Secrets of the Universe

by Paul Murdin

  The B2FH paper proposed that stars make helium by ‘burning’ hydrogen, which is the most simple and abundant element in the Universe. The burning converts helium to carbon, carbon converts helium to oxygen, oxygen and subsequent burning produces neon, silicon and finally iron. When a star explodes as a supernova the explosion irradiates elements such as carbon and oxygen in the body of the star. Other elements are produced on the surfaces of red-giant stars – dramatic proof of this was provided in 1952 by Mount Wilson astronomer Paul Merrill, who discovered the spectral lines of the element technetium in some red giants. Technetium is radioactive and its longest-lived isotope decays relatively quickly – in a matter of a million years. Since red giants are much older than this, the technetium must have been made inside them.

  The elements that are made in stars are dispersed into interstellar space by supernova explosions, stellar winds and planetary nebulae. There, they mix with hydrogen gas and form clouds that may eventually condense into stars. This is the origin of all the chemical elements that make up the Earth and all that is on it, including ourselves. In the words of Carl Sagan, ‘We are star stuff.’

  Inside the Sun

  Whispers and rings

  There’s not the smallest orb which thou behold’st

  But in his motion like an angel sings.

  William Shakespeare, The Merchant of Venice, c. 1599

  We cannot look at the interior of the Sun directly, but over the past century we have developed ingenious ways to ‘see’ what happens inside it. Astronomers first captured neutrinos, incredibly small particles generated inside the Sun, by burying a vast quantity of dry-cleaning fluid in a gold mine. Studies of solar earthquakes showed that the Sun rings like a gigantic bell and provided clues to its makeup.

  Astronomers who wanted to understand what happens inside the Sun faced one big problem: none of its internal workings could be seen, because the Sun is completely opaque. At first only the Sun’s surface characteristics, and its global properties – such as its diameter and the amount of energy that it radiates – could be determined by direct observation. However, we now know what happens inside the Sun thanks to three lines of astronomical enquiry. First, ingenious mathematical calculations built up a theoretical picture of the Sun’s interior. This picture was verified by enormous efforts to capture a tiny quantity of the neutrino particles, and later by measuring the sound waves generated by motions inside the Sun.

  Our understanding of the inner workings of the Sun is the result of one of the great feats of modern mathematical reasoning. From the 1920s astronomers knew the physical conditions inside the Sun by calculation and from the 1930s they knew that nuclear reactions were the source of the Sun’s energy. In the 1950s they had begun to understand the way that stars evolve in relation to one another from observations of star clusters. These calculations built up confidence in astronomers’ theoretical knowledge of the Sun’s interior.

  The physics suggested that the Sun would be an abundant source of neutrinos. Neutrinos are small nuclear particles, whose existence was suggested by the Austrian-Swiss physicist Wolfgang Pauli in 1930 to explain some details of nuclear reactions, and confirmed experimentally in 1956. Neutrinos are made in the nuclear chain reaction inside the Sun that makes the Sun’s radiation from hydrogen. The nuclei of the hydrogen atoms in the Sun become free protons. Two protons combine, one of them changing to a neutron by emitting a neutrino and a particle called a positron. The reaction continues to its conclusion when another proton sticks to the pair, forming a helium nucleus, containing a pair of protons and a neutron. Two similar helium nuclei collide and two protons are ejected, leaving behind a helium nucleus with a pair of protons and a pair of neutrons.

  The net result of this chain is that four protons make a helium nucleus, releasing energy. The neutrinos escape, also carrying off small parcels of energy. The numbers of neutrinos given off by the Sun is immense – about ten billion pass through every square centimetre of the Earth every second. There are floods of them, but they are whisper-quiet and can travel through a light year (ten trillion kilometres) of material without interacting with it in any detectable way. Neutrinos travel so fast that it takes only eight minutes for them to reach the Earth. Despite the astonishing speed and elusiveness of solar neutrinos, it is possible to build detectors that do catch some.

