Relativity Lite

54 | Relativity Lite BLACK HOLES Stars maintain their size by balancing the inward gravitational force with the outward heat pres- sure supplied by nuclear fusion. In the core of the star (which is under high pressure due to the outer mass of the star pressing inward), pairs of hydrogen nuclei called protons are squeezed together to form deuterons, a pair of which are then transformed into one helium nucleus, a sort of process that gives off energy. This fusion will continue for 10 billion years in a star the size of our Sun until the hydrogen in the high-pressure core is used up. For another 150 million years, * helium is fused into carbon, but there is never enough heat to fuse carbon into the heavier elements. Without these nuclear fires, the Sun will no longer have the heat pressures to maintain its present size (large enough to fit a 1.3 million Earths inside), and it will ultimately shrink to about the size of the Earth. Such a star, called a white dwarf , maintains the new size only by the quantum-mechanical pressure of electrons that do not like to occupy the same quantum state, in part their position in space. We call this quantum effect a degeneracy force . † For a star four times more massive than our Sun, there is sufficient pressure in the core to fuse carbon into oxygen, neon, and so on until iron is produced. The star then resembles an onion with hydrogen as the outer layer and iron at the core. Once iron is produced, it is no longer possible to fuse nuclei together to form heavier elements and give off energy . So the element production stops and the star begins to shrink. For these more massive stars, the quantum mechanical electron pressure (degeneracy force) is not enough to keep the star from collapsing past the white dwarf stage. The electrons will be jammed into the protons, converting them to neutrons (and neutrinos, such as those recorded in Supernova 1987a ‡ ), and the star collapses inward until it is about 40 km across. At this point, the neutrons stop the collapse because they, too, do not like to occupy the same region of space. This star is called a neutron star , and it is so dense that “a thimbleful of neutron-star matter brought back to Earth would weight 100 million tons.” § In the rapid gravitational collapse, there is a rebounding of the outer layers, a shock wave that has enough extra heat and pressure to transform some of the iron into heavier elements such as zinc. Thus, the heavy elements in your body were literally produced in a stellar explosion called a supernova . One or more supernovas some 6–10 billion years ago scattered this material into the interstellar clouds of dust that eventually formed our Sun and planets. The interstellar shock wave of such explosions also helps trigger the coalescence of the solar systems out of this dust. * Eric Chaisson and Steve McMillan, Astronomy Today , 2nd ed. (Prentice Hall, Upper Saddle River, NJ, 1996), p. 428. † For moderate-mass stars like our Sun, fusion of helium into carbon begins after the core collapses into this degenerate state . Because the pressure in this nearly degenerate gas increases only slightly with temperature (Donald D. Clayton, Principles of Stellar Evolution and Nucleosynthesis [McGraw-Hill, New York, 1968], p. 103) the core does not reexpand for hours. But the fusion rate goes up with temperature to the 40th power (R. Robert Robbins, William H. Jeffreys, and Stephen J. Shawls, Discovering Astronomy [Wiley, New York, 1995], p. 388), so the core is a runaway fusion bomb for a few hours, called a helium flash . Finally, the carbon left in the core from helium burning shrinks until electron degeneracy sets in again and, for a moderate-mass star, never gets hot enough to fuse into heavier elements. ‡ K. Hirata et al., Phys. Rev. Lett. 58 , 1490 (1987). § That is, a density of 10 14 g/cm 3 . William J. Kaufmann III, Discovering the Universe (W. H. Freeman, New York, 1989), p. 289.