how supernova's happened

one a second or so, somewhere in the world, a star blows itself to smithereens, blossoming momentarily to a brilliance greater than a billion alone.

Nobody understands how these events, among the most violent in nature, actually happen. But, until recently, that does not make much unless it is a professional arcane and messy branch of science known as nuclear astrophysics.

Lately, however, supernovas have become signal events in the life of the cosmos, as told by modern science.

Using a particular species of supernova, Type 1a, as cosmic distance markers, astronomers have concluded that a mysterious "dark energy" is wrenching space apart, a discovery that has thrown physics and cosmology into an uproar.

Consequently, the fate of the universe - or at least our knowledge of it - is at stake, and understanding supernovas has become essential.

Astronomers are busy on many fronts trying to figure out the details of these explosions scanning the heavens for harvest more of them in place, peering at the remains of ancient supernovas to seek a clue to their demise, to exploit networks supercomputers when calculating Since reactions in the heart of hell.

This has resulted recently in a kind of two-steps forward, one step back progress, encouraging astronomers that they are on the right track, generally, with their theories, but at the same time reiterated the complexities and baffling puzzles when A pinning it is the details of what happens in the explosions.

Last month members of an international team of astronomers led by Dr. Pilar Ruiz-Lapuente of the University of Barcelona announced that he had found a star speeding away from the site of a supernova blast seen in 1572 by the astronomer Tycho Brahe. This supernova, which appeared as a "new star" in the constellation Cassiopeia, was one of the earliest studied by astronomers, and helped shatter the Aristotelian notion that the heavens above the Moon are immutable.

The newly discovered star, presumably the companion of the star that exploded, supports a long-held notion that such explosions happen in double star systems when one star accumulating matter from the other reaches a critical mass and goes off like a bomb.

Meanwhile, members of a group of astrophysicists using a network of powerful supercomputers to simulate supernova explosions say that succeeded for the first time in showing how such a star could blow up.

In the course of 300 hours of calculation at the University of Chicago's Center for Astrophysical Thermonuclear Flashes, otherwise known as the Flash center, as the bubbles thermonuclear fury rise from the depths of the star like a deadly jellyfish and then sweep around the Surface and clashing in a apocalyptic detonation that Dr. Donald Lamb, a Chicago astrophysicist, called "totally bizarre and novel."

If it is true, the Chicago results could help explain not only how stars explode, but why the explosions are almost but not exactly alike, allowing astronomers to better calibrate their measurements of dark energy.

Many supernova experts said, however, that such computer simulations were more of a good beginning of a definitive answer. Dr. J. Craig Wheeler of the University of Texas called the Flash center work "a courageous calculation," but added that many details needed to be filled in. "I do not think this is the end of history", he said. The history of Type 1a supernovas, experts have long agreed, begins with a dense cinder known as a white dwarf, composed of carbon and oxygen, which is how moderate-size stars like the Sun, having exhausted their thermonuclear fuels of hydrogen and helium, end their lives.

If it happens to be part of a double star system, the white dwarf can accumulate matter from its companion until it approaches a limit, known as the Chandrasekhar mass-about 1.4 times the mass of the Sun.

At that point, so the story goes, the pressure and density in the previously dead star will be great enough to reignite the star and thermonuclear reactions ripple upward, the transformation of carbon and oxygen in elements heavier and heavier, ripping the dwarf white, and more than His companion goes flying off.

Until recently, however, there was little evidence of this. Two white dwarfs could collide, for example, and blow up. In this case there would be no survivor.

Tycho Brahe the supernova has now offered new evidence for the former model, white dwarf bomb.

The supernova, which is one of the few of Type 1a's, which have occurred in our galaxy, and so astronomers have long tried to find his companion. This star, astronomers reasoned, should be zinging along on its neighbors, as a result of having been released, like a stone from a slingshot, from its orbit around the white dwarf died suddenly.

The site of the explosion of supernova is marked today by a small scruff of X-rays and radio waves in the sky.

Near the center of this patch, the team has found a sunlike star moving three times faster than its neighbors.

The star has the right characteristics to have been to give the material to the white dwarf that exploded, but the identification is not maximum, a member of the team, Dr. Alex Filippenko of the University of California at Berkeley, said , explaining in an e-mail message that "it is" possible "that the star just happened to be zoom through that region and is not related to the supernova."

A measure of possibility, the astronomers say, is that most are the observations of supernova ash pollute the outer layers of the star. But this is probably too much to be desired, said Dr. Stan law of the University of California at Santa Cruz, pointing out that the explosion could have blown the outer layer of the star, and all the ashes, out in space.

"This star sat next to, and for a while in the more powerful thermonuclear explosion in the universe, 2.5 million, trillion, trillion megatons," said Dr. patiently.

