Secrets of the Universe

by Paul Murdin

  The explosion causes the old star to disintegrate (plate XVIII), although usually not completely – it may leave a black hole or a neutron star. The body of the star, including all the elements made inside it by nuclear fusion, is dispersed into space as it disintegrates. Eventually, this material mixes with interstellar gas, and congeals to form new stars and planets. This happened in our own Solar System before our present Sun was formed, and elements from ancient supernovae constitute the physical makeup of our own planet and everything on it – including our own bodies. We are made of stardust.

  The supernova explosions just described are called Type II supernovae. There is an alternative kind. Brahe’s supernova was a so-called Type Ia supernova – its progenitor star was a white dwarf star. White dwarfs are held up against the force of gravity by a force of quantum mechanical origin called degeneracy. As discovered by Subrahmanyan Chandrasekhar, this balance only works if the white dwarf is not too massive. If the white dwarf feeds on material donated from a close companion star, its mass increases and may surpass the upper limit. The star collapses in a supernova explosion, in which the white dwarf is completely disrupted. Its companion star, the one that donated the extra material, is released like a stone from a slingshot. The companion star released by Brahe’s supernova was discovered in 2004 by a team led by Spanish astronomer Maria Pilar Ruiz-Lapuente. She used the William Herschel Telescope on La Palma in the Canary Islands to identify the star, and later confirmed her discovery by observing the star more closely with the Hubble Space Telescope. ‘Here we have identified a clear path: the feeding star is similar to our Sun, slightly more aged,’ Ruiz-Lapuente reported. ‘The high speed of the star called our attention to it.’

  In the case of Type II supernovae, the progenitor stars are actually too massive to support themselves from the outset. Massive stars are ticking time-bombs, and all of them eventually collapse and become supernovae. Because stars are long-lived, in a galaxy the size of the Milky Way there is on average only one supernova every fifty years. But this does mean that if you watch fifty galaxies, you will find one supernova exploding every year, and if you watch a thousand galaxies, you will find a couple of dozen every week. This approach has made it possible for the Hubble Space Telescope to find and study many faint supernovae at great distances. Astronomers working on the Supernova Cosmology Project and the High-Z Supernova Search Team use large ground-based telescopes and powerful automatic image analysis systems to search fields crowded with galaxies that lie just ahead in the path of the HST. When the teams locate supernovae in these galaxies, they can direct the HST to study them in detail, discovering more of the secrets of these explosive dying stars.

  Supernova 1987A

  The whisper and the vision

  So when, by various turns of the Celestial Dance,

  In many thousand years

  A star, so long unknown, appears,

  Though Heaven itself more beauteous by it grow,

  It troubles and alarms the World below.

  Abraham Cowley, ‘Ode to the Royal Society’, 1667

  Most supernovae are in distant galaxies and therefore rather faint. The appearance of a bright, nearby supernova like Tycho Brahe’s ‘new star’ is always an occasion for excitement, and a source of valuable astronomical information. The most recent supernova that could be seen on Earth with the unaided eye was called SN 1987A (indicating that it was the first supernova of 1987). In 2005 astronomers studying the supernova witnessed an astoundingly beautiful and unprecedented event.

  SN 1987A was discovered by Ian Shelton at 05:40 GMT on 24 February 1987 at Las Campanas Observatory in Chile. He had taken a photograph of the Large Magellanic Cloud and developed it before going to bed. To his surprise there was a black spot on the photograph, which, at first, he thought was some sort of blemish. Then he realized that it was actually a bright star where none was indicated on the charts. It was a supernova.

  Shelton wanted to share his discovery, and went to another telescope to talk to his colleagues. A fellow astronomer, Oscar Duhalde, mentioned that he had seen the star earlier that night with his own eyes while strolling about outside during a break, but when he returned to the telescope he had been immediately harassed by another researcher in the queue for the equipment and had forgotten to mention the strange object. Later it transpired that the supernova had in fact been photographed the day before by astronomer Robert McNaught in Australia, but he had put off looking at his photographs and thus failed to discover the nova before Shelton.

