Secrets of the Universe

by Paul Murdin

  Herschel’s son, John Herschel, made a special point of mapping the nebula very carefully in 1826 so that any changes could be proved in future observations. A decade later he remapped the Orion Nebula and found that his father had been mistaken about the changes: he saw no differences between the new map and the one he had made ten years earlier: John Herschel’s findings were confirmed when the first photographs of the Orion Nebula were made independently in 1880–83 by two pioneers of astronomical photography, the American amateur astronomer Henry Draper, and the English retired businessman Andrew Ainslee Common, who photographed the nebula to test a new technique. Draper made the first photograph of the Orion Nebula in 1880 in a 50-minute exposure. Common was an engineer and used his engineering skill to build a telescope that could track a nebula smoothly and accurately enough for its image to be recorded on the photographic emulsions available in the 1880s, which required long exposure times. His observatory in Ealing near London contained a 36-inch reflecting telescope. In 1885 it was bought by Edward Crossley, a member of the British parliament, and given by him to Lick Observatory ten years later, where it is still in use under the name of ‘the Crossley Reflector’. Common wrote about his photograph: ‘Although some of the finer details are lost in the enlargement, sufficient remains to show we are approaching a time when photography will give us the means of recording, in its inimitable way, the shape of a nebula and the relative brightness of the different parts in a better manner than the most careful drawing.’ Draper’s and Common’s images proved that the brightness of the Orion Nebula was unchanging; it is large and far away.

  Because the Orion Nebula is so bright, it has always been a natural target for people testing new technology. In 1864 William Huggins deployed his new astronomical spectroscope on the brightest nebulae. Instead of the continuous rainbow spectrum of colours that would indicate that the light from the nebula was similar to sunlight, and therefore made by stars, Huggins saw the individual spectral lines of a glowing gas. Astronomers had speculated that all misty ‘nebulae’ would eventually be found to be a mass of stars like the Andromeda galaxy or the Pleiades. Huggins’s result showed that this was not true – some nebulae, like the Orion Nebula, were truly gaseous, although they may contain a few individual stars.

  Huggins was especially puzzled by a strong green line in the nebula’s spectrum. Between 1880 and 1889, he and his wife Margaret persistently tried to photograph the spectrum, and discovered that the strong green line did not have a wavelength that coincided with any known terrestrial element. The Hugginses incorrectly attributed the line to a new cosmic element, which they called ‘nebulium’. The light represented by the mysterious line gives the nebula a green colour when it is viewed by the naked eye, although photography or the more modern electronic detectors show the nebula as reddish, because the technological processes emphasize the red light generated by burning hydrogen more than the human eye does.

  The Orion Nebula contains not only gas, but also copious amounts of dust. One especially dusty region shows as a protrusion of dark material into a bright nebula near to the Orion Nebula that is illuminated by the star σ (sigma) Orionis. The dark nebula and its distinctive horse-head shape were discovered in 1888 by Williamina Fleming. She had emigrated to the USA in 1878 from her native Scotland, and after her husband abandoned her with a young child, supported herself by working as a maid in the home of Edward Pickering, the director of the Harvard College Observatory. Eventually Pickering asked her to take on clerical work at the observatory, and then promoted her to scientific assistant. The story is that Pickering exhorted his male assistants to achieve more with the admonition ‘My maid could do better!’ and then found that, indeed, she could. Fleming discovered the Horsehead Nebula as a dark indentation recorded on a photograph taken by Edward’s brother, William Pickering.

  The Orion Nebula and the Horsehead Nebula are just two features in a vast complex of dust, gas and stars that eventually became known as the Orion Giant Molecular Cloud. It is 1,500 light years across and covers the entire constellation of Orion. The bright object that Herschel and the Hugginses had seen is only a hollow dent on the surface of the enormous cloud, like the white flesh of an apple left after the first bite from its red surface. The inner surface of the dent on the surface of Orion’s Cloud is illuminated by the four Trapezium stars.

