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Fire or ice: supernova or white dwarf in .NET Printing ANSI/AIM Code 128 in .NET Fire or ice: supernova or white dwarf




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Fire or ice: supernova or white dwarf using visual .net toinclude barcode code 128 for asp.net web,windows application MSI Plessey ing the coll Code-128 for .NET apse has produced neutrinos that have already removed some energy, so the bounce will be a little weaker. What is more, at the point of maximum density something new happens.

Neutrinos, which can pass through normal matter virtually without scattering, become trapped: the density is high enough to scatter them many times as they move through the star. The effect of this is that neutrinos quickly come into thermal equilibrium with the hot neutron matter, and a neutrino gas builds up. Much of the kinetic energy of infall is converted into neutrino energy.

When the bounce starts and the density goes down a little, these neutrinos can suddenly escape, carrying away a great deal of energy. This is a sort of shock absorber, which prevents the star from rebounding back to its original white dwarf size again. Most of the star is now trapped at the enormous density of the neutron star.

But the rest of the material of the star has also been falling in, and begins to hit the outer layers of the core, just as the neutrinos are beginning to expand away. The neutrino gas runs into the infalling envelope of the star, and what physicists call a shock wave develops. Familiar examples of shock waves are the sonic boom, the bow wave in the water in front of a fast-moving ship, and the tidal bore found on some rivers, as in Figure 5.

4 on page 44. What happens after the shock forms seems to depend sensitively on details of the nuclear physics, much of which is not yet fully understood. But computer simulations suggest that, at least in many cases, the neutrinos remain trapped long enough to help push the shock outwards into the infalling envelope, with enough energy to blow the envelope away.

The expanding envelope is heated by the shock, so that nuclear reactions take place in it at a very rapid rate. When the shock reaches the outer boundary of the star, the star suddenly brightens up, and we see a supernova. Meanwhile, at the center, the collapsed core either settles down into a neutron star, or if much further material from the envelope falls down onto it collapses again to a black hole.

We will discuss both possible outcomes in later chapters. It should not be a surprise that the envelope can be blown away by the neutrinos. The energy released by the collapse of the core is enormous.

When we study general relativity later in this book we will learn how gravity can convert mass into energy. Gravitational collapse to a neutron star converts a larger fraction of the mass of the core into energy than happens in a nuclear reactor or nuclear bomb, and much of this energy is carried away by the shock. The envelope has been sitting in a relatively weak Newtonian gravitational eld, and it is no match for the thundering impact of the shock.

Despite the fact that the envelope may contain ten or twenty times the mass of the core, it blows away at a high speed. What we have described is called by astronomers a supernova of Type II. Supernovae of Type II are among the most spectacular events visible in optical telescopes.

The most recent one visible to the naked eye from the Earth was the supernova of 1987, called sn1987a (see Figure 12.7). Located in the Large Magellanic Cloud, which is a small galaxy in orbit about our own Milky Way galaxy (see 14), it seems to have occurred in a blue giant star of about 20 solar masses.

It. A shock deve lops when an object (here the expanding core) moves into a uid (the envelope) with a speed faster than the local sound speed. The uid cannot move out of the way fast enough and a large density difference develops just in front of the object..

Figure 12.7. VS .

NET USS Code 128 The supernova of 1987 in the Large Magellanic Cloud was accompanied by the formation of these extraordinary rings when light from the supernova hit shells of gas that the original giant star had expelled during a phase of mass-loss. The supernova light caused these shells to glow in uorescence. Image courtesy of nasa/stsci.

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