  Shelton’s discovery made it possible to identify the first neutrino particles from a supernova. When a Type II supernova collapses, its protons and electrons are jammed so close together that they merge and form neutrons. This nuclear reaction produces huge numbers of elusive particles called neutrinos. Some of these neutrinos were caught in a neutrino detector in Gifu, Japan, called Kamiokande (Kamioka Nucleon Decay Experiment). In its latest version, called Super-Kamiokande, it is a cylindrical tank, 40 metres tall and 40 metres in diameter, filled with 50,000 tonnes of purified water. Hemispherical photomultiplier tubes lining its inside wall catch flashes of light produced when neutrinos are absorbed by the water. The main use of Kamiokande was to catch neutrinos originating from the Sun. Upon learning of Shelton’s discovery, the Japanese scientists searched their computer files and discovered an unusual burst of neutrinos at 07:35 GMT on 23 February, the day before the explosion had made the nova bright enough to be noticed. An American team operating a neutrino detector in a salt mine in Ohio then searched their records, and reported that their detector had recorded a burst of eight neutrinos at the same time – although these were mere whispers compared with the enormous scale of the explosion. The neutrinos from SN 1987A were the first from outside the Solar System to be discovered, providing remarkable insight into the conditions in the hitherto unseen interior of a supernova explosion.

  Neutrinos are by-products of the nuclear reactions that build up the heavier elements from hydrogen and helium inside stars. One of the main types of nuclei made in a Type II supernova is called nickel-56, which eventually becomes cobalt-56 and finally iron-56 through the process of radioactive decay, emitting gamma rays in the process. At first these gamma rays are absorbed by the expanding body of the exploding star. But eventually the explosion thins out and the gamma rays can escape. Gamma rays from SN 1987A were discovered by a satellite-mounted gamma-ray detector called Solar Max, which had been designed to study the Sun. It was only by chance that the satellite was in orbit at the time the supernova went off and that its design let it detect gamma rays from a completely different celestial object.

  As SN 1987A faded away it became easier to probe into the neighbourhood of the star, and astronomers realized that something else about it was interesting. In 1989 Joe Wampler discovered that the spectral lines of a small nebula were visible in the supernova’s spectrum. The spectral lines were confirmed by an orbiting satellite called IUE. The next year the Hubble Space Telescope imaged a ring that surrounded the supernova, and, in 1994, discovered that the ring was in fact a symmetrical, hollow, three-dimensional bipolar structure, shaped like two glass tumblers set bottom to bottom. This small nebula had been produced by the star that exploded as SN 1987A during previous phases of its life, some 20,000 years before the explosion witnessed by Shelton. By 1998 the HST was able to see that the exploding supernova was about to crash into this smaller nebula. By 2005 the whole of the central ring was involved in the crash, and the nebula lit up like a celestial firework display. One problem that SN 1987A threw up was that even thirty years after the event, no neutron star has appeared as the material from the supernova dissipated, as had been predicted. Possibly the supernova made a neutron star and then, after material immediately fell back onto it, the neutron star collapsed further to a black hole.

  Cepheid Variable Stars

  Beats of a star’s heart that measure the Universe

  As tho’ a star, in inmost heaven set,

  Ev’
n while we gaze on it,

  Should slowly round his orb, and slowly grow

  To a full face, and there like a sun remain

  Fix’d – then as slowly fade again,

  And draw itself to what it was before.

  Alfred, Lord Tennyson, ‘Eleänore VI’, 1832

  Cepheid variable stars have a regular ‘heartbeat’. Thanks to the work of pioneering female astronomer Henrietta Leavitt, this stellar heartbeat makes it possible to measure the size of the Universe.

