Secrets of the Universe

by Paul Murdin

  Planetary nebulae are shells of illuminated gas that surround old stars. They were discovered by the English astronomer William Herschel, who in 1785 referred to the ‘planetary’, or ‘disc-like’, appearance of the object we now call NGC 7009, or the Saturn Nebula – it reminded him of the appearance of the planet Uranus. In 1790 he wrote that it was ‘a most singular Phenomenon! A star…with a faint luminous atmosphere, of a circular form’, adding that there could be ‘no doubt of the evident connection between the atmosphere and the star.’ He later discovered a similar nebula, NGC 6543, which also had a central star.

  Herschel speculated that planetary nebulae were ‘generating stars’ (that is, stars in the process of being born) and that ‘the further condensation of the already much condensed luminous matter may be complete in time.’ He was articulating what became known as the Nebular Hypothesis of the origin of stars. He was actually completely on the wrong track: planetary nebulae are formations associated with the end of the life of stars, not their birth.

  The gaseous nature of the nebulae was established by William Huggins. He became a pioneer in stellar spectroscopy after selling the family business at the age of thirty to pursue his interest in astronomy. In August 1864 he examined the planetary nebula in the constellation Draco (now identified as NGC 6543, or the Cat’s Eye Nebula) with his spectroscope. He saw a single green emission line, which was reminiscent of the spectral lines that he had seen in gas discharge tubes in his laboratory. He had discovered that planetary nebulae are made of low-density gas. Later he identified other spectral lines in the nebula, some of which were generated by hydrogen.

  In 1918 Lick Observatory astronomer Heber Curtis found that all planetary nebulae had central stars, which were visible if photographs could be taken at sufficient depths inside the nebulae. The stars are all white-hot, emitting copious amounts of ultraviolet light, which turned out to be key to answering the question of why the planetary nebulae shine. The mechanism was discovered by the Dutch astronomer Herman Zanstra in the 1920s, while he was a post-doctoral fellow. He showed that the amount of ultraviolet light emitted by the central star of a planetary nebula was identical to the amount emitted by the gaseous parts of the nebula: the energy from the star and the energy from the nebula matched. Every ultraviolet photon (light particle) emitted by the central star ionized one atom of hydrogen in the gas. When the ionized atom recombined, it emitted one photon of visible light.

  Zanstra’s theory accounted for the most abundant element in the nebulae, namely hydrogen, but other spectral lines remained unidentified. Lick Observatory astronomers William Campbell and James Keeler had identified some as being from helium soon after that element was discovered in the Sun in the late nineteenth century; Huggins had attributed other lines to a new element that he called ‘nebulium’. But Henry Norris Russell, the director of the Princeton Observatory, remarked on the failure to reproduce the ‘nebulium’ lines in the spectrum of any other material that had been investigated, and concluded that they ‘must be due not to atoms of unknown kinds but to atoms of known kinds shining under unfamiliar conditions. The suggestion is tempting that the nebular lines may be emitted only in gas of very low density.’

  The American physicist and astronomer Ira Bowen confirmed Russell’s guess in 1927 by discovering that in certain circumstances in space, common elements such as oxygen and nitrogen can emit spectral lines that they do not generate in the laboratory. Such spectral lines are described as ‘forbidden’. On Earth, oxygen atoms never get a chance to emit these spectral emissions because collisions with other atoms interrupt the process, but in space the time between collisions is longer and the atoms have sufficient time to release their radiation. The gradations of colour in the nebula illustrate the decrease in temperature of the gases away from the centre.

  The central stars of planetary nebulae are very hot, so they look rather faint because most of their energy is radiated as invisible ultraviolet. During the formation of planetary systems, the central stars are typically at the stage in their life cycles where they are transitioning from red giants to white dwarfs. The Ring Nebula (M57) is centred on a faint star that is on its way to becoming a white dwarf and has a surface temperature of 120,000 K.

