Secrets of the Universe
Page 20
Despite their careful planning, Mayor and Queloz had not expected to be successful so soon. Their original programme essentially decided their careers, since they could only be sure that they had found a twin of Jupiter by observing at least two periods – amounting to twenty-four years! But within only eighteen months they had discovered their first planet, orbiting the star 51 Pegasi, 45 light years distant. Even more surprisingly, the star’s oscillation was much greater than expected, and its period was very much shorter – 4.2293 days! The planet, 51 Peg b, is roughly the same mass as Jupiter. Its orbit is close to circular and the orbital period of four days indicates that it lies much closer to its sun than does Jupiter to our Sun – in fact, closer than any planet in our Solar System. The distance of 51 Peg b to its parent star is only 1⁄20 of the Earth–Sun distance.
As news of the discovery spread around the world, Mayor and Queloz’s observations were swiftly confirmed during a brief four-day observing run with the powerful Lick Observatory telescope conducted by Geoffrey Marcy of San Francisco University, Paul Butler of the University of California, and a team from the High Altitude Observatory and the Harvard-Smithsonian Center for Astrophysics. Like Mayor and Queloz, Marcy and Butler had been monitoring solar-type stars for radial velocity variations indicating the presence of jupiters, but 51 Pegasi was not on their original observing list because of a mistake in the catalogue from which they compiled their programme. After hearing of the Swiss discovery and altering their expectations accordingly, the team quickly found further examples of exoplanets, some of them in archives of previous observations that they had not yet examined closely because they thought there was no rush.
The first technique used to discover exoplanets uses spectroscopy, a technique that gives an important property of the exoplanet, namely an estimate of its mass. It is not a method, however, that readily lends itself to the discovery of lots of planets, because you can examine only one star at a time. A second technique that has been used to discover exoplanets is to look for ‘winking stars’: stars whose light is periodically dimmed a little by the transit of planets across their faces. The dark planet obscures a small fraction, typically less than 1%, of the bright star. Since astronomers can image thousands of stars in one picture, they can examine many stars for the periodic winks – the hard part is to measure the brightness of the stars accurately enough and to seek the significant, periodic dips in brightness. This technique results in a complementary property of the exoplanets that it finds, namely their size. Furthermore, by scrutinizing the spectrum of the star for the effects of the transiting exoplanet, it is possible to determine properties of the atmosphere of the planet. For example, some exoplanets have expanding atmospheres, boiling off as a result of heat from the parent star.
The transiting technique has been used by exoplanet-hunters to find planets. Some of them are ground-based, like SuperWASP (Wide-Angle Search for Planets), an international project that uses two robotic observatories located in the Canary Islands and South Africa. This programme uses two arrays of eight wide-angle cameras to image the sky repeatedly as it passes overhead, and has found about 200 exoplanets so far. But because the accuracy with which astronomers can measure the brightness of stars from the ground is limited by the trembling of the atmosphere, the most productive method to discover exoplanets is from space. There have been two space observatories devoted to this method, the pioneering French CoRoT (Convection, Rotation et Transits planétaires) spacecraft, active between 2006 and 2012, and the incredibly productive NASA Kepler mission, which between 2009 and 2018 stared at a field of stars in Cygnus and detected more than 5,000 possible planets, with about 3,000 planets so far confirmed.
A third technique to discover exoplanets is to image them, but this is very difficult because exoplanets are faint and are close to a very bright star. The technique has been successful in few cases (plate XVII).
Due to technical constraints, only very specific kinds of exoplanets have been discovered thus far, representing a minuscule proportion of the total. Virtually all of the stars that are currently known to have exoplanetary systems are no more than 3,000 light years away – and therefore relatively bright; you need a lot of light to measure accurately the small changes in a star produced by a planet. Most of the known exoplanetary systems contain large planets orbiting a central star, because only the largest planets can be discovered – Neptune-sized or bigger. ‘Jupiters’ are the most commonly found. Often the jupiter in the extrasolar planetary systems is in an orbit much closer to its sun, and thus hotter, than the Jupiter in our Solar System. Extrapolating from the sample that we have, it seems that there are as many planets in the Galaxy as stars – roughly half the stars have no planet, roughly half the stars have an average of two planets each. It seems that the most common planets in the Galaxy as a whole are so-called ‘super-earths’, twice the mass of our own Earth. It is not known why we do not have a super-earth in our Solar System.
