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The brochure of the John von Neumann Institute for Computing
is available in English and in German. It can be ordered at the NIC secretariat
(nic@fz-juelich.de).
deutsche Broschüre (pdf) | English brochure (pdf)
 Astrophysics
Human beings are made up almost exclusively of elements that were distributed by stellar
explosions, as is our planet Earth. The evolution of very massive stars and their subsequent
explosions have produced all the heavy elements from carbon on upwards, and a substantial
fraction of helium. This production was very rapid in the early phases of the Universe. And
yet the only star that we understand reasonably well is the Sun. The Sun will never explode,
although it surely will eject large amounts of gas near the end of its normal life, which will
then make the Earth uninhabitable. We still do not understand the explosion mechanism of
very massive stars.
Stars, and our understanding of their formation, their life and their death,
are being tested best
with the Sun; and from the Sun we can also learn and check the microphysics that may rule
stellar explosions. The Sun inspired the early evolution of the study of
the physics of ionized
gases, and the Sun is still the test bed for all attempts to understand ionized gases and
especially their behavior in magnetic fields. This may, on the one hand, lead to an
understanding of where magnetic fields actually come from, and, on the other, what drives
stellar explosions, and finally, what drives the relativistic jets that
emanate from the vicinity
of black holes. Most current attempts to understand these relativistic jets are based on our
models for the magnetic Solar wind. And finally, emulating the Sun in the laboratory, using
magnetic fields instead of gravitation as confinement, may lead to an inexhaustible energy
supply for humanity, only limited by thermodynamics.
The work of Kliem and his collaborators is one example of how we can learn more about
magnetic fields in general from the Sun and its magnetic field phenomena.
Stellar explosions have shaped the evolution of planets like the Earth; Gamma Ray Bursts are
a special and rare kind of stellar explosion, visible for just seconds all
across the Universe,
and we have little idea of how to connect such a flash in gamma rays with stellar explosions
of massive stars. We do not even approximately agree on why massive stars explode, which
must rank as one of the most important unanswered questions in Astronomy, Cosmology and
Physics.
The work of Janka and his group is an attempt to bring one concept to its logical conclusion,
namely that the neutrino production coupled with a full 3D treatment may lead us to an
understanding of this physics. This is very plausible given that we have already seen these
neutrinos.
Galaxies, like our Milky Way, are made up of large numbers of stars, and almost all galaxies
have a massive black hole at their center. The formation and evolution of these black holes
poses a major riddle. Black holes are the most efficient thermodynamic machines we know of
in the Universe, and yet we do not really understand gravity. Black holes probably drive
relativistic jets through twisted magnetic fields. The interaction of dense stellar
systems with
black holes, the evolution of binary black holes, and the rotation of black holes are key
questions today in General Relativity and Astrophysics.
The work of Spurzem and his collaborators is one example of how to deal with stellar systems
and black holes, trying to use the observations through cosmic time (by looking deeply into
the Universe) to learn more about black holes.
The early evolution of the Universe, its first stars, galaxies and black holes, closely linked to
its structure formation - the Universe looks like an arrangement of many spider webs or soap
bubbles - is now accessible through the study of the spatial microwave background
fluctuations which represents the radiation left over from the Big Bang. The existing
observations already allow us to determine the cosmic parameters with very high precision,
and yet we understand less and less of the underlying physics. Dark energy, dark matter and
invisible baryonic matter make up almost 100 percent of what seems to be present in the
cosmos.
The work of Gottlöber and his collaborators on clusters of galaxies and their evolution is an
attempt to shed light on these questions.
In all such endeavors supercomputer simulations are the key to success; the world is just so
complex that even when the principle is simple, the sheer numbers of particles,
stars, or phase
space elements make the use of supercomputers indispensable.
Ultimately, we wish to comprehend the world around us, and use this understanding for the
well-being of humanity.
(Peter L. Biermann, MPI for Radioastronomy, Bonn, and Department of Physics and
Astronomy, University of Bonn )
Magnetic fields on the Sun are sheared and twisted in the process of energy storage and
eventually pass through an as yet unidentified instability at which point they open into
interplanetary space, eject the plasma trapped in them, and cause a flare.