But the details of that explosion, invisible, which happens in a second or so, are still a mystery.

The light show seen by astronomers from radiation released by radioactive nickel, cobalt a decade, and then iron over the days and months after the cataclysm, releasing gamma rays that strike the ashes of the shattered star and make them more luminous glow briefly A galaxy .

Because all Type 1a supernovas start from the same point, astronomers have tried to use them as cosmic geodetic markers, standard candles whose distances can be deduced from the way they appear bright.

But the supernovas are not standard enough. They vary their brightness by about 40%, which is similar enough to prove that the expansion of the universe is speeding up and that dark energy exists, astronomers say, but it is not enough to define the details about crucial the strength of this strange force and how the transition could be cosmic time, and thus whether the universe will ultimately rip apart or come together in a "great weakness."

In order to reduce the uncertainties in their measurements astronomers need to know how or whether to correct their comments for the differences in things such as age and chemical composition of the parent white dwarfs.

The problem is that there are two ways for the star to burn: like a flame, which is called deflagration, and as an explosion, detonation, in which the combustion spreads as a shock.

And neither type of burning, by itself, can easily explain what astronomers have seen in supernova explosions.

The slow burn, deflagration, results in more of a crew of an explosion, they say. It does not produce enough nickel to generate the light seen by astronomers and leaves much the star of hydrocarbons. Moreover, the parts that are burned are all jumbled, while supernovas in the sky seem to be well stratified, with the thick elements, like iron and nickel in the center, and light, such as silicon, sulfur and magnesium on the outside .

If the supernova consists simply of a detonation, on the other hand, the star turn that all nickel, and that would result in too much light.

Consequently, over the past 10 years, many theorists have adopted a "Goldilocks" model of the explosion, in which the star burns in the flame mode for a while, slowly expanding, and then explode when the density of the star has fallen to the value to take the right amount of nickel.

"The polenta should be the right temperature," said Dr. Wheeler, who described recent three-dimensional simulations by Dr. Vadim Gamezo and Dr. Elaine Oran, both of the Naval Research Laboratory in Washington, and Dr. Alexei A. Khokhlov of the University of Chicago, as "state of the art."

None of these "delayed detonation" models to explain why or when the star would be detonated.

Scientists have had to put that into the calculation by hand. Finding a natural trigger for the detonation is the "silver cup" of our profession, Dr. Wheeler said, adding that the automobile companies spend millions to the problem of ignition in car cylinders.

This is where the center of Flash calculations come in. "It turns out that you need to have an explosion walls," explained Dr. Lamb. But a star has no walls. So how do explode?

The Flash group, led by Dr. Tomasz Plewa of Chicago and the Nicolaus Copernicus Astronomical Center in Warsaw, was investigating what would happen if the white dwarf has started fires burning in a manner not exactly at its center, an event unlikely in the case of Stella a real subject of turbulence - but a bit off-center. In addition to Dr. Lamb, the group included Dr. Alan C. Calder Chicago.

The result was a bubble of flame rising from the depths and then sweeping around the star to become its own wall, crashing into itself at a temperature of 03 million degrees and crushing density, moderate, Chicago physicists say, trigger detonation.

"We watched with eyes and jaws dropped as the curious thing is done," said Dr. Lamb.

But if nature really works this way or not, Dr. Lamb and others agree, is yet to be defined, and it is far from a complete theory.

For one thing, the group has not yet been able to make three-dimensional calculations of the actual detonation. These calculations could could be compared to observations.

As Dr. Lebanon, said in an e-mail message, "only as a type of supernova explodes 1a is one of the most complicated things in a big world."

His group uses supercomputers to study small patches of turbulent flame front-only one or two in yard-high resolution.

Dr. David Arnett, a supernova expert at the University of Arizona, said that such simulations were a way to test ideas and see them was a product of theoretical 'intuition.

"Massive calculation does not provide the answers so much as it provides an extension of our imagination," he wrote in an e-mail message. "For some years there has been talk of how the calculation is the third leg 'of science: theory, experiment and computer simulations. I think that the work of Flash is a concrete example of this at work, and in reality work. "

Meanwhile, supernovas real threat to confound the theoretical. The carbon at the center of the star could "ash" before it burns, putting it on a path to liquidate as something other than nickel, in the end, based on recent observations of two supernovas from Dr. Ken Nomoto of the University of Tokyo and the his group using the Subaru Telescope on Mauna Kea in Hawaii. Evidence is sketchy, but that would mean that most of the models, including the Flash center increased bubble, it is a mistake, he said.

But we should not be discouraged. "We have come along way," said Dr. Wheeler. Referring to the problem of ignition, he said, "We do have a long way before we knew this was a problem."