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Current Conventional Supernova Model

Current theoretical models of supernovae Type-II explosions postulate that the source of explosive energy in an approximately 20 solar mass star, at the end of its lifetime, is entirely gravitational. This energy is suddenly released as the iron atoms in the inner core of the star catastrophically collapse into an extremely small sphere during a time period of 10 to 100 milliseconds, turning potential energy into kinetic energy. Just before the collapse, the star has used its last available fusion source, silicon, to fend off the gravitational pressure. But this source material lasts for only for about a day, producing iron, which can no longer undergo fusion. The central iron core radius builds up rapidly until the electron shells surrounding each nucleus can no longer keep adjacent atoms apart. This point is called the Chandrasekhar limit, which is of the order of 1.5 solar masses. The approximate mass distribution just prior to the collapse is shown below [1].

Note that the time it takes to collapse the core is calculated from a mathematical model of the star. There is no actual data available to test this model, except for neutrinos that were measured on Earth over a time interval of 12 seconds after they had travelled from SN1987A.

A somewhat more accurate physical picture of the initial state is shown below [2], which illustrates that the various shells of matter have vastly different sizes. The radius of the iron core is about four orders of magnitude smaller than the outer radius of the star. But when the core collapses, it reduces its radius by another four orders of magnitude! The collapse begins at the core center and works its way out. Due to inertia, the core actually collapses away from the adjacent shells which cannot move that fast. The problem is how to convert the energy of an inward collapse into an outward-directed explosion that blows the outer layers of the star away from the center.

There has been a change in the postulated mechanism, discussed below.

In the previously accepted model, it was assumed that the iron atoms first photo-dissociated into helium nuclei, and then most of the protons were converted to neutrons by electron capture. The remaining neutrons coalesced to form a dense single giant nucleus called a neutron star. An outward-directed shock wave was created when the collapsing layers reached nuclear density, and the shock wave then propagated outwards at sonic speed. Neutrinos were assumed to take away most of the energy, but they were initially trapped. Computations showed that the shock wave soon stalled, but was ultimately reinforced by neutrino heating, and this drove the outer layers of the star away.

 Detection of neutrinos on Earth is taken as proof that the postulated mechanism is correct, although the computations are still not very satisfactory in explaining all the experimental observations for different stars and the total energy release. The entire process is diagrammed in the figure below [2].

It is readily apparent that the central core must survive in this model, because the shock wave begins at its surface. The gravitational collapse energy is assumed to be the source of the explosion, and some of this is assumed to be the source of shock wave reheating. Neutron stars, also called pulsars, are usually observed at the site of a supernova, but not always. No pulsar has yet been detected in the case of SN1987A, but this may be because we are not correctly positioned to see its radiation.

It is now recognized that the neutrinos, rather than reheating the shock wave, actually drain it of energy and cause it to stall [3]. "Researchers found that less than a thousandth of a second after the shock wave is generated, a flood of tiny, nearly massless particles called neutrinos escapes from the center of the star. The neutrinos, born in the collapsing core, drain energy from the shock wave. The shock stalls, and - at least in the computer - the supernova is a dud."

Astronomers once thought this shock would be enough to tear the star apart and generate the explosion, says Adam Burrows of the University of Arizona. Turns out it's not so simple. Now Burrows and his colleagues are working with a computer model powerful enough to simulate how the core shakes and churns during the collapse, and they've finally seen how a collapsing star could turn around and explode. The turbulent infalling gas starts shaking the core, causing it to pulsate. Raining down from the star's outer layers, the gas wraps around the core, dancing over its surface and penetrating its depths.

"The core is oscillating, and the stuff falling onto the core is exciting it," says Burrows. In about eight-tenths of a second, the oscillations are so intense they send out sound waves. The waves exert a pressure that expels material, reinforcing the shock wave created by the star's collapse. They also amplify the core's vibrations in a runaway reaction, says Burrows, "until the star finally explodes."

Burrows acknowledges that sound waves may not be the full story. But his model tends to produce a lopsided explosion, and stars do indeed explode asymmetrically, with more punch in some directions than others. That was true for supernova 1987A, recorded 20 years ago, the closest and brightest supernova since 1604. Astronomers also have found that some of the neutron stars left behind by supernovas zip along at 500 miles a second, as if the explosion had imparted an enormous kick in one direction.

They hope that the detection of gravitational waves will prove this idea to be correct, even though gravitational waves have never been detected, and may not even exist.

Approximately 20 neutrinos were detected on Earth during a twelve-second time-interval, which is taken as experimental proof that neutrinos were released from SN1987A by electron capture and delayed in the dense neutron star core until they escaped and reheated the shock wave.

The question that now has to be asked is, did the core collapse of SN-1987 actually take place in 12 seconds rather than in milliseconds? Was the shorter time required in order to form a shock wave to make the rest of the postulated explosion mechanism work? Does this neutrino data actually conflict with the accepted explanation?

 

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References

1) Hans A. Bethe and Gerald Brown, "How a Supernova Explodes", Scientific American, 252, 5, pp60-68, May1985

2) Paul Murdin, End In Fire, Cambridge University Press, 1990

3) W. Zhang and S. Woosley, "Bang, the Cataclysmic Death of Stars", National Geographic, pp 78-95, March 2007.