After it became clear that supernovae are the key to understanding the "dark energy" that rips apart space, they themselves became one of the main targets of the study of the universe
Denis Overbey, Haaretz, Walla News!
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Every second or so, somewhere in the universe, a star explodes into pieces and shines for a few moments with a light many times greater than that of a billion suns.
No one understands how these events, among the most violent in nature, actually take place. However, until recently they did not arouse special interest, except among those involved in the mysterious field known as nuclear astrophysics. Recently, as modern science teaches, supernovae have been given the status of key events in the history of the universe.
Astronomers who used a particular type of supernova, known as 1a, to indicate cosmic distance, concluded that a mysterious "dark energy" is twisting and tearing apart space. Following the discovery, which caused a great uproar in the world of physics and cosmology, the fate of the universe, or at least the knowledge we have about it, is uncertain. The need to understand supernovae has become extremely essential.
In trying to understand these explosions, astronomers are working on many fronts. They scan the sky to discover more and more events, examine the remains of ancient supernovae in search of clues to their circumstances and harness networks of supercomputers to calculate, moment by moment, the happenings in the heart of hell.
The results led to progress, according to the principle "two steps forward, one step back", and encouraged the astronomers to believe that they were theoretically on the right path. But they also highlighted the complications involved in the research and raised questions when it was necessary to determine the details of what happened at the time of the explosions.
Last month, an international team of astronomers, led by Dr. Pilar Ruiz-Lapuente from the University of Barcelona, announced the discovery of a star moving away from the supernova explosion site reported in 1572 by the astronomer Tycho Braha. This supernova, which appeared as a "new star" in the constellation Cassiopeia, is one of the first to be studied by astronomers. She helped disprove Aristotle's assertion that "the sky above the moon does not change".
The star that was discovered was most likely the companion of the star that exploded. Its discovery supports the conventional view that such explosions occur in double-star systems when one of them, accreting material from the other, reaches a critical mass and explodes.
At the same time, astrophysicists operating a supercomputer network for the purpose of simulating supernovae claimed that they had succeeded, for the first time, in showing how such a star might explode.
In 300 hours of calculation done at the Astrophysical Center for Thermonuclear Flashes at the University of Chicago, also known as the "Flash" Center, the scientists observed thermonuclear bubbles that rose from the depths of the star like a deadly jellyfish, wrapped around its surface and collided with each other in an apocalyptic explosion that, according to Dr. Donald Lamb, was "Completely strange and innovative". The results obtained in Chicago could explain not only the explosion of the stars, but also why the explosions are almost completely similar to each other. This will allow astronomers to calibrate the dark energy measurements.
Many experts claim that computer simulations of this type are considered more of a "successful start" than a final answer. Dr. Craig Wheeler from the University of Texas defined the center's work as a "brave calculation", but added that many details are still missing. "I don't think this is the end of the story," he said.
The experts outline two possible scenarios leading to a type 1a supernova. Both focus on "white dwarfs": average-sized stars, like our Sun, that have used up their thermonuclear fuel (hydrogen and helium) - and are dying. According to the first scenario, two white dwarfs merge and explode. In that case they have no remnant left. The second scenario talks about a double system where there is a white dwarf and a "living" star. The white dwarf absorbs material from its companion until it reaches the limit known as the Chandrasekhar mass - 1.4 times the mass of the Sun. At this point the pressure and density of matter in the white dwarf is great enough to reignite it. The thermonuclear reactions, the scientists say, propagate upward, turning the carbon and oxygen into heavier elements, which tear the white dwarf apart as its companion flies away.
Until recently, the evidence for the second model of supernova formation was few (it should be noted that the evidence for the first model is also not many). Tycho Brahe's supernova revealed new evidence for the model that the white dwarf absorbs material from a star and turns into a bomb. The sun-like star discovered by the researchers - suspected of being the material donor to the exploded white dwarf - is moving at three times the speed of its neighbors, away from its previous orbit around the white dwarf. The star in question had all the characteristics of a "material donor", but the identification was not absolute. According to Dr. Alex Filippenko from the University of California at Berkeley, "it is possible that the star passed by chance in the region and that it is not related to the supernova."
A faint possibility, according to astronomers, is that further observations will reveal that the supernova ash contaminates the outer layers of the star. But the expectation seems to be too great, said Dr. Stan Wesley of the University of California, Santa Cruz. He said the explosion appeared to have scattered the outer layers of the star, including the ash, far into space. "This is a star that was near, or inside, the largest thermonuclear explosion in the universe, of 2.5 million-trillion-trillion megatons," Wesley said.
However, the details of the explosion, which occurred within about a second, are still shrouded in fog. The source of the light that the astronomers saw was the radiation released by radioactive nickel, which in the days and months after the explosion turned into cobalt and then into iron, and released gamma rays that hit the ashes of the shattered star and caused it to glow for a while.
Since all type 1a supernovae are created in the same way, astronomers tried to use them as "standard candles" - objects whose brightness is known and therefore can be used to measure distances to the ends of the universe. But the supernovae are not exactly alike. The differences in their light intensity can reach about 40%. This fact may prove the very existence of dark energy and the acceleration of the expansion of the universe, but these are not enough to provide unequivocal details about the strength of the strange force and its variation in the space of cosmic time.
To reduce the uncertainty in the measurements, astronomers need to know how or whether to make corrections in the search for changes in issues such as the age and chemical composition of the white dwarf. The problem is that a white dwarf can burn in two forms - as a flame or as an explosion, in which the burned parts are dispersed as a wave of the page.
In the last decade, many theories have been developed, according to which the white dwarf may burn like a flame for a while, expand slowly and explode when the density of the material in it drops to the level required to produce the appropriate amount of nickel. "The porridge must be at the right temperature," said Dr. Wheeler. Neither model predicts why or when the star will explode. The scientists must take this into account in their calculations. Finding the natural cause of the explosion is the prize in our profession, said Wheeler, who mentioned that car manufacturers have spent millions to solve the ignition problem.
According to a message sent by Dr. Wesley via e-mail, "Understanding the cause of a type 1a supernova explosion is one of the most complicated things in the world." At the same time, real supernovae threaten to confound the theorists. The carbon in the center of the star may burn without a flame before it decays into a material other than nickel. This is clear from an observation made by Dr. Wesley on two supernovae, with the help of the giant telescope in Cerna, Chile. The evidence is weak, but it may be possible to interpret the existing models as invalid, said Dr. Wheeler. But he does not despair. "We have come a long way", he said and added, referring to the ignition problem, "we had to go a long way before we realized that this was the issue".
Yadan Astrophysics 2 - stars and galaxies
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