MPA: Death of a star in three dimensions

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MPA: Death of a star in three dimensions

Post by bystander » Tue May 11, 2010 9:32 pm

Death of a star in three dimensions
Max Plank Society - 11 May 2010
New computer models show in detail how supernovae obtain their shape

Researchers at the Max Planck Institute for Astrophysics in Garching have for the first time managed to reproduce the asymmetries and fast-moving iron clumps of observed supernovae by complex computer simulations in all three dimensions. To this end they successfully followed the outburst in their models consistently from milliseconds after the onset of the blast to the demise of the star several hours later. (Astrophysical Journal, May 10th, 2010)
Image
Fig. 1: Three-dimensional explosion simulation at about 0.5 seconds after core ignited. The
bluish, almost transparent surface is the shock front with an average radius of 1900 km. (MPA)
Massive stars end their lives in gigantic explosions, so called supernovae, and can become - for a short time - brighter than a whole galaxy, which is made up of billions of stars. Although supernovae have been studied theoretically by computer models for several decades, the physical processes happening during these blasts are so complex that until now astrophysicists could only simulate parts of the process and so far only in one or two dimensions. Researches at the Max Planck Institute for Astrophysics in Garching have now carried out the first fully three-dimensional computer simulations of a core collapse supernova over a timescale of hours after the initiation of the blast. They thus could answer the question of how initial asymmetries, which emerge deep in the dense core during the very early stages of the explosion, fold themselves into inhomogeneities observable during the supernova blast.

While the great energy of the outburst makes these stellar explosions visible far out into the Universe, they are relatively rare. In a galaxy of the size of our Milky Way, on average only one supernova will occur in 50 years. About twenty years ago, a supernova could be seen even with the naked eye: SN 1987A in the Tarantula Nebula in the Large Magellanic Cloud, our neighbouring galaxy. This relative closeness - "only" about 170,000 light years away - allowed many detailed observations in different wavelength bands over weeks and even months. SN 1987A turned out to be a core-collapse supernova, a so-called Type II event. It occurs when a massive star, which is at least nine times heavier than the sun, has burned almost all its fuel. The fusion engine in the centre of the star begins to stutter, triggering an internal collapse and thus a violent explosion of the entire star. In the case of SN 1987A the star had about 20 solar masses at its birth.
Image
Fig. 2: A star dies in 3D: These snap-shots show the outward mixing of certain elements in the supernova explosion from two different viewing directions, 350 seconds after core ignition in the upper two panels and after 9000 seconds in the lower two panels, when the shock has broken out of the stellar surface. The surfaces denote the outermost radial locations of carbon (green), oxygen (red), and nickel (blue) with a constant mass fraction. (MPA)

Three-dimensional Simulations of Mixing Instabilities in Supernova Explosions

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Nature: Model stars set to explode

Post by bystander » Wed Jun 02, 2010 4:37 pm

Model stars set to explode
Nature - 02 June 2010
Realistic computational models of supernovae might soon solve a long-standing mystery.

Giant, mushroom-shaped blobs of nickel speed out from the blistering core of the supernova at 4,000 kilometres per second, piercing the smooth, ballooning sphere of hydrogen. The effect looks something like a cosmic hernia, but astrophysicist Hans-Thomas Janka couldn't be more pleased with what his computer model is showing.

In a paper published in the 10 May issue of the Astrophysical Journal, Janka and his colleagues from the Max Planck Institute for Astrophysics in Garching, Germany, used their model to address a long-standing puzzle: how do heavier elements, synthesized in the core of the massive, dying star, get out of the explosion before the lighter stuff that sits in the star's outer shells?

But the simulated explosion, with its colourful protrusions, is significant for another reason. It marks the first published, nearly complete, three-dimensional (3D) model of a supernova. Simpler one-dimensional (1D) and two-dimensional (2D) models, which assume that an explosion unfolds symmetrically, fail to get ticking star bombs to blow. So a handful of computational astrophysics groups, including Janka's, are moving up to the third dimension. The computational demands of tracking a complex and rapidly evolving explosion in which densities, temperatures and velocities are all at physical extremes are themselves astronomical. And the researchers have yet to tackle the toughest challenge: modelling the first milliseconds of the explosion in the innermost core of a supernova.
Image
(H.-Th. Janka, Max Planck Institute for Astrophysics)
This three-dimensional model shows the evolution of a supernova in its first half-second.
The explosion has a 200 km radius in the first frame; in the last frame, it is 1,900 km.

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