  In 1784 English astronomer John Goodricke discovered that the star Delta Cephei was a variable star. It pulsates, expanding and contracting like a heart beating, the regular changes in size causing its brightness to increase and fade between magnitude 3.6 and 4.4. Goodricke estimated the period of its brightness cycle as 128 hours and 45 minutes: 5.36634 days.

  Delta Cephei became the prototype for explaining similar pulsating variable stars, which were subsequently given the generic name of ‘Cepheids’; other examples have periods that range from a few days to several hundred days. These stars are heat engines: they convert heat energy that they make in their interior into mechanical motion that moves their outer layers up and down. In the 1930s the English astronomer Arthur Stanley Eddington explained how this worked in theory; the details were confirmed by the Russian astronomer S. A. Zhevakin in the 1950s. Essentially, there is a valve mechanism in the star. The valve is closed when the star is small, causing pressure to build up and the star to expand. When the valve opens, heat and pressure are allowed to escape and the star shrinks back to its initial size.

  The ‘valve’ is a layer of ionized helium in the upper layers of the star. When the star is smallest, the helium is opaque and traps radiation. As the radiation increases, it lifts the ionized layer, so that the helium recombines and becomes transparent, releasing the radiation and causing the outer surface of the star to fall again.

  Cepheid variable stars are fascinating in themselves, but they are especially important in astronomy because they are ‘standard candles’ that can be used to measure the distance of other galaxies. Normally the brightness of a star is only a rough indicator of its distance, as stars’ brightnesses vary according to their sizes. But the special qualities of Cepheid stars make it possible to estimate their intrinsic brightness more precisely. One of the main projects of the Hubble Space Telescope, led by American astronomer Wendy Freedman, was to discover and measure the brightness of Cepheid variable stars in external galaxies in order to calculate their distances from the Earth and from each other. The ultimate goal of Freedman’s team was to measure the size of the Universe. Henrietta Leavitt’s discovery made this possible.

  Henrietta Leavitt was born in Lancaster, Massachusetts, in 1868 and died too early, of cancer, in 1921. She studied at the Society for the Collegiate Instruction of Women (later Radcliffe College and now part of Harvard University) and in her final year took an astronomy course, which enthused her about the subject. An illness left her deaf, but, after she had recovered, she became a research assistant at Harvard College Observatory at a salary of 30 cents an hour, working for its director Edward Pickering. It was not a time when women in the field were encouraged to carry out independent research, although Leavitt was undoubtedly of the highest intellect and capability. In her work for Pickering she discovered 2,400 new variable stars, doubling the number then known. Some 1,800 of them were found on photographs of the Magellanic Clouds taken at the Boyden Observatory then at Arequipa, Peru. Some proved to be Cepheid variables.

  In 1908 Leavitt discovered that the brighter Cepheids in the Magellanic Clouds took longer to complete their brightness cycles. Leavitt did not know that the Clouds were galaxies separate from the Milky Way, but she did reason that all the Cepheids in each Cloud were at the same distance from Earth, so their longer periods were not an illusion caused by distance but must somehow relate to their average light output. Leavitt’s discovery became known as the period-luminosity relation, and demonstrated that Cepheids were ‘standard candles’ that could be used to measure distances by comparing the apparent and intrinsic brightnesses of recognizable stars. Good standard candles are consistent (neighbouring stars of roughly similar brightness), highly luminous (for observation at great distance) and reliably recognizable. Cepheids are easy to pick out in the sky because of their variability. Once you have determined the period of a Cepheid, this Cepheid is the same average brightness as others of the same period. You need to anchor the relationship somehow. This has in the past been done by locating Cepheids in close clusters of stars whose distance you can find by other methods, but recent space experiments have been making it possible to triangulate directly to some Cepheid stars.