  The curious shapes and colours of the dust clouds in planetary nebulae inspire fanciful names. The Hubble Space Telescope has a beautiful gallery of images of planetary nebulae (plates XXIII and XXIV). Some planetary nebulae, like the Ring Nebula (M57), look circular and may be spherical. Another explanation for the shape is that we are viewing it down the axis of a three-dimensional barrel-like structure. Other planetary nebulae have complex shapes, often with some sort of symmetry like an hourglass (these are called ‘bipolar nebulae’). Even some circular planetary nebulae like the Owl Nebula have double features (like the ‘owl’s eyes’) that suggest they are bipolar nebulae seen mostly end on. The Cat’s Eye Nebula (NGC 6543) is more complicated. It is basically a bipolar structure lying within a series of spherical shells, all centred on a star turning from a red giant to a white dwarf. When it was a red giant, the star ejected some of its body into space at 1,500-year intervals, like a dog shaking dry its coat after a swim. The Butterfly Nebula (NGC 6302) is a bipolar planetary nebula, the two ‘wings’ of the butterfly shape being the two lobes of the bipolar shape. Its central star is hidden by an equatorial disc, which, unusually, contains significant quantities of icy dust. It is a mystery how this cold material has survived for so long so near a star that appears to have a temperature in excess of 250,000 K. It could be that discs like this play a part in making the bipolar shape of planetary nebulae, throttling the outflow from the red giant stage of their central stars.

  The Origin of the Stars and the Planets

  The solar nebula, proplyds and planetesimals

  This world was once a fluid haze of light,

  Till toward the centre set the starry tides,

  And eddied into suns, that wheeling cast

  The planets…

  Alfred, Lord Tennyson, ‘The Princess’, 1847

  The discovery of the origin of the Sun and its Solar System, and of other stars and planetary systems, is a story of inspired guesses. The theoretical investigations of an international who’s-who of astronomers and astrophysicists dating back over 300 years were gradually proved by observations with spacecraft and detectors. The Sun and the Solar System formed from the collapse of part of a very dense, very cold cloud of interstellar dust and gas. The main part of the cloud condensed to become the Sun; orbiting lumps gathered surrounding gas and dust and grew into planets.

  The explanation of the origin of planetary systems, first put forward by the Swedish scientist Emanuel Swedenborg in 1734 and the Prussian philosopher Immanuel Kant in 1755, is called the ‘nebular hypothesis’. The nebular hypothesis was given support as a theory by the French mathematician and astronomer Pierre-Simon Laplace in 1796. He had proved that the Solar System was stable, with the orbits of planets oscillating by small amounts within their average values. The current shape of the Solar System, in which the planets all orbit in the same direction around a flat disc, reflects the way that it originally formed. In his book Exposition du système du monde (The System of the World), Laplace put forward the idea that the planets condensed out of a flat nebula that was whirling around the Sun.

  Laplace added, in support of his theory, the observations by the English astronomer William Herschel in 1786 of nebulae that showed a single star embedded in the centre of a nebula. Herschel interpreted these as planetary systems seen in an early stage of development. In a striking image, Laplace compared them to the saplings in a forest of mature trees. In fact, the objects discovered by Herschel were not embryonic planetary systems at all – the first such system was discovered by Caltech astronomers Eric Becklin and Gerry Neugebauer in 1966. They used a newly developed infrared detector in a laborious scan, point by point, of a region of the Orion Nebula. During this scan they found a strong source of infrared radiation. Invisib
le to the unaided human eye, the ‘BN Object’ is the size of a planetary system. The infrared radiation comes from dust shrouding a new-born star inside. The dust traps the star’s radiation and is heated to a temperature of about 700 K – at this temperature the dust radiates most strongly in the infrared spectrum.

  The InfraRed Astronomy Satellite (IRAS) discovered further examples of proto-planetary systems in 1983, including discs of warmed dust grains orbiting the stars Vega, Zeta Leporis and Beta Pictoris. The disc of Beta Pictoris was photographed by Paul Kalas and Dave Jewitt in 1996, using a special camera attached to a small telescope on Hawaii – the clear skies at this high-altitude site concentrated the starlight into a small area. The starlight in this small area was blocked by a central obstruction. This made it possible to see the faint light from the dust.

