by Paul Murdin
About the Author
Paul Murdin, OBE, is the author of The Secret Lives of Planets. He has worked as an astronomer in the UK, the USA, Australia and Spain, and discovered the first stellar black hole, Cygnus X1. He is a fellow of the Royal Astronomical Society, former president of the European Astronomical Society and Senior Professor Emeritus at the Institute of Astronomy at the University of Cambridge. In 2012 Asteroid 128562 was named ‘Murdin’ by the IAU in honour of his contributions to the field.
Other titles of interest published by
Thames & Hudson include:
What Shape is Space?
Will AI Replace Us?
Dinosaurs Rediscovered
Comet
See our websites
www.thamesandhudson.com
www.thamesandhudsonusa.com
CONTENTS
Introduction
BEFORE THE TELESCOPE
DISCOVERING THE SOLAR SYSTEM
THE DYNAMIC UNIVERSE
OUR GALAXY AND ITS STARS
THE UNIVERSE AND ITS GALAXIES
FUTURE DISCOVERIES
Glossary
Further Reading
Acknowledgments
Illustration Credits
Index
Introduction
The Australian Astronomical Observatory is located on a mountain near the small town of Coonabarabran in rural New South Wales, Siding Spring, in the Warrumbungle range, on the edge of a national park 400 kilometres northwest of Sydney. The seemingly interminable plains of the outback stretch to the western horizon. In the foreground, across a deep cut valley, are the volcanic hills, dykes and plugs of the range, with fanciful names: the Breadknife, Belougery Spire, Crater Bluff. Eucalyptus trees stand tall on the steep sides of the mountains, difficult for all to clamber through, except for kangaroos, koalas and wallabies. Brightly coloured lorikeets, cockatoos, galahs and rosellas fly singly and in flocks through the tree canopy, quarrelling with each other and their neighbours. The telescope buildings contrast with this natural scene; their hemispherical domes are painted white to reflect the heat of the Sun during the day in order to keep the telescopes cool at night. The whole area lies within a light-pollution protection zone, and the skies above the telescopes are in one sense dark, in another sense brilliant with the splendour of the Southern Hemisphere stars, especially in the southern winter when the centre of the Milky Way arches across the zenith.
Right after the telescope was completed, I was lucky enough to join the first group of six scientific staff of the Observatory, then called the Anglo-Australian Observatory. Its 4-metre telescope was at that time the largest in the Southern Hemisphere. It was built to a very high specification, and equipped with sensitive instrumentation, including what were then innovative computer-controlled electronic detectors. What it revealed, almost wherever it pointed, were new discoveries.
I well remember one such discovery. I had been working for twelve hours non-stop through the night on the identification of an X-ray source – that is, I set myself with the telescope to track down the optical star that was responsible for a beam of celestial X-rays that had been detected by a satellite with a telescope sensitive to X-rays.
I found it. I had also discovered, fairly clearly, that the star was the result of a relatively recent supernova explosion that had taken place perhaps 3,000 years ago. I was also able to estimate that the star was about 2,000 light years away.
I finished the night’s work as the day dawned, helped the telescope operator to shut down and left the observatory building in the golden light of the rising Sun to walk to the Lodge and a welcome sleep. The kangaroos and wallabies were finishing their night’s grazing and loped away unhurriedly from my path across the grass, towards their own beds in the bush. Kookaburras were greeting the dawn with mad laughter, which echoed in the valleys below. Large, black birds called currawongs were waking up in the gum trees with a dawn chorus of melodious warbling and chortling.
I was feeling tired, but it was a lovely morning and I had had a successful night. I was the only person in the world who knew what I knew about the star and I felt pretty good.
As I walked along the path an even more thrilling thought struck me. Light from the explosion of the supernova had taken 2,000 years to get to Earth, and it had travelled on for a further 3,000 light years. It was now ranged on the surface of a sphere 5,000 light years in radius. Outside this sphere the supernova had not yet happened.