  After her untimely death, Leavitt’s work made it possible for modern astronomers to measure the size of the Universe and discover the location of objects within it. Astronomers Ejnar Hertzsprung and Harlow Shapley found that the Magellanic Clouds were outside the Milky Way galaxy, at distances currently estimated as 160,000 light years from Earth. The Andromeda galaxy – the nearest galaxy of the same size as ours – is 2.5 million light years from Earth. Wendy Freedman’s team has used Hubble Telescope observations of Cepheid variable stars to measure the distance of thirty-one galaxies, out to distances of 70 million light years. The distance calibrators for more distant galaxies can be referenced against this scale. However, although the accuracy of the Cepheid distance scale has been vastly improved over the past century, there remain worrying discrepancies in the scale of the Universe when the Cepheid distance scale is compared to the distance scale measured in other ways. The discovery of gravitational waves from merging black holes offers the exciting prospect of being able to help astronomers with the distance scale, because the distance of the mergers comes naturally out of the observations. But it replaces one problem with another: identifying which out of millions of galaxies the merger is taking place in, so as to determine its redshift to relate to the distance. The argument about the expansion scale of the Universe is not yet over!

  Exoplanets

  Other worlds beyond ours

  For there is a single general space, a single vast immensity which we may freely call Void; in it are innumerable globes like this one on which we live and grow.

  Giordano Bruno, On the Infinite Universe and Worlds, 1584

  Until a few years ago astronomers knew of one planetary system in the Universe – our own Solar System. This changed in the 1990s, with the discovery of the first of a number of large, Jupiter-sized planets orbiting central stars. Yet none of these exoplanetary systems seems to have formed in the same way as our own Solar System.

  For three thousand years philosophers had been theoretically convinced that there were other worlds like our own in existence. More recently, astronomers had realized that planetary systems are a necessary consequence of the formation of stars, and had detected proto-planetary discs orbiting young stars, for instance, in the Hubble Space Telescope images of the Orion Nebula. By the last decade of the twentieth century it was, in fact, becoming worrying that no actual planets had been found orbiting other stars.

  Finally, in 1992, Aleksander Wolszczan, a Polish-born American radio astronomer, was timing the rapidly rotating pulsar PSR 1257+12 and he noticed that its pulses alternately arrived earlier and later than expected. The pulsar was being pulled nearer to and further from Earth by three earth- and moon-sized planets in orbit around it. Wolszczan had discovered the first planetary system other than the Solar System. But Wolszczan’s system exists in circumstances completely different from our own: it is the remains of a supernova explosion in a binary star, and is a second-generation planetary system, formed not at the birth of a star but at its demise. The system’s central star was as unlike the Sun as it is possible to get.

  In 1995 two teams of astronomers discovered three planetary systems orbiting stars that were much like our own Sun. Two Geneva Observatory astronomers, Michel Mayor and Didier Queloz, made the first discovery. In April 1994 they h
ad embarked on a programme to detect any radial velocity variations in a list of 142 nearby Sun-like stars which could be due to the gravitational pull of jupiters (large gas planets). The analogy of our own Solar System guided the search. Although we typically say that the planets orbit around the central Sun, this is actually a simplification. In fact, the Sun and the planets orbit around their common centre of mass. Because the Sun is so much more massive than any planets, the Solar System’s centre of mass is actually inside it, so the Sun scarcely moves in its orbit – its motion of 13 metres per second is not much faster than world sprinting records and is more of a slow quiver than an orbit. This delicate motion of the Sun is mostly caused by the gravitational effect of the most massive planet in the Solar System, Jupiter. The main period of the Sun’s motion is therefore the same as the orbital period of Jupiter, namely twelve years.

  To find signs of this ‘quivering’ motion in other stars, Mayor and Queloz developed a spectrograph capable of detecting such small oscillations. They studied the 142 bright, nearby solar-type stars that had been selected because they showed, at coarser accuracy, no large velocity changes that would suggest they were members of double-star systems. It seems that planets survive only in orbit around single stars: astronomers calculate that a planet in a doublestar system would loop in complicated orbits among the two stars, and in a relatively short time would be ejected from the system.