These new systems of planets, large and close to their central stars, contradicted astronomers’ theories about the formation of our own Solar System. Large planets are not supposed to be so close to their suns, but are expected to orbit the colder outer reaches of a planetary system, like Jupiter, Saturn, Uranus and Neptune, which retain their gases because they are massive and cold. Astronomers had believed that planets as near to their suns as the ones discovered recently would be small terrestrial planets, resembling Mercury. The reason seems to be that new-born planets migrate in towards their sun, the bigger ones swallowing up smaller inner ones. But something happened in our own Solar System to stop or even reverse the migration of Jupiter. This is good for us: otherwise our Earth would not have survived.
The Energy of the Sun and Stars
Discovery of nuclear fusion
That evening after we had finished our essay, I went for a walk with a pretty girl. As soon as it grew dark, the stars came out, one after another, in all their splendour. ‘Don’t they sparkle beautifully?’ cried my companion. But I simply stuck out my chest and said proudly: ‘I’ve known since yesterday why it is they sparkle.’ She didn’t seem the least moved by this statement. Perhaps she didn’t believe it. At that moment, probably, she felt no interest in the matter whatever.
Fritz Houtermans, quoted in Robert Jungk, Brighter than a Thousand Suns, 1958
How does the Sun shine, and how long has it been shining? Could the fuel ever run out? These questions have perplexed scientists since they first began to comprehend the awesome size and age of the Solar System. The secrets of the Sun’s power system – for better or for worse – have made nuclear technology possible on Earth.
As soon as the scale of the Solar System and the distances of the stars became apparent, it was clear that the amount of heat and light emitted by the Sun and stars was enormous. When astronomers calculated the mass of the Sun using Newton’s laws of gravity, it, too, was enormous. If the Sun is a kind of normal fire, providing power by chemical means (for example, by chemically combining carbon and oxygen to carbon dioxide, as happens when wood or coal is burnt), there is a lot of fuel available to supply the power that we see. But for how long could the fuel last?
Presumably the Sun is at least as old as the Earth – the one depends on the other. In 1650, to determine the age of the Earth, Archbishop James Ussher published an analysis of the chronology of events in the Bible, which was reproduced in the standard edition of the Bible used in England for centuries and so became widely accepted. He set the date of the creation of the Earth at 4004 BCE. If the Sun really had been created along with the Earth just 6,000 years ago, and chemical energy was the source of its power, the amount of material consumed as the Sun shone was only a small fraction of its total mass. But in the nineteenth century, British geologists like Charles Lyell and John Phillips began to estimate that the Earth was actually millions of years old, based on calculations of how long it would take for sedimentary rocks to be laid down from sea deposits or for rocks to be eroded away.
This created a problem for the idea that chemical energy was the source of the Sun’s power – a conventional fire or chemical reaction would not be able to continue burning for millions of years. The discrepancy was made worse when calculations by physicist Lord Kelvin and biologist Thomas Huxley suggested that the Earth was actually hundreds of millions of years old. Even this seemed short, considering the length of time needed for the evolutionary processes that Charles Darwin envisaged in 1859 to produce the variety of living species on Earth. The German physicist Hermann von Helmholtz and the Canadian astronomer Simon Newcomb temporarily rescued the chemical energy theory at the end of the nineteenth century by suggesting that the Sun’s energy was also supplied by gravitational contraction, calculating that the Sun’s lifetime could be consistent with the Earth’s if they were both at least hundreds of millions of years old. But in the twentieth century scientists began to realize that the Earth was much older even than this.
