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CHRISTMAS LECTURE The beginning and the end of the universe Joint Meeting with the William Herschel Society Prof. Sir Martin Rees, Astronomer Royal and Royal Society Research Professor at Cambridge University, on 14 December 2001 Professor Sir Martin Rees won the 2001 Cosmology Prize when the following Citation was written: "Sir Martin Rees, Astronomer Royal and Royal Society Research Professor at Cambridge University, is renowned for his extraordinary intuition in unravelling the complexities of the universe. He has been a leader in the quest to understand the physical processes near black holes and is responsible for major advances in our understanding of the cosmic background radiation, quasars, gamma-ray bursts, and galaxy formation. He has contributed to almost every area of cosmology and astrophysics and has been an inspiring leader, eloquent spokesperson, and patient guide for astronomers all over the world. Through his public speaking and writing he has made the Universe a more familiar place for everyone". Sir Martin was introduced by Victor Suchar, who organised the event. The lecture was illustrated with a large number of slides and followed by questions, which the speaker answered fully. At the end of his presentation a Vote of Thanks was proposed on behalf of both organisations by Professor Francis Ring, Chairman of the William Herschel Society. Instead of an abstract of the lecture he presented, Sir Martin Rees has supplied a copy of a paper on the same subject which was published recently in "The Age of the Earth: from 4004BC to AD2002", CLE Lewis & SJ Knell (eds.), a Special Publication (190, 275-283, 0303-8719/01/$15.00), © by The Geological Society of London 2001 with permission to re-print it.
Abstract: This paper attempts to set the Earth in a cosmic perspective. It discusses the Sun's life cycle ***** Whilst this planet has been cycling on according to the fixed law of gravity, from so simple a beginning, forms most wonderful ...have been and are being evolved. These are the famous closing words of Darwin's On the Origin of Species, but astronomers aim to go back before his `simple beginning', to set our entire Earth and Solar system in a broader context, stretching back to the birth of our galaxy, perhaps even to the initial instants of a `Big Bang' that set our entire universe expanding. Darwin guessed that it would have required hundreds of millions of years to have transformed primordial life (formed, he surmised, in a `warm little pond') into the amazing varieties of creatures that crawl, swim or fly on Earth. And this concept did not overly concern him, because such timespans had already been invoked by geologists to account for the laying down of rocks and moulding of the Earth's surface features. Other contributors to this volume (Dalrymple 2001; Lewis 2001; Shipley 2001) have described how Kelvin estimated the Earth's age. His inferences about the age of the Sun were actually rather more firmly based than those about the Earth: if the gravitational energy released by its continuing contraction was supplying the heat radiating away, it would deflate in ten million years. Kelvin's views carried great weight. But it was perhaps fortunate for his reputation that he included an escape clause: his conclusion regarding the age of the Sun only held good, he said, provided that there was no other power source `prepared in the storehouse of creation' (Thomson 1862, p.393). As was realised in the 1930s, fusion of hydrogen is sufficient to sustain the Sun for ten billion years. Our knowledge of atomic and nuclear physics is now sufficient to give us a (broadly uncontroversial) quantitative picture of the Sun's life cycle. The proto-Sun condensed from a cloud of diffuse interstellar gas. Gravity pulled it together until its centre was squeezed hot enough to trigger nuclear fusion of hydrogen into helium at a sufficient rate to balance the heat shining from its surface. (Any deuterium in the original cloud would have been burnt at an earlier stage in the contraction.) Less than half the Sun's central hydrogen has so far been used up: it is already 4.5 billion years old, but will keep shining for a further 5 billion years. It will then swell up to become a red giant, large and bright enough to engulf the inner planets, and to vaporise all life on Earth. During this `red giant' phase, lasting some five hundred million years, hydrogen will continue to burn in a shell around the helium core. Next, the Sun will undergo a more rapid convulsion, triggered by the onset of helium fusion in its core. This blows off some outer layers - about a quarter of the Sun's mass altogether. The residue will become a white dwarf - a dense `stellar cinder' no larger than the Earth, which will shine with a bluish glow, no brighter than today's full Moon, on whatever remains of the solar system. Our Sun has more time ahead than has so far elapsed; as explained later, our entire universe could have an infinite future ahead of it. So we may still be near Darwin's `simple beginning': if life is not prematurely snuffed out, our remote progeny will surely -in the aeons that lie ahead -spread far beyond this planet. Even if life is now unique to the Earth, there is time enough for it to spread through the entire galaxy, and even beyond. The structure and life cycle can be computed for a star of any mass. The output of such calculations allows us to infer the ages of star clusters, which contain a population of coeval stars of different masses. The key idea is that heavier stars use up their core hydrogen fuel, and `burn out', more quickly than lower-mass stars. The older a system is, the fainter (and lower-mass) will be the brightest stars that are still in the hydrogen-burning phase. Particularly interesting in this regard are the so-called `globular clusters' - each a swarm of up to a million stars, of different sizes, held together by their mutual gravity - which are believed to be the oldest stellar systems of all. The uncertainties in the age estimates stem partly from the theoretical models themselves, but also from the difficulty of inferring stellar masses from observed brightness and colours. Estimated ages are up to 13 billion years, with, however, an uncertainty of at least 10%. Such estimates are of course crucial to cosmology, because it would be embarrassing if the inferred age of the entire universe (in other words the time since the Big Bang) were not comfortably higher than the age of the oldest stars. As I shall comment later, there now seems no such paradox. Not everything happens slowly. Massive stars Stars more than ten times heavier than the Sun use up their central hydrogen hundreds of times quicker than the Sun does - they shine much brighter in consequence. Gravity then squeezes them further, and the centres get still hotter, until helium atoms can themselves stick together to make the nuclei of heavier atoms. A kind of `onion skin' structure develops: a layer of carbon surrounds one of oxygen, which in turn surrounds a layer of silicon. The hotter inner layers have been transmuted further up the periodic table and surround a core that is mainly iron. When their fuel has all been consumed, big stars face a crisis. A catastrophic infall compresses the stellar core to neutron densities, triggering a colossal explosion - a supernova. The outer layers of a star, by the time a supernova explosion blows them off, contain the outcome of all the nuclear alchemy that kept it shining over its entire lifetime. Elements beyond the Fe peak can be built up during the explosion itself. Work over the last 40 years - taking account of different types of stars, different nuclear reactions - has shown that the calculated `mix' of atoms is gratifyingly close to the proportions now observed in our solar system. This story is well authenticated by detailed modelling of the expected relative abundances of elements and isotopes, and also by spectroscopic evidence from the oldest stars (which contain less processed material). It is also found that the abundances are higher in gaseous environments like the galactic centre, where reprocessing would be fast, and lower in locations like the Magellanic Clouds where it is slower . Our galaxy is like an ecosystem, recycling gas through successive generations of stars, gradually
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