Secrets of the Universe

by Paul Murdin

  Some of these interstellar particles have been brought back to Earth by the Stardust spacecraft, which between 2000 and 2004 deployed a sticky gel in interplanetary space and at Comet 81P/Wild (also known as Wild 2, pronounced ‘Vilt Two’) and returned to Utah in January 2006, so that the material that had been collected on the gel could be analysed. Most particles were from the comet, but some have proved to be interplanetary and interstellar grains. The types of interstellar dust that have been discovered include tiny diamonds and larger graphite grains that are made in supernovae, as well as silicon carbide, aluminium oxide, spinel and titanium oxide grains made in the atmospheres of red giant stars before they turn into planetary nebulae.

  Interstellar grains are little factories where molecules are built in space. They have sticky surfaces, with atomic-scale electrical charges that provide attractive ‘hooks’ that latch onto passing atoms in the interstellar gas. When two such atoms – for example two hydrogen atoms (H + H) – are brought together side by side on the surface of a grain, they may loosen their hold on the grain and clutch each other to make a molecule – for example molecular hydrogen (H2) – giving up excess energy to the grain. The hydrogen molecule will then drift off the dust grain and, eventually, become part of a giant molecular cloud, and participate in the formation of stars and planetary systems. Like termites in the vast areas of the savannah, interstellar grains, small but numerous as they are, play an important part in the ecology of space.

  XVI The Cosmic Microwave Background. The Planck satellite made this image of the remnant fireball of the Big Bang.

  XVII The star Fomalhaut is blocked out in this image by the Hubble Space Telescope, so that we can see the dusty disc that surrounds it and the planet that orbits it, shown at four positions that span eight years of its 2,000-year orbital period.

  XVIII Cassiopeia A. This remnant of a supernova that exploded 300 years ago emits light (Hubble Space Telescope image, orange), infrared (Spitzer Space Telescope, red) and X-rays (Chandra X-ray Observatory, blue and green).

  XIX The Millennium Simulation. Matter, dark matter and dark energy are mixed in a computer calculation to show how they evolve and generate galaxies, linked together in a ‘cosmic web’.

  XX Saturn’s rings. The orbiting particles that constitute Saturn’s rings are colour-coded by size (green less than 1 centimetre, purple from 5 centimetres up to several metres across).

  XXI Gravitational waves. Two orbiting neutron stars coalesced in 2017 and emitted a spiralling pattern of gravitational waves, detected by the LIGO interferometer (left). The matter from the neutron stars is shown right, ejected in a more chaotic pattern.

  XXII The Orion Nebula. New-born stars embedded within a giant molecular cloud have blown a cavity whose inner surface they illuminate. Atoms of gas glow with spectacular colours.

  XXIII The Helix Nebula. This dramatic planetary nebula is centred on the star that produced it. The star is making the transition from red giant to white dwarf.

  XXIV MyCn18. This young planetary nebula is shaped like an hourglass with an intricate pattern of ‘etchings’ in its walls.

  XXV The Antennae. Two galaxies have collided, with their gas surging together and forming a sparkling starburst of bright new stars.

  XXVI The Whirlpool Galaxy, M51. The first galaxy in which the characteristic spiral shape was recognized has a small companion at the end of one spiral arm.

  XXVII Centaurus A. Lobes and jets emanate from the galaxy’s central black hole. The image is a composite of images obtained with infrared (orange), X-rays (blue) and light (true colour).

  XXVIII A cloud of electrons surrounding the black hole in M87 emits radio waves, which have been pictured in a magnified image by the Event Horizon Telescope. The event horizon shadows some of the radio waves in this, the most detailed image yet taken of a black hole.

  THE UNIVERSE AND ITS GALAXIES

  Hydrogen

  The most abundant element in the Universe

  Credit: NASA Ames

  Carl Sagan, Frank Drake and Linda Salzman Sagan, Plaque from the Pioneer 10 spacecraft, 1971

  Hydrogen is the most abundant element in the Universe. It was made in the Big Bang, and condensed to form large-scale gaseous structures, which in turn produced galaxies and stars. Our own Galaxy contains hydrogen both in stars and in interstellar space. Under the shadow of Nazi occupation, a group of Dutch astronomers worked in secret to identify the radio signature of this interstellar hydrogen. After the war, their discoveries would make it possible to map the entire Galaxy.

