So far, we have been discussing the dynamics of what are referred to as main sequence stars. Such stars burn protons on their neutrogenic shells and gradually accumulate neutrons in their cores. As mentioned, nuclear fusion is not a relevant phenomenon, and these stars do not burn at different temperatures depending on which elements they happen to be fusing. Our sun, for example, has always burned and will continue to burn at an almost perfectly constant temperature until it begins its death throes, solving the Faint Young Sun problem in astrophysics.4
4 According to the current nuclear fusion model of stellar evolution, the Sun has been gradually warming since its formation. This is a problem because one or more unlikely scenarios must be assumed in order to explain how a star, at ~70% of the sun’s current intensity, could have supported the life we know existed on earth as early as 3.5 billion years ago.
There is an obvious tipping point that pushes a star off the main sequence—namely, the moment in its evolution at which the diameter of its neutron core becomes equal to the diameter of its neutrogenic shell. This is when interesting things start to happen. In the supermassive stars we are considering here, the protons in the core are converted to neutrons fairly quickly because of the low ratio of mantle matter to core surface area. When the core runs out of protons, the shell simply vanishes. After all, the shell is nothing more than the locale at which protons are converted into neutrons. No protons means no shell. When the shell vanishes, there are no longer any expanding partons to either push down against the neutron core or push up against the infalling mantle. This moment (Figure 3.4), though brief, triggers the most spectacular phenomenon in the universe. With the disappearance of the shell, the neutron core is, in essence, exposed. That is, it instantly begins behaving as a supermassive neutron star.
A main sequence star (a) is stable and has a gradually expanding neutron core. When all of the protons are converted to neutrons, the shell vanishes (b) and the mantle is pulled violently onto the core’s surface.
A neutron star of ten million solar masses possesses more energy than anything since the Big Bang itself. The instant the shell disappears, our newly minted neutron star pulls the mantle down onto its surface with cataclysmic ferocity, sending a tremendous shock wave down into the star. The mantle matter rebounds off the surface and is catapulted into space (Figure 3.5), while the shock wave bounces off the center and back to the surface. When it hits the surface of the neutron core, it rips off a significant fraction of the core’s outer skin and sends it flying out into space right into the receding mantle matter. The neutrons from the core’s skin and the protons from the mantle are mixed together in copious amounts, creating most of the complex atoms currently found in our galaxy. What happens next is even more spectacular.
After it hits the surface of the supermassive neutron core, the hydrogen mantle rebounds (c) into space in the form of a dense cloud. The shock wave from the collapse of the mantle then bounces off the center of the star and back to the surface, ripping off a large quantity of neutrons (d) and catapulting them into the proton cloud, resulting in the nucleosynthesis of most of our galaxy’s complex elements.
Only some certain fraction of the neutrons ripped off the core will find happy homes within stable atoms. The rest—possibly the majority—either remain completely exposed to space or find themselves in various unstable neutron-rich isotopes. In either case, there is only one possible fate for all of these extra neutrons: decay. Over the course of the next few minutes, all of the neutrons that did not end up in atoms will decay all the way down to the level of undifferentiated spacetime (Figure 3.6). Recall that neutrons, having less intrinsic mass than protons, cannot decay into protons, an issue I will address in much more detail later. Incredible as it sounds, a quantity of neutrons that can be measured in solar masses is completely converted from mass to energy in just a matter of minutes. In normal stars, this event is what we know as a supernova. In supermassive galactic stars, it is known as a gamma ray burst.
Any neutrons that did not find their way into stable atoms will decay in just a few minutes, resulting in a colossal explosion (GRB), the force of which compresses the center of the core all the way down to the level of undifferentiated spacetime, creating a black hole. Outside of the GRB, the newly created atoms are blasted out into the young galaxy.
The cloud of decaying neutrons is roughly spherical and so there are two interesting regions of the explosion, both inside and outside of the sphere. Inside the sphere, the force of the blast is focused directly onto the very center of the neutron star. If the blast is strong enough (as it always is in a galactic star) it will collapse some fraction of the star all the way down to the level of spacetime, crushing the partons and creating a black hole in the center. If it is not strong enough (as in much smaller stars), it will simply leave the neutron star behind. Neutrons are not created by smashing electrons (which are not discrete particles of matter) into protons (which already possess more intrinsic mass than neutrons to begin with). Outside of the neutron sphere, the newly created atoms are blasted with tremendous force out into the galaxy. It is this rapid nucleosynthetic process (similar to the current r-process), and not nuclear fusion, that accounts for the abundances of various elements in the universe.
I said the cloud of neutrons around the star was roughly spherical. In fact, the mass tends to concentrate more in the galactic plane than around the poles. This happens because as the mantle collapses onto the surface of the neutron core, it spins down toward the equator in order to conserve angular momentum. As a result, the subsequent GRB has a slight bias in the plane of the galaxy and is not entirely spherical. But it is not, as the current model argues, concentrated into two narrow jets emanating from the poles. That theory came from the inability of the standard model to accommodate anything large enough to generate the power of a spherical GRB. If it were concentrated in two polar jets, most of the complex atoms created in the process would be blasted away at right angles to the plane of the disk, significantly impoverishing the host galaxy and making our metal-rich solar system, for example, much harder to explain.
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