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Fate has something very different, and very dramatic, in store for stars which are some 5 or more times as massive as our Sun. After the outer layers of the star have swollen into a red supergiant (i.e., a very big red giant), the core begins to yield to gravity and starts to shrink. As it shrinks, it grows hotter and denser, and a new series of nuclear reactions begin to occur, temporarily halting the collapse of the core. Then something bad happens. Silicon fuses to Iron-56. Up until now, all these fusion reactions had liberated energy. Fusing iron does not liberate energy. Even worse, because of the vast pressure and temperature, iron is actually forced to fuse. The supernova explosion is less than a fraction of a second away. Iron takes in energy when it fuses, it also takes in electrons. This energy and those electrons had been helping to support the star against it's own gravity. Now, with the support gone, the envelope of the star comes crashing down onto the core at a fine fraction of the speed of light. The rebound can be seen clear across the universe. |
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Fate has something very different, and very dramatic, in store for stars which are some 5 or more times as massive as our Sun. After the outer layers of the star have swollen into a red supergiant (i.e., a very big red giant), the core begins to yield to gravity and starts to shrink. As it shrinks, it grows hotter and denser, and a new series of nuclear reactions begin to occur, temporarily halting the collapse of the core. Then something bad happens. Silicon fuses to Iron-56. Up until now, all these fusion reactions had liberated energy. Fusing iron does not liberate energy. Even worse, because of the vast pressure and temperature, iron is actually forced to fuse. The supernova explosion is less than a fraction of a second away. Iron takes in energy when it fuses, it also takes in electrons. This energy and those electrons had been helping to support the star against it's own gravity. Now, with the support gone, the envelope of the star comes crashing down onto the core at a fine fraction of the speed of light. Then through a process which we do not fully understand, there is a rebound can be seen clear across the universe. |
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So what, if anything, remains of the core of the original star? Unlike in smaller stars, where the core becomes essentially all carbon and stable, the intense pressure inside the supergiant causes the electrons to be forced inside of (or combined with) the protons, forming neutrons. In fact, the whole core of the star becomes nothing but a dense ball of neutrons. It is possible that this core will remain intact after the supernova, and be called a neutron star. However, if the original star was very massive (say 15 or more times the mass of our Sun), even the neutrons will not be able to survive the core collapse and a black hole will form! |
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So what, if anything, remains of the core of the original star? Because we do not have a good understanding of the actually explosion mechanism, it is not entirely clear. It is known that in some supernova, the intense gravity inside the supergiant causes the electrons to be forced inside of (or combined with) the protons, forming neutrons. In fact, the whole core of the star becomes nothing but a dense ball of neutrons. However, it is still an open question as to whether or not all supernova do form neutron stars. It is also believed that if the stellar mass is high enough that the star will collapse into a black hole. However, our understanding of stellar collapse is not good enough to know whether it is possible to collapse directly to a black hole without an supernova or if there are supernova which then form black holes, or what the exact relationship is between the initial mass of the star and the final object that remains. |
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