Finished a reread of Poul Anderson's Mirkheim, the novel that closes out his Polesotechnic League stories. Man, I remember this book as being a whole lot better than it was this time around. Although maybe that's because the whole idea behind the planet Mirkheim is so incredibly cool. Well, if you're me, anyway.
Contrary to sappy poetic license, the stars are not eternal. As the average star ages, it's core will eventually start to run out of the hydrogen it's been fusing into helium all these years. The fusion reaction will then move up into the shell of hydrogen that immediately surrounds the core, leaving behind a huge pile of helium. Up until now, the helium has just been sitting there minding its own business because the hydrogen kept it fairly spread out, but now the helium starts to contract, becoming hotter and more dense in the process. As the helium becomes more dense, its gravity increases, compacting the adjacent shell of fusing hydrogen down on top of it, thus speeding up the hydrogen fusion process. This increases the temperature of the core, which does a couple of things. The outer shell of the star expands to become a red giant. (Aldebaran, in Taurus, is an example of a red giant. Eventually the sun will become one, too, expanding enough to incorporate the orbits of Mercury, Venus, and maybe even the Earth. Don't forget your sunscreen.) Meanwhile, down in the core, all that helium starts to fuse, forming elements like oxygen and carbon, which can, in bigger stars, continue on to form even bigger atoms. This process is called stellar nucleosynthesis, and in larger stars it can eventually produce atoms as big as iron (atomic number = 26, i.e., 26 protons).
The thing is, stellar nucleosynthesis can't make any atoms bigger than iron. The iron nucleus is extremely stable, and it takes more energy to fuse an iron nucleus than the reaction returns, so it just doesn't happen. You can't make nickel (at.no. = 28), or copper (29), or silver (47), or gold (79), nor anything else in the upper three quarters of the periodic table that way. The only way to make the higher elements is to bombard the nuclei of the smaller atoms with high energy neutrons. It's like pushing a snowball down a hill, with the neutrons acting as the snowflakes it picks up as it rolls along. Along the way, some of the absorbed neutrons convert into a proton and an electron, increasing the atomic number incrementally. And as with a snowball, the bigger the hill, i.e., the more energy available, the more neutrons a nucleus can potentially pick up. That takes a lot of energy, way more than normal stellar fusion can provide. In fact, the only way to generate enough energy to even get the ball rolling is to, er, blow up the star.
Now I know what you're thinking. We can't be going around the universe blowing up stars willy-nilly just so we can harvest a few scattered atoms of gold here and there to make into a nice pair of earrings for Aunt Martha. Well we don't have to. Some stars will do it on their own without us having to lift a finger. Stellar lemmings, really. All you need is a star of sufficient mass, say, ten times that of the sun.
Take Rigel, for example. Rigel is Orion's left knee. In fact, according to the Wikipedia, the name Rigel comes from the Arabic description "Rijl Jauza al-Yusra," the "left foot of the Central One." It's one of the brightest stars in the sky. Rigel is classified as a blue supergiant, which as you might guess means it's really, ridiculously big, about 20 times the mass of the sun. It's surface temperature is about twice as hot as the sun, which is why it's blue instead of yellow, and it puts out about 60,000 times the amount of light the sun does, which is why we can see it so well from almost 800 light years away.
Rigel is big enough and hot enough that it will produce huge amounts of iron via stellar nucleosynthesis as it ages. The iron will collect in its core and just sit there, since the iron can't fuse any further. Iron is very dense, as stellar materials go, so as it collects, the gravity of the core will increase dramatically, causing it to compress. Since there is no fusion going on to produce energy that could counteract this compression, eventually the matter in the core will get squeezed until the only thing keeping the nuclei from touching are some squashed electrons (electron degeneracy pressure). When the mass of the core gets to be about 1.44 times the mass of the sun (Chandrasekhar's limit), even the electrons will get squeezed out, and the core will collapse in on itself. As it does, the iron nuclei will break apart into neutrons and helium nuclei. Electrons and protons will merge to produce even more neutrons, along with a bunch of neutrinos. The neutrinos will then escape from the core, carrying somewhere in the neighborhood of 1046 joules of gravitic potential energy with them. (This is equivalent to the caloric content of sixteen thousand billion billion billion billion twinkies, which makes for one helluva hill.) They will pass through Rigel's outer shell like Sherman through Georgia, transferring all that energy to the shell in about ten seconds, producing the Rigel-shattering kaboom that signals the birth of the galaxy's newest supernova. The kaboom will also signal the start of supernova nucleosynthesis as scads of high energy neutrons start slamming into the atomic nuclei of the shell. The process is known to produce heavy atoms up to and including elements like Plutonium (94) and Californium (98). And that's not even the beauty part...
The beauty part has to do with some esoteric nuclear chemistry, and something called the Island of Stability. Now, the identity of an element is determined by the number of protons in the nucleus. Protons are positively charged, so any nucleus bigger than hydrogen will also need some neutrons to keep those positive charges separated, or else there'll be trouble with a capital T, and that rhymes with B, and that stands for boom. In most elements, the number of neutrons the nucleus can accommodate can vary some. These variations are called isotopes, and not all of them are stable. There aren't many combinations of protons and neutrons that will produce a stable (i.e., non-radioactive) atomic nucleus. If you plot the number of neutrons in the nucleus versus the number of protons for all the elements, you get a long peninsula shaped outcropping of stable nuclei starting at hydrogen (1), and stretching out into a sea of instability until it ends at bismuth (83). No element larger than bismuth has a stable isotope.
Except, when I was in college, I remember reading an article in which it was speculated, based on then new quantum mechanical calculations, that certain isotopes of some as yet undiscovered very heavy atoms (atomic numbers 114, 120, and 126) could be stable, or at least not very unstable. This is the so-called Island of Stability, somewhere off the tip of the peninsula. There has been rampant speculation over the years as to the properties of these very dense metals. The fly in the ointment is that we don't know of any natural sources of these elements, and making them a few atoms at a time in a collider just isn't a terribly efficient way to produce the stuff for commerce.
One place they could be produced in quantity is in a supernova. The trouble with that is that the products of a supernova are just an expanding cloud heading out into space. That's not very useful, either. What you need is something nearby the star that could capture some of those heavy atoms. A planet would be nice, but the supernova is likely to vaporize anything in the system. Unless...
Suppose there was a really big gas giant, one even bigger than Jupiter, almost big enough to become a star itself, out on the edge of the system. The supernova blast would rip most of it away, too, but the core might survive to be bathed in all those heavy atoms. That planet would become a treasure trove of all sorts of useful stuff. Gold! Silver! Unbihexium!
That's Mirkheim, the macguffin of an otherwise pretty boring book. And if you've read this, you don't need to read the book.