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The Cosmos, 4th ed. by Pasachoff & Filippenko Chapter 13 Concept Review » Stars have different fates depending on the mass with which they were born. Lightweight stars are those with mass up to about 10 (but perhaps only 8) times the Sun’s mass, while heavyweight stars have larger masses (introductory section). When the Sun and other low‐mass stars exhaust their central hydrogen, they will swell and become luminous red giants as their cores contract and the outer layers expand (Sec. 13.1a). The outer layers subsequently drift off as planetary nebulae, which glow because their gases are ionized by radiation from the hot dying star (Sec. 13.1b). The remaining core continues to contract until electrons won’t be compressed further (they become “degenerate”), and the core becomes a white dwarf (Sec. 13.1c). The entire postmain‐ sequence evolution of the Sun can be conveniently traced on a temperature‐luminosity (Hertzsprung‐Russell) diagram (Sec. 13.1d). The evolution of other solitary (single) stars is similar, but differs in detail. » If a star is in a binary system, its evolution can be sped up by the transfer of matter from a companion star filling its Roche lobe, the region in which the companion’s gravity dominates (Sec. 13.1e). The flowing gas forms an accretion disk around the recipient star because of the rotation of the system. Matter falling onto a white dwarf from a companion star can release gravitational energy suddenly, or even undergo rapid nuclear fusion on the white dwarf’s surface, flaring up to form a nova (plural novae). » Heavyweight stars, having more than 8 to 10 times the Sun’s mass, become red supergiants (Sec. 13.2a). Heavy elements build up in layers inside these stars. When the innermost core builds up enough iron, it collapses and the surrounding layers rebound (explode); the result is a Type II supernova (plural supernovae), a type of core‐collapse supernova that has hydrogen in its spectrum. The remaining compact object is a very dense neutron star, consisting almost entirely of neutrons. If the evolved massive star had previously lost its outer envelope of gases, it would explode in a similar manner, but hydrogen lines would be absent in its spectrum; these are known as Type Ib (helium present) and Type Ic (no helium) supernovae. » Hydrogen‐deficient Type Ia supernovae, in contrast, occur when a carbon‐oxygen white dwarf in a binary star system receives too much mass from its companion to remain in that state and undergoes a nuclear runaway that completely destroys the white dwarf (Sec. 13.2b). This happens a little bit below the Chandrasekhar limit, the theoretical maximum mass of a white dwarf (about 1.4 solar masses). In some of these white‐dwarf supernovae, the companion appears to be a relatively normal star, but in others it may be a second white dwarf that merges with the first one. In both scenarios for white‐dwarf supernovae, the explosion does not produce a compact remnant (neutron star or black hole), unlike the case in core‐collapse supernovae. » The last bright supernovae to have been seen in our Galaxy were in 1572 and 1604 (Sec. 13.2c). Many supernovae are discovered each year in other galaxies, however. © Cambridge University Press, 2013 Page 1 The Cosmos, 4th ed. by Pasachoff & Filippenko » Supernovae in our Galaxy have left detectable supernova remnants, the expanding debris of the explosions. The gases ejected by supernovae are enriched in heavy elements produced prior to, and during, the explosions (Sec. 13.2d). We owe our existence to supernovae, being made of their debris. » The Type II Supernova 1987A, in a satellite galaxy known as the Large Magellanic Cloud, was the brightest supernova since 1604 and provided many valuable insights (Sec. 13.2e). The star that exploded was a massive, evolved star, as predicted, although its exact nature (blue, instead of red, supergiant) was a surprise. Neutrinos from the supernova, as well as the detection of radioactive heavy elements, show that we had the basic ideas of the theory of Type II supernovae correct. Cosmic rays are particles accelerated to high energy, in many cases from supernova explosions (Sec. 13.2f). » The cores of massive stars, after the supernova explosions, consist of degenerate neutrons that cannot be compressed further; they are called neutron stars (Sec. 13.3a). Some give off beams of radio radiation (and sometimes other electromagnetic waves as well) as they rotate like a lighthouse, and we detect pulses in their brightness. We now know of thousands of these pulsars (Sec. 13.3b). They are explained by the lighthouse model, in which two oppositely directed beams along the magnetic axis are seen only when they rotate into our line of sight (Sec. 13.3c). The discovery of a very rapid pulsar in the Crab Nebula, a young supernova remnant, provided strong support for the hypothesis that pulsars are rotating, magnetized neutron stars (Sec. 13.3d). » Most pulsars slow down as they radiate energy (Sec. 13.3e). Some pulsars, however, spin very rapidly, up to a few hundred times per second. They were probably spun up by accretion of gas from a companion star. » Binary‐star systems consisting of a pair of neutron stars, at least one of which is a pulsar, are proving very useful for testing the general theory of relativity (Sec. 13.3f ). Their orbital periods are getting shorter at a rate that agrees with the idea that energy in the form of gravitational waves is given off. Observational facilities have been built to directly detect gravitational waves, so far without success. » Strange planets have been discovered around one pulsar, but they must have formed after the supernova explosion (Sec. 13.3g). Neutron stars in binary systems can be studied in a way other than their existence as radio pulsars: They give off x‐rays emitted by hot gas in an accretion disk fed by the companion star (Sec. 13.3h). © Cambridge University Press, 2013 Page 2