Kirschner, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Although the star was first studied in 1969 by a Case Western Reserve University astronomer, Nicholas Sanduleak, it appears on photographic plates that are up to 100 years old. There were no detectable changes in those images, so ``there was no clue on the outside that the star would blow up,'' Dr. Kirschner says.
One puzzle scientists are grappling with is the star's color: Until now, supernovae were thought to develop from much larger red supergiants. While some researchers are working on models to help explain a blue supergiant's explosion, others see evidence that 20,000 to 30,000 years before it blew, Sanduleak -69 202 may have briefly expanded into a red giant.
According to Kirschner, evidence comes from observations he's made using the International Ultraviolet Explorer satellite of a shell rich in nitrogen that lies about one light-year from the supernova. Before a red giant explodes, some of its atmosphere escapes the relatively low gravity at the star's bloated surface. The position and velocity of the shell suggest that it came from the Sanduleak star, leading many astronomers to conclude that the star went from blue to red and back to blue before it exploded.
Why it would do that remains something of a mystery.
Stanford Woosley, an astrophysicist at the University of California at Santa Cruz, points to the relatively low metal content of the Large Magellanic Cloud as a possible factor. It may also have to do with conditions inside the star's gaseous envelope. But given the myriad interactions in that region, he says, calculating what goes on there is ``like trying to calculate the weather.''
After its first 9 million years, the star's core begins to run out of hydrogen. Gravity squeezes the core, compressing the helium ``ash'' that has accumulated over eons. The temperature and density rise until the helium fusion reaction ignites, producing carbon. The core temperature has shot up to about 300 million degrees C. A cubic inch of the core, which once weighed 3.3 ounces, now weighs about 36.1 pounds.
The helium burns for about a million years before it too is nearly exhausted. Gravity squeezes, and the core, now largely carbon, ignites, yielding neon, magnesium, and sodium. The star is into its last thousand years. The core temperature reaches 700 million degrees. A cubic inch of the core now weighs 2.7 tons. Energy losses grow, as energy is carried off by small massless, chargeless particles called neutrinos. Neutrinos interact with matter so weakly that they zip through the substance of the star as though it wasn't there.
As the carbon exhausts itself, the core contracts until the neon fusion reaction ignites, forming oxygen and magnesium. The core burns for several years at temperatures of 1.5 billion degrees. The core becomes so dense that a cubic inch of it weighs 1,804 tons.
The core continues to contract, igniting the oxygen, which forms silicon and sulfur. The temperature now hits 2.1 billion degrees. The star has entered its last year of existence. The core's density holds at its previous level for a while longer.
Once the oxygen is largely depleted, silicon forms the core's last sphere of defense against gravity. Silicon and sulfur nuclei begin to fuse, sealing the star's fate by forming members of the iron group of elements - iron, nickel, chromium, and others. The core temperature hits 3.5 billion degrees, and a cubic inch of the stuff weighs 18,040 tons. The rest of the star's life is now measured in days.
Finding direct evidence of this stellar alchemy is one of the biggest coups so far. When the star exploded, nuclear reactions formed radioactive nickel, cobalt, and titanium. The nickel decays into cobalt and then into iron, giving off gamma rays in the process. As the gamma rays work their way out through the material ejected by the blast, some lose enough energy to become X-rays. Thus, the first hints of elements formed through this process came from Japan's Ginga X-ray satellite and an X-ray telescope on the Soviet space station Mir. Then, in December, the National Aeronautics and Space Administration announced that its Solar Maximum Mission satellite and balloon experiments flown form Alice Springs, Australia, had detected the gamma rays.
The catch, says Dr. Woosley, is that this radiation was spotted months before it was predicted. This leads him and others to conclude that the expanding shell of ejected material may have holes or clumpiness that allow the X-rays and gamma rays to escape, or there may be some mixing between layers of ejected material. Scientists using optical and infrared telescopes have also seen signs of calcium and oxygen.
As the mass of the iron ``ash'' at the heart of the core reaches 1.4 times that of the sun, the final collapse takes but a fraction of a second. The core's temperature soars to more than 100 billion degrees C.