Magnetohydrodynamic simulations of the kink instability of a twisted magnetic flux rope
yield very good agreement with observations of solar eruptions and suggest this process as a
mechanism for their initiation.
The figure shows field lines of an unstable twisted flux rope in perspective
and vertical views.
Twice the initial height is reached at this moment, and a helical shape develops, as is often
observed in erupting prominences. Two groups of field lines, J-shaped in the bottom
projection, pass through a current sheet that forms below the rising flux rope
and is the site of
the enhanced dissipation which causes the X-ray flare. The characteristic S- or inverse-S-
shaped X-ray source at the onset of solar eruptions, illustrated by an image from the Yohkoh
satellite, corresponds to the double J-shaped structure in the simulation.
Interestingly, the erupting flux rope is not visible in X-rays because the currents do not
steepen here as much as in the current sheet.
(Bernhard Kliem, Tibor Török, Astrophysical Institute Potsdam)
Supernova explosions are among the most powerful phenomena in the universe. They end the
lives of massive stars and play a crucial role in the cosmic cycle of birth and death of the stars
which breed the heavy chemical elements. Computer models are indispensable to obtain a
better understanding of the complex processes which cause the explosion and lead to the
observed properties of supernovae.
The sequence of snapshots shows the beginning of the explosion at about 0.05, 0.15, 0.45 and
1.0 seconds after the core of a 15-solar-mass star has collapsed to a neutron star with a radius
of only 20 km. The bubbles enclose the (invisible) neutron star at the center and contain hot,
buoyant gas that drives the fast expansion of the explosion wave. The displayed region has a
diameter of 400, 500, 3000, and 20 000 km, respectively. The gas bubbles are heated by
elementary particle reactions, which provide the energy of the explosion in this three-
dimensional simulation.
(Leonhard Scheck, Konstantinos Kifonidis, Hans-Thomas Janka, Max Planck Institute for
Astrophysics, Garching)
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In these simulations a large number of particles (namely the stars) interact only by their
gravitational force. Relaxation and heat conduction, provided by the cumulative effect of
small-angle two-body encounters between particles, compete with external influences such as
the gravity of a supermassive black hole of a few million solar masses,
as has been detected at
the center of our Milky Way. In this example, we followed a star cluster, similar to
a globular cluster, as it approaches the central parsec of our Milky Way, is deformed by tidal
forces of the central black hole (first picture) and is finally distorted
into a spiral configuration
of tidal arms with the supermassive black hole (second picture, black hole not highlighted but
its position can be inferred from the convergence point of the two spiral arms). These figures
are snapshots from a movie produced using the special interface of the VISIT software of NIC
Jülich for our N-body simulation codes. The pictures contain a color code depicting the local
stellar density, and small velocity arrows highlighting the velocities of individual stars. To
avoid overcrowding of the picture not all stars were plotted. In other simulations, we use
similar methods to study the evolution of binary supermassive black holes embedded in dense
stellar clusters (with possible generation of gravitational waves due to merging of the two
black holes), and also the stability and formation of extrasolar planetary systems.
(Rainer Spurzem, Gabor Kupi, Patrick Glaschke, Christoph Eichhorn, Astronomisches
Rechen-Institut, Heidelberg; Chingis Omarov, Fessenkov Observatory, Almaty Kazakhstan)
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Clusters of galaxies consist of hundreds or thousands of galaxies spread over a few million
light years. The space in between the galaxies is filled by a hot, X-ray-emitting gas. One
aspect of our study is the relation of observable properties, such as the luminosity and
temperature of the gas, to the distribution and evolution of the dark matter in the cluster.
Cosmological simulations with box sizes about 300 million light years across and several ten
million particles in the cluster area have been carried out. They reveal that the dark matter
forms clumpy structures over all mass scales. In contrast, the adiabatically evolved gas is
much more smoothly distributed in the cluster. One can clearly see the gas streams which
have been stripped from the galaxies moving at high velocity in the cluster potential. The high
degree of detail that can be appreciated in these pictures is due to the unprecedented force and
mass resolution of this simulation.
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(Stefan Gottlöber, Astrophysical Institute Potsdam; Gustavo Yepes, Universidad Autonoma
de Madrid; Matthias Hoeft, International University Bremen)
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