  The first direct images of planetary systems were made with the Hubble Space Telescope in 1992 by Robert O’Dell of Rice University and his colleagues. The images showed dust discs silhouetted against the luminous background of the Orion Nebula and other nebulae. O’Dell’s wife named the objects ‘proplyds’ – a contraction of ‘proto-planetary discs’. Their dust is concentrated in a disc rotating around a central star, just as Laplace visualized the solar nebula.

  The way that the central mass of a proplyd contracts as a proto-star was first calculated by the Japanese astrophysicist Chushiro Hayashi in 1960. The star system does not immediately settle down but goes through paroxysms during which it ejects a stellar wind in every direction. It might also squirt jets of material from its poles. The material is ejected as a result of the rapid spin that the star has acquired as it contracts from the interstellar cloud from which it has formed. The slow rotation of the proplyd suddenly increases, just as, in a final flourish at the end of their dance routine, an ice skater will spin faster as they bring their arms closer to their body. The ice skater then brakes by grinding her skates into the ice; the star brakes by throwing off material. This quickly clears out a lot of the nebula in the neighbourhood of the newly formed star. But before it is cleared away, the nebula makes planets, which are massive and not so susceptible to the scouring force of ejected material from the new-born star.

  The nebula surrounding the condensing star is mostly made up of hydrogen and helium. It contains dust grains that were made in old stars. These dust grains found their way into interstellar clouds, and thence into the nebulae orbiting the proto-stars – these are the dust grains discovered by astronomers in the BN Object, and imaged by the Hubble Space Telescope. Some of the material in the gas cloud assembles into molecules, which condense as ice on the surface of the dust grains. Still other material condenses into crystals. When the star switches on its nuclear reactions, it radiates energy, which melts the ice from the dust and blows away more gas in the inner, warmer parts of the solar nebula.

  The dust that is left collides during its revolution round the star and sticks together, consolidating into larger lumps, centimetres to metres in size. According to the calculations proposed in 1969 by the Russian theorist Viktor Safronov, and in 1973 by Caltech theorists Peter Goldreich and William Ward, the disc then condenses further, with the lumps flowing past one another in streams that merge like the wake water behind a boat. This forms lumps that are kilometres in diameter. These lumps are called ‘planetesimals’. Some of them survive in the Solar System as comets and asteroids. The gravity of the planetesimals is high enough to attract others – a large planetesimal is better able to do this. The more a planetesimal grows, the more its gravity increases, and the faster it grows – it is an accelerating process, called ‘accretion’. The bodies formed in this way are called ‘proto-planets’. Some of them survive as larger asteroids.

  Our inner Solar System once contained about a hundred proto-planets formed by accretion. The accretion process stops when the proto-planets have emptied their immediate neighbourhood of raw materials. Small fragments that are left over from all this are called chondrites – some fall to Earth from time to time as meteorites. Astronomers estimate their age by looking at the radioactive elements that are trapped in the meteorite material. This is the main way that astronomers establish the age of the planets, the Solar System and the Sun. It is a surprisingly accurately known number – 4.555 billion years old.

  The larger gas planets in the outer Solar System formed by the same process, although material in the more distant part of the gas cloud is less affected by the heat of the star. Jupiter grew the fastest and the most, and its gravity disturbed the orbits of the inner proto-planets, some of which jaywalked across the main stream or revolution of the others and collided. These collisions caused some proto-planets to aggregate with the terrestrial planets – Mercury, Venus, Mars, and the Earth and Moon. Other collisions shattered proto-planets, creating the most common type of asteroid. The fragments of shattered proto-planets rained down in the ‘Heavy Bombardment’ and made the large craters on the Moon and Mercury. One impact produced not one single planet, but a ‘twin planet’, the Earth and the Moon.

  Interstellar Dust

  Curtains of diamonds and graphite

  Until a person has thought out the stars and their interspaces, he has hardly learned that there are things much more terrible than monsters of shape, namely, monsters of magnitude without known shape. Such monsters are the voids and waste places of the sky.

