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By Root 914 0

and there was a clue. If there can never be a white dwarf star with a mass larger than 1.4 solar masses, what happens to stars that are bigger than that? Putting aside the possibility that big stars can shed material so that they sneak in under Chandrasekhar’s limit, two alternative fates await. In both cases, the large initial mass means that the electrons eventually start to move around at close to the speed of light as the collapse continues. Once that happens, there really is nowhere else to go; their pressure will never be sufficient to resist the force of gravity. For these massive stars, the next stop is a neutron star, in which nuclear fusion steps in for a final time. The protons and electrons move so fast that they reach a point where they have sufficient energy to initiate proton-electron fusion, producing a neutron. The reaction is the reverse of the radioactive beta decay process, whereby a neutron spontaneously decays into a proton and an electron with the emission of a neutrino. In this way, all of the protons and electrons gradually convert into neutrons and the star is nothing but a ball of neutrons. The density of a neutron star is phenomenal: A single teaspoon of neutron star matter weighs more than a mountain. Neutron stars are stars that are more massive than our sun yet are compressed to the size of a city.11 Many of the known neutron stars spin at phenomenal rates and blast beams of radiation out into space like cosmic lighthouses. These stars are known as pulsars, and they are truly wonders of the universe. Some known pulsars are approaching twice the mass of our sun, measure only 20 kilometers in diameter, and spin more than five hundred times every second. Imagine the violence of the forces on such an object. We have discovered wonders beyond imagination.

Beyond neutron stars, a final fate awaits the biggest stars. Just as the electrons can approach the speed of light in white dwarfs, the neutrons in a neutron star can bump up against the limit Einstein imposed on them. When this happens, no known force will prevent complete collapse, and the star is destined to form a black hole. Today our knowledge of the physics of space and time inside black holes is incomplete. As we shall see in the final chapter, the presence of mass causes spacetime to warp away from the Minkowski spacetime that we have become so familiar with, and in the case of a black hole, that warping is so extreme that not even light can escape its clutches. In such extreme environments, the laws of physics as we currently know them break down, and figuring out the way forward is one of the great challenges for twenty-first-century science, for only then will we be able to complete the story of the stars.

7

The Origin of Mass

The discovery of E = mc2 marked a turning point in the way physicists viewed energy, for it taught us to appreciate that there is a vast latent energy store locked away inside mass itself. It is a store of energy much greater than anyone had previously dared imagine: The energy locked away in the mass of a single proton is approaching 1 billion times what is liberated in a typical chemical reaction. At first sight it seems we have the solution to the world’s energy problems, and to a degree that may well be the case in the long term. But there is a fly in the ointment, and a big one too: It is very hard to destroy mass completely. In the case of a nuclear fission power plant, only a very tiny fraction of the original fuel is actually destroyed; the rest is converted into lighter elements, some of which may be highly toxic waste products. Even within the sun, fusion processes are remarkably ineffective at converting mass into energy, and this is not only because the fraction of mass that is destroyed is very small: For any particular proton, the chances of fusion ever taking place are exceedingly remote because the initial step of converting a proton into a neutron is an incredibly rare occurrence—so rare, in fact, that it takes around 5 billion years on average before a proton in the core of the