After Henri Becquerel and Marie and Pierre Curie discovered radioactivity, the British physicist Lord Rutherford developed a technique for using radioactive decay to measure the age of rocks, by which a young American chemist, Bertram B. Boltwood, discovered that some rocks were as much as 1–2 billion years old. How could the Sun keep shining for such a length of time?
The answer was discovered by two physicists of the University of Göttingen, Fritz Houtermans and Robert d’Escourt Atkinson. As they passed their summer holiday in 1927 on a walking tour, they discussed the problem of the source of the Sun’s energy. They knew about the physical conditions inside the Sun from the work of the British astrophysicist Arthur Stanley Eddington: a high density and a high temperature created a high pressure inside the Sun, which countered the force of gravity that was drawing the material of its body tightly together. Atkinson knew of Einstein’s formula for converting mass to energy, E = mc2, and understood that what was then called atomic transmutation was possible. The atoms (or, as we now know, their broken-down nuclei) in the centres of stars and the Sun are frequently colliding together because of the high densities and high temperatures. If the collisions transformed some atoms from one kind to another, losing mass in the process, atomic (or ‘nuclear’) energy would be produced. ‘This might be the source of the Sun’s energy. ‘Let’s just work the thing out, shall we?’ said Houtermans. ‘How could it happen in the Sun?’
The two young scientists discovered how the fusion of light elements into heavier ones could fuel the Sun. Atkinson later learned that the Sun was mainly hydrogen, and realized that the source of the energy was specifically the conversion of four hydrogen atoms to one helium atom. A helium atom is 0.7% lighter than four hydrogen atoms. This tiny amount of excess mass, multiplied many times over by the astonishing number of hydrogen atoms present in the Sun and the frequency of their collisions, provides the Sun’s energy. 400 million tonnes of mass disappears from the Sun every second and is transformed into solar energy – this attrition rate can be kept up for billions of years.
Houtermans and Atkinson’s work was followed up in 1939 by the German-American physicist Hans Bethe. He discovered the exact process that enables hydrogen fusion in the Sun. It is called the CNO cycle, since the hydrogen nuclei are fused together in successive stages with carbon, nitrogen and oxygen as intermediate steps. Bethe was awarded the Nobel Prize in 1967 ‘for his contributions to the theory of nuclear reactions, especially his discoveries concerning the energy production in stars’. The details of what happens inside the Sun have been confirmed with astonishing accuracy by the detection of neutrinos, tiny particles given out in the nuclear processes, which travel from the Sun’s interior and have been detected on Earth using specially built neutrino detectors.
In the decades after Bethe’s discoveries, physicists from many nations were organized into programmes that attempted to reproduce on Earth what happens in the stars, finding ways to release nuclear energy slowly in a reactor or suddenly in a bomb. For better or for worse, many of these projects have been successful. The discovery of the energy source of the Sun and other stars may well prove to have been the most momentous secret of the universe to be uncovered.
The Origin of the Elements
Making star stuff
We are bits of stellar matter that got cold by accident, bits of a star gone wrong.
Sir Arthur Stanley Eddington, The New York Times Magazine, 9 October 1932
‘All the innumerable substances which occur on Earth – shoes, ships, sealing-wax, cabbages, kings, carpenters, walruses, oysters, everything we can think of – can be analysed into their constituent atoms,’ wrote James Jeans. ‘It might be thought that a quite incredible number of different kinds of atoms would emerge from the rich variety of substances we find on Earth. Actually, the number is quite small. The same atoms turn up again and again, and the great variety of substances we find on Earth result, not from any great variety of atoms entering into their composition, but from the great variety of ways in which a few types of atoms can be combined.’ The remarkable fact is that all but one of the elements in the Universe – and therefore the elements that constitute the building blocks of our own bodies – are made inside stars.