Now, 5,000 light years sounds like a large distance – and indeed it is. However, it is not large compared with the size of the Galaxy, and although there are some stars within this sphere, there are not many. Some of them, like our Sun, may have planets, and some of the planets may be habitable, and some of them may have life on them, and some of this life might be intelligent, and some of the civilizations on these planets might be interested in astronomy and might have seen the supernova – or, as I had, the X-ray source – and followed the trail to the interesting star. But there was also a good chance that there was only one such civilization – the one here on Earth. If there was indeed only the one astronomy-curious civilization within the 5,000-light-year sphere – our own – I was not only the one person in the world who knew what I knew, I might be the one person in the Galaxy, or indeed in the Universe, who knew what I knew. I floated happily to bed and, pleased with myself, slept soundly, treasuring in my dreams the secret that I had unlocked from the Universe.
I was happy to learn during the research for this book that other scientists had felt the same sense of exhilaration when they had cracked a problem and made a cosmic discovery. Einstein’s worry about his own theory of General Relativity was assuaged by his discovery of the reason for a twist in the orbit of Mercury and, for a few days, he was beside himself with ‘joyous excitement’. Henry Norris Russell described the discovery of white dwarfs with Edward Pickering and Williamina Fleming: ‘I knew enough, even then, to know what it meant… At that moment, Pickering, Mrs Fleming and I were the only people in the world who knew of white dwarfs.’ Watching the transit of Venus in 1639, William Crabtree ‘stood for some time motionless, scarcely trusting his own senses, through excess of joy’.
Discoveries in astronomy challenge our fundamental assumptions about the Universe. They alter our perception of matter, time and distance; they transform how we view our history and future as a species. Where the astronomers of antiquity spoke of fixed stars, we speak of whirling galaxies and the death and birth of stars in supernovae. Where we once considered the Earth to be the centre of the Universe, we now see it as a small planet among millions of similar systems, a few of which might also hold life. These dramatic shifts in perspective hinge on thousands of individual moments of discovery, moments when it became clear to an observer that a component of the Universe – from a tiny subatomic particle to a supermassive black hole – was not as it once seemed. Each is a revelation that unlocks yet another of the infinite secrets of the Universe.
My own discoveries were more modest, at the scale of the quanta of astronomy – over 50,000 pages per year of similar-scale discoveries in astronomy are published every year. This book is about major discoveries that unlocked major secrets of the Universe – not run-of-the-mill discoveries in astronomy, but the big ones. I have selected them not only because they are important, but also because the people involved were interesting, because the knowledge that they encompass illustrates the range of astronomy, or because the stories behind them illustrate how science, and in particular astronomy, works.
Science is a cyclic process that oscillates between the real world (‘observation’ or ‘experiment’) and the picture of it in the scientist’s head (‘theory’). A scientific discovery might be the revela
tion of one of the secrets of the Universe by finding something in the real world. A secret might also be revealed by bringing together such a compelling set of interpretations that the picture in one person’s head is accepted by most people as a portrait of the real world. Galileo saw mountains on the Moon. Copernicus pictured the Sun to be at the centre of the Solar System. Both were discoveries; one of them an observation, one of them a theory.
For the man in the street, the word ‘theoretical’ sometimes carries a sense of derision – you can’t trust something that is ‘just a theory’. For scientists, the word can mean something that is as solid as the chair on which I am sitting, so theories can definitely be discoveries too. But perhaps to be a discovery a theory needs an extra something. Either it brings together a number of previously unrelated phenomena with such clarity that everybody is convinced it is right, or it points to some phenomenon that has not been seen yet but which turns out, when looked for, to be so.