  The first solar-neutrino detector was built by Brookhaven National Laboratory physicist Raymond Davis, Jr, following technical suggestions from the notorious Italian-born physicist Bruno Pontecorvo (who later defected to join the Soviet nuclear programme) and American physicist Luis Walter Alvarez. Three American nuclear astrophysicists – William Fowler, Alistair Cameron and John Bahcall – had insisted that it was practical to try to catch neutrinos, as the vast numbers constantly released by the Sun overwhelmed the small chance for each one that it would slip through a neutrino detector unnoticed. In the bowels of the Homestake Gold Mine, in Lead, South Dakota, deep enough underground to avoid interference from cosmic rays, Davis installed a tank containing 615 tonnes of carbon tetrachloride, a solvent normally used for dry cleaning. Solar neutrinos were captured on the chlorine atoms in the solution and converted to argon atoms. These argon atoms were flushed out of the tank every two months and counted.

  The original estimates were that just seventeen argon atoms would be produced in the tank in each extraction run, but in fact, in the first experiment in 1968, lasting six months, even fewer neutrinos were seen. As Davis repeated his experiment with improved equipment, the question became ‘where are the missing neutrinos?’ – this became known as the ‘solar neutrino problem’. Another neutrino detector called Kamiokande, built and operated by Japanese astrophysicist Masatoshi Koshiba, was able to determine the trajectory of the incoming neutrinos. It not only confirmed in 1989 that Davis had detected neutrinos from the Sun and that there were fewer than expected, but was able to prove that the neutrinos it captured really came from the Sun. The Sudbury Neutrino Observatory (SNO), a neutrino observatory located 2,100 metres underground in Vale’s Creighton Mine in Ontario, Canada, detected solar neutrinos through their interactions with a large tank of heavy water – about 300 million dollars’ worth, loaned by Atomic Energy of Canada. It likewise found that solar neutrinos were missing.

  At first, some physicists thought that the discrepancy between observation and theory had arisen because astronomers’ standard calculations relating to the solar interior must be flawed. There were no missing neutrinos, but somehow astronomers had overestimated the numbers of neutrinos that the Sun was making. The astronomers rejected this, in part because they had found another way to look inside the Sun, to check their theories about its makeup and to solve the mystery of the missing neutrinos. The approach they used was called ‘helioseismology’. Helioseismology is the study of oscillations in the body of the Sun, which resemble earthquakes studied by seismologists on Earth. In the general turmoil of motion of hot material in the Sun’s interior, the Sun generates sound waves whose resonances travel across the body of the Sun, and its surface oscillates up and down. The Sun rings, like a bell quietly singing as it is brushed by a succession of impacts from a stream of sand grains.

  Caltech physicist Robert Leighton discovered the surface oscillations of the Sun in 1960, and measured the oscillation periods at about five minutes. In the 1970s UCLA physicist Roger Ulrich suggested that the duration, frequency and tone of these oscillations could provide clues to the composition of the Sun’s interior. Ulrich pointed out that the frequencies at which the Sun rings depend on the time it takes sound to cross the Sun. This in turn depends on the composition, temperature and density structure of the solar interior. The sound waves thus carry information about the interior of the Sun to the surface where it can be seen, just as the oscillations of earthquakes carry information about the interior structure of the Earth.

  Individual earth-based telescopes had a great limitation: they could not observe the Sun after it disappeared daily below the horizo
n at night. Astronomers therefore set up networks of ground-based solar telescopes around the world to measure the frequencies of solar oscillations more accurately – the networks have names like GONG (Global Oscillation Network Group of the US National Solar Observatory), BiSON (Birmingham Solar Oscillations Network) and HiDHN (High Degree Helioseismology Network) – but intermittent cloud still interfered with their observations.

  The Solar and Heliospheric Observatory (SOHO) satellite, a joint project involving the European Space Agency and NASA, avoided even this limitation. It has been staring at the Sun continuously from space since its launch in 1995. The comprehensive observations of the SOHO satellite provided new data on the temperature inside the Sun, and the way that its interior rotates slower than its surface layers, generating a hot layer inside the Sun that is the ultimate cause of sunspots and prominences on its surface. SOHO also proved that the standard calculations that had been used to measure sound-speed at various depths in the interior of the Sun were 99.9% accurate. The conclusion was that astronomers knew rather well how many neutrinos the Sun was making, and Davis’s ‘missing neutrinos’ were not the result of a miscalculation of solar conditions.