  There are three types of interstellar hydrogen in space. One type (called ‘H II’) is ionized hydrogen, which is present in areas where stars are forming and becomes visible to the naked eye when it is excited by ultraviolet light emitted by hot stars. By locating clouds of ionized hydrogen, William W. Morgan, Stewart Sharpless and Donald Osterbrock were able to plot the location of nebulae in a 1951 map of the Galaxy, which was the first to show its structure and scale.

  Outside the nebulae, away from the stars that are sources of ultraviolet light, are vast amounts of a cool, invisible form of non-ionized hydrogen gas called ‘H I’, or neutral hydrogen. The existence of neutral interstellar hydrogen was confirmed by radio astronomers in the 1950s, following a remarkable prediction made by a group of astronomers who met in secret during the Nazi occupation of the Netherlands. Their work was published only after the Second World War ended in 1945.

  When Germany invaded the Netherlands, Hendrik van de Hulst was an astronomy student studying under the astrophysicist M. Minnaert in Utrecht. Minnaert was sent to a detention camp for the duration of the war after protesting the treatment of his Jewish academic colleagues and Van de Hulst fled to Leiden, where he studied under Jan Oort. Oort encouraged his astronomical students to concentrate on theoretical studies while the war was under way, since observing with telescopes at night while the curfew was in effect caused suspicion on the part of the occupying forces, and was therefore highly dangerous. Through Bart Bok, a Dutch astronomer working in the USA, copies of Grote Reber’s papers on radio astronomy had been smuggled into the country, and in the spring of 1944 Oort decided to hold a colloquium on Reber’s findings.

  Although the occupying forces had banned most public gatherings, to counter conspiracy and resistance, Oort arranged a meeting of the Astronomenclub (Dutch Astronomy Club), where Van de Hulst presented calculations to support his theory that hydrogen emits radio waves that have a distinct signature. He chose to study hydrogen because it was the most abundant element in the Universe.

  Hydrogen atoms consist of an electron that is in orbit around a proton. Both the electron and the proton have a spin, and the axes of the spins can be parallel or antiparallel. Van de Hulst found that if the electron flips spontaneously from a parallel to an antiparallel spin, the hydrogen atom emits a pulse of radio waves that have a wavelength of 21 centimetres. Although these electron ‘flips’ happen only once every 11 million years in an individual hydrogen atom, the number of hydrogen atoms in space is so large that a hydrogen cloud produces measurable amounts of 21-centimetre radiation every second.

  After the war had finished, Oort and his colleague Lex Muller attempted to prove Van de Hulst’s theory by isolating the 21-centimetre radiation with a radio receiver, but suffered a setback when their equipment in Kootwijk was destroyed in a fire. The spectral line from the hydrogen radio emission was finally discovered in 1951 by American radio astronomers Harold Ewen and Edward Mills Purcell (later a Nobel laureate for his work on the fundamental physics of the hydrogen atom), using a radio telescope built at weekends by Purcell with a grant of only $500 (equivalent in purchasing power to about $5,000 today). This discovery was confirmed by Oort and Muller after they repaired their receiver, and subsequently reconfirmed by Australian radio astronomer Frank Kerr in Sydney. All the discoveries were published together as a set of three papers in the magazine Nature.

  The first maps of the Galaxy, drawn according to Oort’s pres
cription, were made by Van de Hulst, Muller and Oort in 1952. In 1958, Oort, Kerr and Dutch-American astronomer Gart Westerhout mapped virtually the entire Galaxy by combining observations of hydrogen radio emissions from astronomers in Australia and the Netherlands. They correlated the neutral hydrogen with the hot stars and nebulae mapped by Morgan, Sharpless and Osterbrock and confirmed that our Galaxy has spiral arms. Its structure is similar to the spiral galaxy M51 (plate XXVI), sketched in 1845 by William Parsons, the Third Earl of Rosse, as he viewed it with his 6-foot telescope, then the largest in the world and known as ‘the Leviathan of Parsonstown’.