  Thomas Hardy, Two on a Tower, 1912

  Imagine a cathedral, with a shaft of sunlight shining through the window. Specks of dust are floating in the sunlight. Then imagine the cathedral cleaned so scrupulously that there is only one speck of dust inside it. This represents the density of grains of dust that float in interstellar space, oxygen- and carbon-rich material expelled from supernovae and the interiors of red giant stars. There is not much dust in space – but there is a lot of space. The number of cathedral-sized volumes that stack one behind another in the line of sight to a star is very large, so the individual dust grains can accumulate to an opaque screen. Interstellar space is indeed filled with stardust.

  In 1847, the Prussian astronomer Wilhelm Struve, who was working at the Tartu Observatory in what is now Estonia, first proposed that something inhabited the space between the stars. He had discovered that the number of visible stars per unit volume in the Galaxy decreases with distance from the Sun. He inferred that the light from distant stars was being absorbed by something in space. Dutch astronomer Jacobus Kapteyn discovered in 1909 that bluer stars moved across the sky more quickly than redder ones. Fast-moving stars are on average closer than slower ones, so Kapteyn concluded that the red stars were more distant, and not only dimmed but also reddened by interstellar absorption, much as dust in the lower atmosphere of the Earth reddens the setting Sun. Similar work was carried out in 1930 by Robert Trumpler, then at the University of California’s Lick Observatory, on clusters of stars – he found that the clusters with smaller diameters are more distant than the larger ones, and fainter than their distances alone would account for because of interstellar absorption by dust.

  In the first two decades of the twentieth century, the American astronomer Edward Emerson Barnard carried out a systematic programme to photograph our Galaxy. In his atlas of the Milky Way he identified distinct dark ‘holes’ in the star clouds. Astronomers since William Herschel had known of the existence of these holes, and for a long time thought they were true voids in the distribution of stars. But Barnard discovered that the holes were ‘obscuring bodies nearer to us than the distant stars’ – dark clouds of unusually dense interstellar dust.

  These dust clouds concentrate towards the plane of the Galaxy, which is why the Milky Way appears to be cleft along its central line when we view it edge-on from Earth. One of the most prominent clouds lies in the Southern Cross and is called the Coalsack. In the culture of some Australian aboriginal peoples, the Coalsack represents the head of an emu defined by the straggling form of the Milky Way between Crux and Scorpio, a unique ‘constellation’ made of dark dust clouds rather th
an stars.

  Some of the dark clouds are very small. IC 2944 is a star-forming region in Centaurus. Silhouetted on the nebula are dense, opaque clouds of interstellar dust, first spotted by South African astronomer A. D. Thackeray from the Radcliffe Observatory, Pretoria, in 1950 and termed ‘Thackeray’s globules’. They may collapse to form proplyds or they may be shredded by intense ultraviolet radiation from the young, hot stars and disperse, rather than forming new stars themselves.

  Dust grains that lie near to bright stars can reflect starlight and form a ‘reflection nebula’. There is a prominent example in the Pleiades star cluster, whose stars illuminate a dark dust cloud that they encountered as they coasted through space. The nature of this nebula, the first reflection nebula found, was discovered by Lowell Observatory director Vesto Melvin Slipher in 1913, who observed the spectrum of the nebula and found that it was identical to the spectra of the brighter Pleiades stars. The Solar System originally formed from interstellar gas and dust. High temperatures destroyed most interstellar dust particles in the solar nebula, but some meteorites (known as carbonaceous chondrites) contain small particles whose composition is not the same as the rest of the meteorite, and which are thought to be interstellar grains. These were convincingly identified in 1987 by University of Chicago physicists Ed Anders, John Wacker and Tang Ming, and Washington University physicist Ernst Zinner, who isolated interstellar diamond and silicon carbide in meteorites by dissolving the rest of the meteorite in acid, a method referred to as ‘burning down the haystack to find the needle’.

  The Ulysses and Galileo spacecraft also detected interstellar grains in the Solar System, using a microphone to detect impacts by interplanetary dust particles. Venturing beyond Jupiter, Ulysses and Galileo encountered a higher than expected number of impacts coming from a particular direction in space and hitting the spacecraft at the same speed. They were from a stationary cloud of interstellar dust particles through which the Solar System is moving at 20 kilometres per second. Before this, it had been thought that the solar wind would stop interstellar dust grains from entering the Solar System, but we now know that the larger grains manage to break through.