The radiation the star counted on to counteract gravity is spent splitting the iron nuclei. Electrons slam into protons to form more neutrons. The collapse accelerates, until the core reaches a density of an atomic nucleus. One cubic inch now weighs 18 billion tons.
The core has reached what Cornell University physicist Hans Bethe calls the point of ``maximum scrunch.'' Like a hard rubber ball squeezed to its limit and suddenly released, the core snaps back, sending out a shock wave that triggers a new round of element-forming fusion as it blasts away the outer layers of material. What remains is a rapidly spinning neutron star, about 44 miles in diameter with a mass of about 1.5 times that of the sun. Some 99 percent of the energy released escapes in the form of neutrinos, which bear information about the conditions that existed at the star's heart at the instant of collapse.
About 170,000 years after the core collapses, first the neutrinos, then the light, hit detectors on Earth.
The burst of neutrinos from the supernova, picked up by underground detectors in Japan and the United States, corresponded ``amazingly well'' to what theory predicted, McCray says. It was direct evidence that the core had collapsed, and what Kirschner calls circumstantial evidence that a neutron star had formed.
What is less clear is the precise mechanism causing the explosion. While all cores bounce, says Woosley, some bounce less than others. In that case, the blast of neutrinos could be responsible for stripping the star of its gaseous layers. He adds that calculations being conducted by scientists as the California Institute of Technology in Pasadena, appear to strengthen the core bounce as the dominant mechanism.
Astronomers are following the supernova's evolution at a range of wavelengths. For example, NASA's Kuiper Airborne Observatory, a converted C-141 Starlifter aircraft, is being used to study the supernova at infrared wavelengths. McCray says that molecules are forming inside the shell, and in April the airborne observatory will try to gather data on the process.
Scientists are also eagerly awaiting the appearance of the neutron star, whose radiation is still blocked by the expanding shell of ejected material.
When it appears, the evidence could take several forms, astrophysicists say. It could come as rapid pulses of low-energy gamma rays or high energy X-rays, formed as the star's rotating magnetic field interacts with the material surrounding it. Based on inferences from data gathered so far, such a pulsar should send out a pulse once every 15 milliseconds,
To gather that evidence, scientists are flying X-ray telescopes on NASA balloons from Alice Springs. NASA recently announced that it was reconfiguring a 1989 Spacelab mission to include an X-ray telescope that was to be flown on a later mission. If the pulsar is oriented so that its X-ray beams don't reach Earth, its energy would still cause the surrounding gas to glow, forming a nebula. Many of the scientists studying the supernova expect to see evidence of the pulsar within one or two years.
Even a supercomputer is being used on the pulsar watch.
Researchers at Los Alamos National Laboratory are using a supercomputer to sort through data from optical telescopes in Chile in hopes that the computer will sort out minuscule changes in the ejecta's light that would indicate the presence of a pulsar. Currently the supernova's light stems from the decay of radioactive elements in the shell. But as that decay radiation fades over time, the pulsar should become more evident.
Over the longer term, the supernova's shell, hurtling through space at 18,750 miles a second, is expected to set off a burst of X-rays in about 10 years as it collides with the slower-moving, nitrogen-rich shell left over from the one-time star's red-giant stage.
Glossary of terms Black hole Formed when an object in space has such a large gravitational pull that it collapses on itself. Even radiation cannot escape from a black hole. Thought to originate from a supernova explosion.
Interstellar material Matter from which stars and planets are formed. Makes up several percent of the galaxy's total mass and consists primarily of hydrogen.
Neutrinos Small, massless, chargeless particles which interact with matter so weakly that they can travel through the universe unhindered.
Neutron star An extremely dense star at the end of its evolutionary life. One possible outcome of a supernova.
Pulsar A rotating neutron star which emits radiation in brief, regular pulses.
Supergiant The largest and brightest type of star, rare because it is formed from the most massive stars. A supernova's precursor.
White dwarf A star of low mass in the final stage of stellar evolution.