but in general the layers are fairly well separated. But this is just the core: the outer layers of the star up to the surface are still almost entirely nonfusing hydrogen. These layers absorb all this heat being created in the core, and, as in their less massive cousins, this gas swells out, becoming grossly extended. Stars in this mass range, though, get far larger than red giants. They can swell to diameters reaching many hundreds of millions of miles, and so we call these bloated beasts red supergiants.
In such a massive star, after millions of years, the fusion cycle is nearing its end. Iron is different from other elements. Unlike hydrogen, helium, and the others, iron resists fusion under almost any circumstances. No normal star in the Universe can produce the temperature and pressure needed to fuse it. At the very heart of the star, deep inside its core, a ball of inert iron just a few thousand miles across sits there ticking like a time bomb. And when enough of it builds up from silicon fusion, the bomb goes off.
RAGE, RAGE INTO THE NIGHT
And now, finally, we have come to the moment of truth. For a year the iron has been accumulating in the massive star’s core, and all that time has been writing the star’s death sentence.
Until this point in the star’s life the core has been generating energy; now this has stopped. Remember, the heat from nuclear fusion is one factor that supports the star against its own crushing gravity.
A second source of support against gravity is the tremendous sea of electrons in the core. In a normal atom, electrons stay connected to the nucleus. However, in the core of a star the conditions are so extreme that electrons are stripped off their atoms. Anytime an electron tries to attach itself to a nucleus, the intense heat and pressure rip it off again.
In the core, electrons are squeezed together very tightly, and weird quantum mechanical effects become important. One of them is called degeneracy, which is similar to electromagnetic repulsion: if you try to squeeze too many of the same kinds of particles together (regardless of charge), they resist it. This resistance is a major source of support for the core. Degeneracy, together with the raw heat from nuclear fusion, keeps the star’s core from collapsing under its own gravity.
The problem is, degeneracy pressure can only withstand so much gravity. As the iron piles up, the core gets more and more massive, and its gravity gets larger and larger. There is a moment when the iron core reaches its tipping point, when its mass is about 1.4 times the Sun’s. At that point, degeneracy loses. It simply cannot hold back all that mass. Previously, when the star was fusing other, lighter elements, this point was never reached; the next element up the chain would start fusing and the star’s core was saved.
But iron won’t fuse, and degeneracy is no longer enough. The core cannot withstand its own titanic gravity, and its support mechanism fails. Catastrophically. The core collapses . . . but this is no gradual deflation, like a balloon losing its air. When the core of a massive star collapses, it collapses. And all hell breaks loose.
The collapse is incredibly fast: in a thousandth of a second—literally, faster than the blink of an eye—the tremendous gravity of the core shrinks it down from thousands of miles across to a ball of ultracompressed matter just a few miles in diameter. The speed of the collapse is breathtaking: the matter falls at speeds upward of 45,000 miles per second. The core heats up almost beyond belief, to a billion degrees. High-energy gamma rays are produced, and these vicious photons are so energetic they can actually destroy atomic nuclei when they collide with them. This process, called photodissociation, rapidly starts destroying the iron nuclei in the core, blasting them into bits of helium nuclei and free neutrons. This actually makes things worse (if you can imagine), since these can absorb even more
In such a massive star, after millions of years, the fusion cycle is nearing its end. Iron is different from other elements. Unlike hydrogen, helium, and the others, iron resists fusion under almost any circumstances. No normal star in the Universe can produce the temperature and pressure needed to fuse it. At the very heart of the star, deep inside its core, a ball of inert iron just a few thousand miles across sits there ticking like a time bomb. And when enough of it builds up from silicon fusion, the bomb goes off.
RAGE, RAGE INTO THE NIGHT
And now, finally, we have come to the moment of truth. For a year the iron has been accumulating in the massive star’s core, and all that time has been writing the star’s death sentence.
Until this point in the star’s life the core has been generating energy; now this has stopped. Remember, the heat from nuclear fusion is one factor that supports the star against its own crushing gravity.
A second source of support against gravity is the tremendous sea of electrons in the core. In a normal atom, electrons stay connected to the nucleus. However, in the core of a star the conditions are so extreme that electrons are stripped off their atoms. Anytime an electron tries to attach itself to a nucleus, the intense heat and pressure rip it off again.
In the core, electrons are squeezed together very tightly, and weird quantum mechanical effects become important. One of them is called degeneracy, which is similar to electromagnetic repulsion: if you try to squeeze too many of the same kinds of particles together (regardless of charge), they resist it. This resistance is a major source of support for the core. Degeneracy, together with the raw heat from nuclear fusion, keeps the star’s core from collapsing under its own gravity.
The problem is, degeneracy pressure can only withstand so much gravity. As the iron piles up, the core gets more and more massive, and its gravity gets larger and larger. There is a moment when the iron core reaches its tipping point, when its mass is about 1.4 times the Sun’s. At that point, degeneracy loses. It simply cannot hold back all that mass. Previously, when the star was fusing other, lighter elements, this point was never reached; the next element up the chain would start fusing and the star’s core was saved.
But iron won’t fuse, and degeneracy is no longer enough. The core cannot withstand its own titanic gravity, and its support mechanism fails. Catastrophically. The core collapses . . . but this is no gradual deflation, like a balloon losing its air. When the core of a massive star collapses, it collapses. And all hell breaks loose.
The collapse is incredibly fast: in a thousandth of a second—literally, faster than the blink of an eye—the tremendous gravity of the core shrinks it down from thousands of miles across to a ball of ultracompressed matter just a few miles in diameter. The speed of the collapse is breathtaking: the matter falls at speeds upward of 45,000 miles per second. The core heats up almost beyond belief, to a billion degrees. High-energy gamma rays are produced, and these vicious photons are so energetic they can actually destroy atomic nuclei when they collide with them. This process, called photodissociation, rapidly starts destroying the iron nuclei in the core, blasting them into bits of helium nuclei and free neutrons. This actually makes things worse (if you can imagine), since these can absorb even more
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