The word ‘discovery’ carries the implication that what is now known was not known before, perhaps not even suspected. It is a bit of a surprise. The most straightforward reason in astronomy for such a surprise discovery is that, like the discoveries that I made with the AAT, it was made possible by some improvement in technical capability. Galileo learned how to make a telescope and used it to point to the sky. What he discovered – the satellites of Jupiter, the phases of Venus, star clusters – confirmed not only that the Sun, not the Earth, was the centre of the Solar System, but also that everything in space operated in much the same way that things did on Earth; there is no difference in principle between the ‘mundane’ (terrestrial things) and the ‘superlunary’ (things beyond the Moon and therefore cosmic). We on Earth are not apart from the Universe, but a part of the Universe.
William Herschel built bigger telescopes that opened the window wider and wider on the Universe; the Hubble Space Telescope blasted open the doors. The development of radio astronomy, X-ray astronomy and infrared and millimetre-wave astronomy in the twentieth century allowed us to see objects in the Universe that are invisible to the naked eye. Martin Ryle’s invention of the technique of Aperture Synthesis Interferometry in radio astronomy made it possible to investigate radio galaxies, which showed that the Universe had a discrete beginning. When they reached the right level of sensitivity, gravitational-wave detectors opened up a completely new window on the Universe. Since 1957, the possibility for spacecraft to carry equipment to the distant reaches of the Solar System has offered new perspectives on the planets.
Discoveries with new equipment are in one sense unexpected but in another sense planned, because the equipment has to be made and deployed. That means having the right idea, gathering together the resources, and carrying out a plan to use the equipment for a specific purpose. William Herschel built a new telescope and found the planet Uranus by systematically searching the sky with it; his sister, Caroline, found her comet by applying the same technique. In modern times the new equipment has to be bought, and that takes money – sometimes lots – so a detailed funding application has to be written, predicting what discoveries will be made with this expensive new telescope or satellite. If you just tell the truth – that the Universe is full of exciting things, and you can find something interesting with every instrumental advance – well, you won’t get funding. You have to at least scope the range of potential discoveries to show that you are serious about your research.
Certainly, in some cases scientists set out to find something specific. Urbain Le Verrier perceived the planet Neptune ‘at the end of his pen; he determined it by the mere force of calculation’, while Daniel Barringer became obsessed with the idea that the Coon Butte crater in Arizona was meteoritic, spurred on by the thought of finding a profitable mass of iron and nickel. Subrahmanyan Chandrasekhar calculated the structure of white dwarf stars as a student exercise that he set himself to pass the time on an ocean voyage and uncovered the reason for black holes. Raymond Davis spent over ten years searching for neutrinos from inside the Sun; his discovery led to the development of a new kind of physics, properly deserving of the award of the Nobel Prize.
Computer modelling has shed new light on known phenomena, making surprising new astronomical discoveries possible. The phrase ‘Garbage in, garbage out’ is well known; you might think that its corollary is ‘Put in the truth that you know, get out the truth that you know.’ But when there is a lot of data or the calculations are complex, computers can reveal unexpected or previously unnoticed features about the real Universe. Computer simulations of the interactions of asteroids and comets led to our present understanding of the Oort Cloud and the Kuiper Belt. Satellites have great difficulty in probing the magnetosphere of the Earth because it is so large that they can only investigate particular parts of it, like the people in the fable who grope the tail, foot, tusks, and trunk of an elephant, but fail to envisage the whole creature; computers are able to assemble these fragments into a complete picture. The Universe is hard to study because you can’t compare and contrast it with other real universes, but the Millennium Simulation models universes that are different from ours, which helps us to estimate how much dark matter and dark energy there are in the real Universe.
Sometimes astronomical discoveries are serendipitous: the right person is in the right place at the right time. Tycho Brahe was returning home from an evening dinner at the time that the supernova of 1572 appeared in the sky and saw it while gazing from his carriage window; 400 years later, Ian Shelton happened to be pointing his telescope in the right direction when Supernova 1987A exploded. The crucial factor was that both discoverers knew about astronomy and understood what they were seeing. Other cosmic discoveries were unexpected by-products of investigations set up for entirely different purposes. Herb Gursky and Riccardo Giacconi made a serendipitous discovery when an X-ray detector on a rocket they had launched to look at the Moon saw a bright source behind it. Jocelyn Bell discovered pulsars as a source of ‘noise’ (‘scruff’, as she called it) during the observation of quasars. In both cases the scientists were remarkably persistent in systematically tracking down the origin of the inconsistency.