  The third form of hydrogen found in interstellar space is in the form of hydrogen molecules (H2), two hydrogen atoms joined together. It was considered likely to be common as far back as the 1930s, long before it was actually discovered. The molecule was observed first with space telescopes since its prominent signatures are masked from ground-based telescopes by molecules in the Earth’s atmosphere. It was first seen in 1970, using instruments borne aloft on rockets. In 1972, the Copernicus satellite detected interstellar molecular hydrogen. Further advances in its study were made by the Far Ultraviolet Spectroscopic Explorer (FUSE, active between 1999 and 2007) and the Hubble Space Telescope. Molecular hydrogen is the most abundant molecule in interstellar space, and accumulates in Giant Molecular Clouds, the places within which new stars and planetary systems are born.

  Hydrogen’s 21-centimetre radio radiation is a universal phenomenon that would presumably be known to any scientific civilization on other planets in the Galaxy. It has thus featured in attempts to communicate with extraterrestrial beings. It has several times been chosen as the radio frequency with which to set up a radio link to such beings – one has to choose one wavelength out of the almost infinite number of possibilities in the radio spectrum, and the 21-centimetre wavelength has been reckoned as the one with the greatest cosmic significance. Additionally, this wavelength was used in the attempt to communicate with extraterrestrial beings through what amounted to sending a letter through the post, using the Pioneer 10 and 11 spacecraft as mail vans. Launched in 1972 and 1973 respectively, they are currently sailing out of the Solar System into interstellar space, carrying plaques with pictorial messages. The intention is that any extraterrestrial beings who might find a spacecraft would be able to get an indication of who made and launched it. The plaques (the design is reproduced at the head of this chapter) depict the location of the Solar System in the Galaxy, relative to a number of pulsars, and show that the spacecraft originated on the third planet from the Sun, having flown past the fourth, fifth and sixth planets. The plaques also depict two people, a man and a woman, the man greeting the finders and the woman passively looking on (the plaque was conceived at a time when feminism had not penetrated very deeply into NASA). The scale of the humans is indicated by the spacecraft and by a diagram of an atom of hydrogen (in the upper left corner of the plaque) emitting 21-centimetre radio waves as it flips its spin axis.

  Galaxies

  Ellipticals, spirals, mergers

  I see beyond this island universe,

  Beyond our Sun, and all those other suns

  That throng the Milky Way, far, far beyond,

  A thousand little wisps, faint nebulae,

  …

  Faint as the mist by one bright dewdrop breathed

  At dawn, and yet a universe like our own;

  Each wisp a universe, a vast galaxy

  Wide as our night of stars.

  Alfred Noyes, ‘William Herschel conducts’, 1922

  Galaxies are in constant motion, speeding outwards as the Universe expands, occasionally colliding with each other and changing shape. Thousands of distant galaxies – representing only a fraction of the total in the Universe – were captured in an astonishing set of images called the Hubble Deep Fields, which reach to the frontier of the visible Universe.

  Elliptical galaxies are smooth and featureless. In three dimensions they are triaxial ellipsoids – aspherical balls with unequal sizes along the three planes of symmetry. Spiral galaxies are flat discs with spiral arms. Lenticular galaxies are a transitional type between spirals and ellipticals. Irregular galaxies are small, without a clear shape, or they are larger and appear to be two galaxies passing close to each other and disrupting any regular shape (plate XXV). In other interactions, a small galaxy might be absorbed by another.