In the eighteenth and nineteenth centuries, chemists realized that all materials were made of molecules, and molecules themselves were made of atoms in fixed arrangements. In the twentieth century the actual physical makeup of atoms was discovered. Each atom is made of electrons orbiting a nucleus, itself made of protons and neutrons. The number of electrons in an atom is equal to the number of protons in its nucleus. The number of neutrons in an atom is roughly equal to the number of protons, but can differ from atom to atom of the same element. Each different nuclear arrangement is called an ‘isotope’. There are about a hundred chemical elements, and the atoms of each are distinguished from the others by the number of electrons. Changes in the arrangement of the electrons produce light; astronomers can see the light with a spectroscope, and, in general, the clearer the spectral signature of a particular element in a celestial body like a star, the more of that element is present in the star – its ‘abundance’.
In her 1925 doctoral thesis, Harvard astronomer Cecilia Payne (later Payne-Gaposchkin) suggested that hydrogen was the most abundant element in the Sun. Reviewing her thesis, the influential Princeton astronomer Henry Norris Russell dismissed the idea, but changed his mind in 1929. In fact 71% of the Sun’s mass is hydrogen, 27.1% is helium, together comprising 99.9% of the number of atoms in the Sun. The remaining 0.1% are (in order of abundance) oxygen, carbon, nitrogen, magnesium, silicon and neon; some seventy further elements have also been identified in the solar spectrum. Payne-Gaposchkin had discovered that the abundance of elements in the stars was broadly the same as in the Sun, so astronomers could treat the Sun as representative of other stars.
After Fritz Houtermans and Robert d’Escourt Atkinson discovered in 1927 that the stars generate energy by nuclear reactions, astrophysicists were able to address the question of where the elements came from and why some are more abundant than others. In 1939 Hans Bethe showed how hydrogen was transformed into helium via a cycle involving carbon, nitrogen and oxygen. This explained one source of helium – but where did the hydrogen, carbon, nitrogen and oxygen come from in the first place? According to a 1948 paper known as ‘αβγ’ (alpha-beta-gamma) after its authors Ralph Alpher, Hans Bethe and George Gamow, the hydrogen was made in the Big Bang and elements were built up in stages from the simplest element, hydrogen, by successively adding neutrons one at a time to make heavier and heavier nuclei, including additional helium.
The ‘αβγ’ theory failed because it could not create elements heavier than lithium. The problem is that there is no stable atom with 8 protons and 8 neutrons, so when you get to 8 neutrons the nucleus decays spontaneously back to 7. Armagh Observatory director Ernst Öpik and astrophysicist Edwin Salpeter found a better explanation in 1951–2: carbon, with 12 protons and 12 neutrons, is made when three helium nuclei (each with 4 pro
tons and 4 neutrons) collide at the same time inside a star. In 1953 British cosmologist and nuclear physicist Fred Hoyle discovered the exact nuclear reactions involved, although the bold predictions he made to tie up the loose ends in his theory were thought by many to be crazy. But he persuaded physicist Ward Whaling in the Kellogg Radiation Laboratory at the California Institute of Technology to perform an experiment, which confirmed his calculations.
Hoyle was the first to establish the currently accepted explanation for how the elements are made inside stars. Although some said that Hoyle’s motivation was to find an alternative to the idea that the elements originated in the Big Bang, he had started work on the topic long before he proposed the rival steady-state theory. In studying the way that stars evolve he had simply asked himself the question, ‘What would be the very last of the nuclear reactions that take place in stars, instead of the first reactions that had so far occupied the attention of astronomers?’
The detailed scheme by which the elements were made in stars was summarized by Margaret Burbidge, Geoffrey Burbidge, William Fowler and Fred Hoyle in 1957. Their seminal paper is known by astronomers as B2FH (‘B-squared FH’), from the authors’ initials, and is one of the most frequently referenced papers in astronomy. The Burbidges provided the astronomical expertise for the paper and Fowler and Hoyle the nuclear physics. In 1983 Fowler received the Nobel Prize ‘for his theoretical and experimental studies of the nuclear reactions of importance in the formation of the chemical elements in the Universe’; it is not clear why the Nobel Prize committee did not also honour Hoyle.