  Assuming that astrophysicists knew about the state of material inside the Sun and nuclear physicists knew how many neutrinos that would create, physicists had to concentrate on why many went missing. Something evidently happened to neutrinos after they had left the Sun. Some of them did not make it across space to the Earth. This explanation was first proposed by Pontecorvo only a year after Davis first found the solar neutrino discrepancy in 1968.

  Neutrinos come in three different kinds, or ‘flavours’. They can oscillate from one ‘flavour’ to another as they travel for eight minutes across the distance between the Sun and the Earth. The neutrino detectors have the capacity to capture and detect solar neutrinos only of the ‘flavour’ generated deep within the Sun. By the time the neutrinos arrived on Earth, many of them had changed by ‘oscillating’ from that flavour to another flavour, so they bypassed the detectors and went missing. The evidence for this happening was discovered and became ever more convincing between 1998 and 2001, and beyond, by the Japanese Kamiokande detector and the Canadian SNO.

  Astronomers were proud that their meticulous work on the Sun had led to a new discovery about particle physics. The importance of this work was justly recognized by the award of the Nobel Prize in 2002 to Masatoshi Koshiba and Raymond Davis ‘for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos’. The director of the Sudbury Neutrino Observatory experiment, Arthur B. McDonald, was likewise awarded a share of the Nobel Prize in Physics in 2015 for the experiment’s contribution to the discovery of neutrino oscillation.

  The Crab Nebula

  A supernova remnant

  And in these Constellations then arise

  New starres, and old doe vanish from our eyes:

  As though heav’n suffered earthquakes, peace or war,

  When new Towers rise, and old demolish’t are.

  John Donne, ‘An anatomy of the World, the First Anniversarie’, 1610

  Imperial astrologers in China and Native Americans in the Southwestern United States recorded the appearance of a ‘new star’ in 1054. Nearly a thousand years later, Knut Lundmark and Edwin Hubble realized that these early observers had witnessed the birth of the Crab Nebula, a magnificent supernova remnant that has intrigued astronomers since it was first mapped in the eighteenth century.

  Like the planet Uranus, the Crab Nebula was found as a result of a systematic whole-sky survey, conducted by an eighteenth-century British doctor, John Bevis, who had an observatory near London. In 1745 he compiled his observations into an atlas called Uranographia Britannica; the etched plates were costly to produce and the printer went bankrupt before printing it, so only a few proof copies survive. On the map of the constellation Taurus, near the star Zeta Tauri, Bevis drew a patch to represent a misty nebula that he had discovered.

  The French astronomer Charles Messier used a copy of Bevis’s atlas on his search for the predicted return of Halley’s Comet in 1758. He found another comet, a new one, which passed through Taurus and drew his attention to the misty nebula. Comets and nebulae look much the same in a small telescope, so Messier, who was known as the ‘Ferret of the Comets’, decided to make a list of known nebulae to avoid confusion. The first item in Messier’s catalogue was M1, Bevis’s nebula, which became known as the ‘Crab Nebula’ after a bizarre sketch made in the 1840s by William Parsons, the Earl of Rosse, after viewing the nebula through his ‘Six-foot’ telescope at Birr Castle in Ireland. Modern pictures show the nebula as a generally oval shape of white light surrounded by a lacy network of filaments. The filaments are fragments of the body of an exploding star, and the white light comes from electrons spiralling around the star’s magnetic field within the filaments.

  The event in which the nebula first appeared was identified by the Swedish astronomer Knut Lundmark in 1931 as he listed the novae that had been recorded by Chinese astronomers and imperial historians. Chinese emperors maintained courts of astrologers who studied the sky in order to infer the future of affairs of state. Some of the signs that they recorded included ‘guest stars’ – temporary celestial phenomena, such as comets or novae. Generally, if the ‘guest star’ does not move for several days or months relative to the other stars it is probably a nova. Number 31 on Lundmark’s list was a ‘guest star’ of July 1054. He noted that M1 was at the same position.