  Mergers like this are going on now in our own Galaxy. The Sagittarius Dwarf Elliptical Galaxy was discovered in 1994 by Cambridge astronomers Rodrigo Ibata, Mike Irwin and Gerry Gilmore, who noted an excess of faint stars grouped just above the plane of our Galaxy at a distance of about 70,000 light years. The Milky Way has disrupted this galaxy into a stream of stars that loop in orbit over the pole of our Galaxy and are merging with it. Our spiral Galaxy has grown by several such mergers in the past, and if it merges in the future with another spiral, it seems likely that both will lose all their spiral features and make an elliptical galaxy – in fact, astronomers James Binney and Scott Tremaine discovered in 1987 that the Milky Way galaxy will collide with and merge with the Andromeda spiral galaxy, M31, in about 2–5 billion years. Our Sun will likely be a red giant at that time. Its fate after the merger is unclear: it may well be flung into very lonely intergalactic space.

  Edwin Hubble did not only classify galaxies, but also discovered their distances from Earth and determined how they were moving. With the 100-inch Mount Wilson telescope, he observed Cepheid variable stars in spiral galaxies, which told him their distances, far outside the Milky Way. He used measurements by Vesto Melvin Slipher of the speed of forty-six galaxies and in 1929 he discovered they were, in general, receding – moving away from our Galaxy. The galaxies’ speeds were proportional to their distances. There was some scatter (the Andromeda Galaxy, for example, is approaching us, which is why it will merge with our Galaxy soon), but, according to Hubble’s determination, the trend line showed that a galaxy at a distance of 3 million parsecs was receding at 500 kilometres per second. This is a factor of 8 too high, according to modern calibrations of the measurement of the distance of galaxies by Cepheids, but the principle remains: the Universe of galaxies is expanding, and the trend line is called Hubble’s Law. It was the first indication that the Universe exploded in a Big Bang.

  The most distant galaxies studied are radio galaxies and the galaxies in the Hubble Deep Fields and similar surveys. In 1995 the Hubble Space Telescope stared at an almost empty area of the Northern Hemisphere sky for ten days to take the deepest picture of the sky obtained up to that time. It did the same thing in 1998 with a similar area in the southern sky, to confirm whether the Universe in that direction was the same as in the north – it was. It capped its own efforts several times with additional deep fields made with cameras that were more sensitive, and sensitive to different radiations. These, the deepest (most sensitive) astronomical pictures ever made, contain the images of several thousand galaxies. Some were formed less than a billion years after the Big Bang.

  The Hubble Deep Fields showed that, in general, galaxies as young as a billion years – the most distant galaxies which the photographs had discovered – are smaller than galaxies are today, 14 billion years after the Big Bang. This is because most galaxies around today have merged with others at some time in their lives – galaxies are now fewer but bigger. The galaxies in the Hubble Ultra Deep Field are also less symmetrical. They are younger and have not settled down to a steady state; they are seen in gawky adolescence rather than mature equanimity.

  Not much further than the distance of the faintest galaxies in the Hubble Deep Fields, there are none visible. This time is called the Dark Ages of the Universe. Astronomers believe that galaxies exist beyond this point, but are cloaked in dust that hides the light of the stars inside. The dust is warmed by the stars and emits infrared and millimetre radiation, and so the galaxies show up only as faint sources of infrared or millimetre radiation. Several such extremely faint sources have been discovered
by the SCUBA cameras on the James Clerk Maxwell Telescope on Mauna Kea, Hawaii. They are so faint that it is extremely difficult to discover much about their properties, even with the Hubble Space Telescope. But the prospects are favourable. Just as the Hubble Space Telescope was targeted at investigating optically visible galaxies, the James Webb Space Telescope (scheduled to be launched in 2021) is aimed at elucidating the nature of their predecessors.

  Magellanic Clouds

  Our neighbour galaxies

  [Our ship was at 37 degrees south latitude when] we sawe a marueylous order of starres, so that in the parte of heauen contrary to owre northe pole, to know in what place and degree the south pole was, we tooke the day with the soonne, and obserued the nyght with the Astrolabie, and sawe manifestly twoo clowdes of reasonable bygnesse mouynge abowt the place of the pole continually now rysynge and nowe faulynge, so keepynge theyr continuall course in circular mouying, with a starre euer in the myddest which is turned abowt with them abowte xi degrees from the pole.