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This chapter discusses some of the other ways mode frequencies can be used, and what additional information they can provide about a star. It first considers how specific combinations of mode frequencies can be used to determine stellar properties. In addition, the properties of a star, such as the acoustic depth of the convection-zone base, are determined by fitting models of the signatures left by acoustic glitches to the frequencies or the second differences. This chapter discusses how one can use signatures of acoustic glitches to infer properties of the star in question. Period spacings of red giants are also potentially very useful, and this chapter discusses how to make use of them.

Central to our feelings of awareness is the sensation of the progression of time. We seem to be moving ever forward, from a definite past into an uncertain future. The past is over, we feel, and there is nothing to be done with it. It is unchangeable, and in a certain sense, it is ‘out there’ still. Our present knowledge of it may come from our records, our memory traces, and from our deductions from them, but we do not tend to doubt the actuality of the past. The past was one thing and can (now) be only one thing. What has happened has happened, and there is now nothing whatever that we, nor anyone else can do about it! The future, on the other hand, seems yet undetermined. It could turn out to be one thing or it could turn out to be another. Perhaps this ‘choice’ is fixed completely by physical laws, or perhaps partly by our own decisions (or by God); but this ‘choice’ seems still there to be made. There appear to be merely potentialities for whatever the ‘reality’ of the future may actually resolve itself to be. As we consciously perceive time to pass, the most immediate part of that vast and seemingly undetermined future continuously becomes realized as actuality, and thus makes its entry into the fixed past. Sometimes we may have the feeling that we even have been personally ‘responsible’ for somewhat influencing that choice of particular potential future which in fact becomes realized, and made permanent in the actuality of the past. More often, we feel ourselves to be helpless spectators - perhaps thankfully relieved of responsibility - as, inexorably, the scope of the determined past edges its way into an uncertain future. Yet physics, as we know it, tells a different story. All the successful equations of physics are symmetrical in time. They can be used equally well in one direction in time as in the other. The future and the past seem physically to be on a completely equal footing. Newton’s laws, Hamilton’s equations, Maxwell’s equations, Einstein’s general relativity, Dirac’s equation, the Schrödinger equation - all remain effectively unaltered if we reverse the direction of time.

Changes in the solar neighbourhood due to the motion of the sun in the Galaxy, solar evolution, and Galactic stellar evolution influence the terrestrial environment and expose life on the Earth to cosmic hazards. Such cosmic hazards include impact of near-Earth objects (NEOs), global climatic changes due to variations in solar activity and exposure of the Earth to very large fluxes of radiations and cosmic rays from Galactic supernova (SN) explosions and gamma-ray bursts (GRBs). Such cosmic hazards are of low probability, but their influence on the terrestrial environment and their catastrophic consequences, as evident from geological records, justify their detailed study, and the development of rational strategies, which may minimize their threat to life and to the survival of the human race on this planet. In this chapter I shall concentrate on threats to life from increased levels of radiation and cosmic ray (CR) flux that reach the atmosphere as a result of (1) changes in solar luminosity, (2) changes in the solar environment owing to the motion of the sun around the Galactic centre and in particular, owing to its passage through the spiral arms of the Galaxy, (3) the oscillatory displacement of the solar system perpendicular to the Galactic plane, (4) solar activity, (5) Galactic SN explosions, (6) GRBs, and (7) cosmic ray bursts (CRBs). The credibility of various cosmic threats will be tested by examining whether such events could have caused some of the major mass extinctions that took place on planet Earth and were documented relatively well in the geological records of the past 500 million years (Myr). A credible claim of a global threat to life from a change in global irradiation must first demonstrate that the anticipated change is larger than the periodical changes in irradiation caused by the motions of the Earth, to which terrestrial life has adjusted itself. Most of the energy of the sun is radiated in the visible range. The atmosphere is highly transparent to this visible light but is very opaque to almost all other bands of the electromagnetic spectrum except radio waves, whose production by the sun is rather small.

The Sun and other stars generate energy by nuclear fusion processes that convert hydrogen into helium. Eddington was first to suggest the conversion of hydrogen into helium might fuel the stars. At the time stars were thought to have a composition similar to that of the Earth, which cast doubt on Eddington’s idea. Cecilia Payne corrected this misconception and showed the Sun and stars are actually formed of hydrogen and helium with only traces of other elements. Bethe and Crichfield devised mechanisms for how hydrogen nuclear fusion takes place in stars. In more massive stars, such as Betelgeuse, helium is converted into carbon and oxygen. We now know that all the elements beyond helium are created in the stars.

The pebble is on the beach, once more, unmarked by its brief contact with human sentience. Almost unmarked. The fingerprints that it lightly bears will, however, be washed away by the next tide. It has a long future, still, but probably not as a pebble—though quite how long it remains as a pebble may well depend on human action. Not on immediate, direct human action—whether it is scooped up by a digger and converted into concrete for a sea-front esplanade, for instance, or even collected as a souvenir by some passing tourist. Either of these fates should cause only a brief deflection from its long-term future (the esplanade is, after all, only a cliff to be attacked by the elements, while beach souvenirs are soon discarded). A larger perturbation of its trajectory more probably hinges on wider human effects—but more of that anon. We might assume, first, that nature runs its course. A pebble on a beach, its natural environment, is changing all the time. Not long ago, it was part of a slab of slate in a cliff, then it briefly became an angular chunk of rock, before the waves and water smoothed it down. They are still smoothing it, wearing away at it, making it smaller. Even the contact with human hands probably removed a grain or two. A pebble has the appearance of permanence, but it is not permanent. How long does it take to wear down a pebble? This can happen astonishingly quickly. Even over a single tide, being washed backwards and forwards by every incoming wave, a pebble can become detectably lighter—by less than one tenth of one per cent, admittedly, but that weight difference can easily be measured using modern electronic scales. Over a season, on an exposed part of the coast, a pebble can lose between a third and a half of its mass. The rates will vary—on a stormy day the banging of pebbles against each other can produce distinct percussion marks on their surfaces, while on a calm day the attrition rate will drop markedly. Night and day, though, the pebble is disintegrating.