  The clinching argument that connected the Crab Nebula with the guest star was American astronomer Edwin Hubble’s measurement of the speed at which the M1 nebula is expanding, growing in size as its filaments rush continually outward from the centre of the explosion. Extrapolating backwards, Hubble found that in 1054 the filaments had been gathered together at the centre, and, in a series of popular essays, pointed out the correspondence with the Chinese record of the nova that had been noted by Lundmark. But Hubble’s conclusion was overlooked until 1942 when it was revived by astronomers Jan Oort and Nicholas Mayall, and a Dutch Sinologist, Jan Duyvendak, who identified other historical records of the nova of 1054 from Korea, Japan and Baghdad.

  In 1955 American astronomer William Miller proposed that a number of ancient rock paintings from Arizona and New Mexico depict the event. For example, on a roof ledge of a cave (now partially collapsed) a member of the Anasazi people living in Chaco Canyon, New Mexico, drew an image of the crescent Moon and a bright star, signing the picture with a handprint. Archaeological dating of the occupancy of the site is a rather long period of time (200 years), but it overlaps the date of the supernova, which was indeed seen in association with the crescent Moon. The evidence is circumstantial, and there are some discrepancies (the crescent is often the wrong way round), but it is a nice story and the star is reckoned by many to be an eyewitness representation of the supernova of 1054.

  The Chinese astronomers had compared the brightness of the guest star of 1054 to other celestial objects like Venus. This made it possible to draw a light curve of the nova and show that it was in fact a supernova, a stellar explosion in which nearly the whole star disintegrates, leaving a black hole or a small stellar cinder, a neutron star. In 1968, while searching for the stellar remnant in the Crab Nebula with a radio telescope at Green Bank, West Virginia, radio astronomers David Staelin and Edward Reifenstein discovered a radio pulsar right in the middle of the nebula – a neutron star spinning on its axis thirty times per second. This was a brilliant confirmation of the bold idea put forward thirty years earlier by Fritz Zwicky that supernovae produce neutron stars.

  The Crab pulsar flashes thirty times per second, and is a rotating neutron star with a diameter of about 29 kilometres. A strong magnetic field is embedded in the neutron star, and generates high-speed electrons that emit radio waves, so that the Crab Nebula is one of the brightest radio sources in the sky, having the designation Taurus A. Because the pulsar is losing energy, its rotat
ion speed is gradually slowing. Occasionally the spin rate suddenly changes, speeding up but then recovering its long-term course. These so-called ‘glitches’ are an effect of the structure of the neutron star. The star has a crust that rotates at a slow rate compared with the interior, lagging behind. Occasionally, the crust cracks, the interior fastens onto the crust and the neutron star abruptly spins up.

  The range of the scales of distance and time in the Crab is remarkable. The pulsar is 29 kilometres in diameter; the Crab Nebula 100 million, million kilometres. The pulsar spins round in one thirtieth of a second; the Crab supernova explosions occurred 30 billion seconds ago. The range of phenomena in the Crab is equally remarkable, to the extent that German-American astronomer Walter Baade suggested that astronomy was divided in two parts – the Crab Nebula, and everything else. It is a living textbook of astrophysics.

  Planetary Nebulae

  Looking into secret places

  On the evening of the 29th of August, 1864, I directed the telescope for the first time to a planetary nebula in Draco [NGC 6543]. The reader may now be able to picture to himself to some extent the feeling of excited suspense, mingled with a degree of awe, with which, after a few moments of hesitation, I put my eye to the spectroscope. Was I not about to look into a secret place of creation?

  Sir William Huggins, The New Astronomy: A Personal Retrospect, 1897

  One August evening in 1864, English astronomer William Huggins used his spectroscope to examine a nebula that he thought was a collection of densely packed stars. To his surprise, the spectrum showed ‘a single bright line only!…The riddle of the nebulae was solved. The answer, which had come to us in the light itself, read: Not an aggregation of stars, but a luminous gas.’ The nebula actually marked the quiet demise of a star much like the Sun, on its way to becoming a white dwarf.