I. Supernovae Yuor book actually has a good, concise explanation of massive-star supernovae (astronomers call them Type II). PAGES 188-189 The explanation of white-dwarf supernovae in the book is a little out of date. But that's mostly because the field has advanced rapidly, and because we have a great supernovae group here at UA, so Jeremy and I have been immersed in the latest thinking on this subject. Massive-star supernovae: As your book describes, as 20 solar mass star evolves very rapidly. The main sequence lifetime is something like 10 million years. He --> C takes 1 million years C --> Ne and Mg takes 100,000 years O --> Si takes 20 years Si --> Fe takes 1 week!!!!!! So all of this evolution is "normal." We go from burning to collapse to new burning to collapse, along with shell sources and envelope expansion and mass loss. But Iron (Fe) is special. If you fuse two elements together to make an element heavier than Fe, YOU DO NOT PRODUCE ENERGY. It takes energy, this is not good for a star! So you have this immensely small, immensely hot gas (billions of degrees) of Iron. The electrons are degenerate, helping to hold this core up against gravity. BUT, everything is so dense that the electrons are driven into the Fe nuclei, making Manganese. We lose electron degeneracy, and things are so hot that the gamma ray photons are so energetic that they have enough energy to split apart the atoms. So we've lost both electrons and radiation pressure (since the gamma rays are busy doing something else). The core colllapses faster and faster, and gets hotter and hotter. This final collapse takes ONE SECOND. Protons and electrons combine to make neutrons. When the "neutron star" thing is made at the center, it can resist gravity. The sudden resistance sends out a shock wave, blasting the rest of the star to smithereens, and supposedly leaving behind a neutron star. Neutrons are catapulted outward at high velocity. They rapidly fuse with the elements in the envelope, making elements that normal stellar evolution cannot make. This new element creation is called the R-PROCESS (R stands for RAPID). In normal stars, the neutrons can combine with "normal fusion" products much more slowly (the S-PROCESS) to make various elements, too. So the elements we see today are, aside from hydrogen and helium, made in the cores of normal stars and in supernovae and in late stages of stellar evolution (s-process). The most massive elements are exclusively made in supernovae. All of the energy for this explosion came from gravitational collapse. So there are predictions: 1) The number of massive stars and the number of supernovae are related. 2) this sort of supernova can only exist where there are massive stars, so it can only happen in star-forming galaxies (most of the stars in the universe are in galaxies, called ellipticals, that are not actively making stars, so you wouldn't expect this sort of supernova in those galaxies. You only expect massive-star supernovae in the spiral arms of actively-star-forming galaxies) 3) you'll see a remnant RAPIDLY moving away from the center of the explosion. 4) a neutron star will be left at the center. It'll be spinning fast, so if we're aligned just right, we may watch the birth of a pulsar. We do see supernova remnants, and indeed they are rapidly expanding (MUCH faster than planetary nebulae expand). We do see a pulsar in the core the the CRAB NEBULA, the remnant of the supernova of 1054. We do see elements not normally made in stars, in fact, the late time brightness of a supernova is caused by decaying radioactive material made during the r-process fusion in the explosion itself. There are a couple of interesting details not mentioned in your book. If all massive stars, say, 15 to 100 solar masses, would explode, we'd make too many heavy elements. Apaprently, some explode as described above, but some explosions never make it to the surface of the star, and end up producing black holes as the outer material falls back on the "neutron star". II. White dwarf supernovae Now, what about white dwarf supernovae? Why do we need white-dwarf supernovae anyway? Well, we see supernovae in all sorts of galaxies, both galaxies that are actively forming stars, and those that aren't. Since massive stars evolve rapidly, if a galaxy isn't rapidly forming stars it doesn't have massive stars, so it can't have massive-star supernovae. So there must be another way to make stars explode. Furthermore, we didn't talk about the "light curves" of supernovae in class. Astronomers had already recognized 2, or more, types of supernovae just from the rate of decay of the light after the explosion, and from the spectra. A model that pretty much works (it has to be right in broad brush, but may still need fine tuning in detail) is the explosion of a C-O white dwarf. The model goes something like this... if you deposit material onto a C-O white dwarf at just the right rate you may avoid making a nova. But eventually this material will contribute to the heating of the core of the white dwarf (it weighs more now). It turns out, I think it's a coincidence, that the temperature at the center of a C-O white dwarf weighing a bit less than the Chandrasekhar mass is enough to ignite the C-O. Since we have a degenerate object, the burning is uncontrolled and an explosion results. We get a supernova and nothing is left behind. This supernova can instantaneously outshine all the rest of the stars in the galaxy it's in, all one TRILLION of them. We cannot see individual stars in galaxies out beyond, to make up a number, about 100 million light years, but we can see supernovae out to billions of light years. So supernovae make good ways to study the distant universe. We won't worry much about coalescing white dwarfs. The idea is basically, what if you don't have accretion. But you still need to get ignition of the C-O core. Perhaps you can make a star explode by slamming two stars together. The way you do that is to have two very close (they're orbiting each other but are very close... we have found a pair of such stars orbiting each other every 2 hours!) white dwarfs, that due to general relativity effects (gravitational radiation), actually spiral in and merge. This merger MIGHT be able to start the explosion. III. Evolution of elemental abundances We've shown that normal stellar evolution makes a set of elements. Slow neutron capture in evolved stars makes a bunch more, and fast neutron capture in supernovae makes the rest. Modellers can estimates how much of each element comes from which mass star, and they do a credible job of modelling what we see. So there might be some primordial star with only hydrogen and helium (it actually may be the case that the primordial stars did not make stars that would still be around today, ie, 1 solar mass stars... but that's another story). As soon as any massive stars gently or explosively spewed their envelopes into space, the interstellar medium was slightly enriched. So each succeeding generation of stars comes from the H and He at the early days of the universe (when the universe was small and dense it was a good fusion reactor, too); and from the pollution from stellar evolution spewed back into the interstellar medium. Each generation of stars, therefore, should be more metal rich than the previous generation. We can test this idea by age dating star clusters (or individual stars, which is much harder) and then measuring the abundance of the elements. The model works quite well. As the late, great Carl Sagan used to say, we are star stuff.