In a lecture in Lille in 1854 the French scientist Louis Pasteur perceptively noted that ‘in the field of observation, chance favours only the prepared mind.’ Usually in astronomy this means the prepared multidisciplinary mind. Astronomy as a subject encompasses the study of everything in space. Physics, mathematics, chemistry, computing, engineering, statistics – all these sciences, and more, are deployed by astronomers to understand what they see and to unlock cosmic secrets. Some of the most important discoveries are collective, the product of investigations made by many different people over several generations, although there is usually one last genius who ties it all together. The laws of the motion of the planets engaged the minds of a stellar system of talents before the proverbial apple dropped and Isaac Newton discovered the theory of gravity. ‘If I have seen further, it is by standing on the shoulders of giants,’ he wrote. The discovery of the greenhouse effect in the atmospheres of Venus and the Earth took 150 years of investigation by dozens of scientists – there was really no one person who made the discovery, now recognized as so momentous that nothing less than the survival of life on Earth may depend upon our understanding of it. By contrast, Special Relativity and General Relativity were the ideas of a single individual, Albert Einstein, working over only a few years.
‘The most exciting phrase to hear in science, the one that heralds new discoveries, is not “Eureka!”, but “That’s funny…”.’ This saying, commonly attributed to science-fiction writer Isaac Asimov, captures the sense that the most important feature of a scientific discovery is the open mind and curiosity of the person who makes it. In this book I have tried to explain what lies behind some of the great discoveries in astronomy, the train of events and thoughts that brought the scientist to exclaim ‘Eureka’ or ‘That’s funny…’ as he or she unlocked one of the major secrets of the Universe. This
book is therefore mostly scientific history – my top cosmic discoveries. However, in the first edition of the book I also identified four major prospects for future discoveries: to uncover the secrets of dark matter and dark energy, to detect gravitational waves and to discover life elsewhere in the Universe – although we may not find what we expect. The challenge for the next generation of astronomers, I wrote in 2008, was to put themselves in the position to uncover these momentous secrets. I hoped that some of them might succeed in my lifetime.
I am lucky enough to have seen one of these four predicted discoveries come to pass: the discovery of gravitational waves. Gravitational waves were first identified in 2015, originating from two colliding black holes. Their existence had been expected – but, as usual, the Universe is cleverer than astronomers, and there was an entirely unexpected feature of the discovery. I have no doubt whatsoever that there will be surprises in the other three future discoveries that I have anticipated, not to mention all those discoveries that I have not written about but are surely still to come.
Paul Murdin, 2019
BEFORE THE TELESCOPE
The Seven Planets
Wandering stars
Day 13 [20 September]. Sunset to moonrise: 8º. There was a lunar eclipse. Its totality was covered at the moment when Jupiter set and Saturn rose. During totality the west wind blew, during clearing the east wind. During the eclipse, deaths and plague occurred. That month, the equivalent for 1 shekel of silver was: barley, [so many] kur; mustard, 3 kur; sesame, 1 pân, 5 minas. At that time, Jupiter was in Scorpio; Venus was in Leo, at the end of the month in Virgo; Saturn was in Pisces; Mercury and Mars, which had set, were not visible.
Cuneiform tablet, 331 BCE
The human fascination with the movement of the planets is almost as old as humanity itself. 25,000 years ago, beside a lake in Africa, a member of the Ishango community recorded the cycles of the Moon; in ancient Babylon, astronomers used their knowledge of the planets to advise their king about affairs of state and the price of barley. For 2,000 years the ‘geocentric theory’ provided the dominant explanation of the Universe, based on the evidence of everyday human observations.
