Astronomers are baffled by what they see-and what they don`t see-in the remains of an exploded star.
A brief glimpse inside the debris of the supernova explosion suggests that the core remaining inside is a bizarre neutron star on the brink of becoming a black hole-with properties that defy most astronomers`
expectations.
There is even a slim chance that the core of the supernova has turned into a black hole already, though experts consider that scenario unlikely.
But their efforts to figure out just what is likely have been frustrated by an inability to get another glimpse inside the exploded star, known as Supernova 1987A.
Since Supernova 1987A burst into view two years ago in the skies of the Southern Hemisphere, scientists have wished they could see what`s inside the bright layers of debris blasted outward by the explosion. That wish came true for seven hours on Jan. 18, when a telescope recorded signs that the supernova contains a pulsar, an extremely compact object that emits pulses of light at regular intervals like a cosmic lighthouse.
Pulsars are believed to be rapidly spinning neutron stars, spheres a few miles across made of material so dense that it weighs about a billion tons per teaspoon. Many pulsars are known, but the one spotted in this supernova appears to be denser, and spinning more rapidly, than any other object ever observed from Earth.
”What we`ve been presented with is the most extreme object in the universe,” Stan Woosley, an astrophysicist at the University of California, Santa Cruz, said at a physics meeting this month in Baltimore. ”And it`s hanging by a thread this side of a black hole.”
Finding a neutron star at the core of a supernova is no mystery-it is just what astronomers expected. But other observations are mysterious.
`There does seem to be a neutron star there, but everything else we`ve been told by the observers has come as a surprise,” Woosley said.
The first surprise, he said, was that the pulsar was seen at all. ”We weren`t expecting it for another year or so,” he said. A second surprise was the brightness of the signal, indicating that this pulsar is more efficient at converting energy into light than expected.
Another mystery is the apparent presence of a small companion, about the mass of Jupiter, orbiting the supernova core at a distance of about twice as far as the distance from the Earth to the sun. Astronomers can`t say what it is or where it came from, though Woosley said it could not have existed before the star exploded. On the other hand, he said, formation of a planet inside a supernova within two years is also difficult to explain.
Perhaps even more surprising is the pulsar`s enormously rapid spin-nearly 2,000 rotations a second, 200 times faster than astronomers would have predicted.
”It was a great surprise to find a millisecond pulsar, let alone the fastest one in the universe so far,” Woosley said.
Solving the mysteries posed by the data will be difficult, though, because of the biggest mystery of all-the signal seems to have disappeared.
”The present largest mystery is why can`t we see the thing,” Woosley said.
Since the sighting on Jan. 18, said Carl Pennypacker of the University of California at Berkeley, repeated attempts to detect the pulsar signal again have failed.
”We`ve seen nothing,” Pennypacker said at the meeting of the American Physical Society in Baltimore. ”I`m worried about where the damn thing has gone.”
Woosley and other astrophysicists said the measurements appear genuine.
”The group that has made these observations has an outstanding reputation,” he said. ”They`ve been very careful. And I think for the time being we have to accept the observations. But our top priority must be to see this thing again.”
Assuming the observations were correct presents difficulties for supernova theorists. At such a high rate of spinning speed, and high density, the pulsar almost has no right to exist. Anything spinning that fast should fly apart unless it were extremely dense yet also very soft.
John Friedman of the University of Wisconsin at Milwaukee said calculations indicate that such an object can barely exist, but only if it weighs more than enough to become a black hole-a region of intense gravity from which nothing can escape. The pulsar may now be spinning fast enough to avoid collapsing into a black hole, but as it slows down, such a death seems inevitable.
”The ultimate fate of neutron stars that are rotating this fast is to collapse to a black hole,” said Friedman. ”It`s conceivable that it would collapse to a black hole in our lifetime.”
Some astronomers contend that the pulsar was not really spinning but vibrating, like a ringing bell. Woosley said he doubted that explanation. Instead, he believes, the pulsar could have been boosted to such a rapid spin as matter from the explosion debris fell back onto the neutron star. The effect would be much the same as the increasing spin of an ice skater pulling in her arms.
If the rapid spin is in fact real, it deepens the astronomical mystery over the nature of millisecond pulsars, the neutron stars that spin rapidly enough to emit 1,000 signals or so a second. Most pulsars spin much more slowly than that.
The pulse from Supernova 1987A came at 1,968.6 times a second, by far a new record. Since most previously known fast pulsars have a companion star orbiting nearby, many astronomers believe they are formed when a neutron star siphons matter from the companion, spinning faster and faster as matter is added.
But Kenneth Brecher, an astrophysicist at Boston University,says pulsars are born spinning rapidly. Those with large magnetic fields would soon slow down, but fast pulsars-like Supernova 1987A-have a weaker magnetic field, allowing a rapid spin rate to continue.
The latest mysteries are not the first surprises from Supernova 1987A, which was first spotted on Feb. 24, 1987, in the Large Magellanic Cloud, a satellite galaxy to the Milky Way. It was a Type II supernova, meaning that the explosion occurred after a heavy star burned up its nuclear fuel and began to collapse under its own weight. For reasons not well understood, such a collapsing star explodes, blowing away its outer layers and leaving a dense core behind.
Oddly, Supernova 1987A was born from a blue supergiant star, whereas astronomers had previously believed Type II supernovae originated only in red supergiants.
A further surprise involved the roughness of the explosion. Previous theories pictured the explosion as a smooth, rapid expansion of the star`s outer layers. Each layer consists of different chemical elements created during the star`s lifetime, wrapped around the star`s core like the layers of an onion.
Observations of radiation from the supernova soon indicated, however, that the layers were mixed up during the explosion instead of expanding smoothly. Furthermore, observations reported at the Baltimore meeting indicated that the expanding shells of matter were broken into threads and clumps.
Instruments in a balloon flight over Australia last year detected gamma rays from the supernova across a relatively wide range of frequencies, indicating that the gamma rays were arriving from different parts of the exploded star. That suggested that the layers of matter were broken into clumps, creating holes through which gamma rays from the distant side of the star could pass, said Scott Barthelmy of the NASA Goddard Space Flight Center. An alternate explanation, he said, is that the star could have been somehow flattened into a disk and tilted so that gamma rays would have to travel different distances to the Earth.
Such an odd geometry might explain the measurements, said Brecher, but he favors the picture of fragmented outer shells.
Clumps and clouds of matter from such a broken shell might be responsible for hiding the pulsar from view, but Woosley believes such an explanation is unlikely. During the seven hours of viewing, the intensity of the signal changed quickly, possibly due to interference from small clouds of matter. But it is hard to envision, Woosley said, a gigantic cloud then coming along and blocking the signal for months. He suspects that the pulses from the supernova are simply not now being emitted.
One possible explanation, he said, is that the pulses were generated by a ”starquake,” similar to an earthquake, and that as the vibrations diminished the signal became too weak to be detected.
A more remote possibility is that the star turned off its signal because it collapsed to form a black hole, which cannot emit signals because of its own immense gravity. Woosley and Friedman both doubted that a black hole has formed already, but Friedman said that there was a ”slim chance” that collapse has already occurred.
Whatever the outcome of future observations, Woosley thinks it unlikely that the mysteries of Supernova 1987A can be explained according to current beliefs.
”We`re going to find that some of our cherished beliefs are wrong,”
Woosley said. ”And that`s always fun.”