  The concept of ‘giant molecular clouds’ (GMC) was proposed in the 1970s by radio astronomers who mapped the radio emission from molecules inside the Orion cloud. The radio emission was first detected in 1963, when Sander Weinreb of the National Radio Astronomy Observatory at Greenbank, West Virginia, and MIT physicist and engineer Alan Barrett identified the presence of the hydroxyl molecule (OH) in the cloud. Over the next few years, ammonia (NH3), water (H2O), formaldehyde (H2CO) and carbon monoxide (CO) were also detected. All these chemicals are commonly found on Earth, where they would normally be broken down by light from the Sun, but in the Orion GMC the massive dark clouds had shielded the molecules from the destructive effects of starlight, allowing them to survive in large quantities. These molecules also play a key part in the evolution of the cloud, and end up in the by-products of the formation of stars – comets and planets – as ‘seeds’ that develop, it seems, into the chemical building blocks of life.

  How do astronomers know that there are stars inside the dark cloud? The dust of the Orion GMC can be very dense and hides the light of most of the stars inside it. Infrared radiation can penetrate the dust, and following the development of the technology in the mid 1960s, astronomers have been able to detect infrared radiation from the hidden stars. Pioneers like Eric Becklin, Gerry Neugebauer, Frank Low and Douglas Kleinmann discovered individual sources of infrared by laboriously scanning the cloud with single detectors in raster scans, just like the survey of the sky that William Herschel made with his conventional telescope.

  These early infrared readings were constantly hampered by the abundant infrared radiation emanating from the Earth itself, but in the 1980s the US military declassified a range of infrared detectors that had been developed during the Cold War to identify the warm engines of approaching vehicles and rockets, and which were capable of taking pictures. Astronomers eagerly exploited their ability to see warm objects in the sky, using them not only in ground-based telescopes but also in satellites, such as IRAS (InfraRed Astronomy Satellite, launched 1983), ISO (Infrared Space Observatory, 1995), SIRTF (the Space Infrared Telescope Facility, later renamed the Spitzer Space Telescope, 2003) and the Herschel Space Observatory (2009). The satellite-mounted detectors operate in the cool of space, carefully avoiding interference from the Earth’s and Sun’s infrared radiation.

  These developments have made it possible to see that the Orion Giant Molecular Cloud, and others like it, contain thousands of stars, which recently formed inside the cloud. Approximately two thirds of these stars are orbited by planet-forming discs – new Solar Systems in their infancy. Herschel’s prescient description of the Orion Nebula as ‘the chaotic material of future suns’ could be extended in modern times to ‘the chaotic material of future suns, planets and life itself’.

  Star Clusters

  Nebulae resolved

  Many a night I saw the Pleiads, rising thro’ the mellow shade,

  Glitter like a swarm of fire-flies tangled in a silver braid.

  Alfred, Lord Tennyson, ‘Locksley Hall’, 1842

  With the invention of the telescope, certain individual stars and nebulae that had been known since antiquity were revealed in fact to be dense clusters of stars. As astronomers mapped these clusters and elucidated their properties, they began to comprehend the astounding size of our Galaxy and to understand how stars age and, eventually, die.

  Some star clusters have been known since antiquity. The Pleiades and the Hyades are readily visible in the constellation of Taurus and are mentioned in the Iliad and the Old Testament. The Praesepe star cluster, known as ‘the Beehive’, was described by the third-century BCE Greek poet Arato
s and by the second-century BCE astronomer Hipparchus. When Galileo turned his telescope on the sky in 1609 he looked at the Praesepe and discovered that it was a star cluster: ‘The nebula called Praesepe is not one star alone, it is a mass of more than forty small stars.’

  Praesepe, the Pleiades and the Hyades are all star clusters in our Galaxy, loose conglomerations of perhaps a few hundred or a few thousand stars. Other star clusters are more densely packed and spherical, containing hundreds of thousands or even millions of stars, orbiting around and through our Galaxy. These dense, mobile clusters are called ‘globular clusters’. The discovery of the first globular cluster was reportedly made in 1665 by the obscure German amateur astronomer Johann Abraham Ihle. In 1677, during an expedition to Saint Helena, Edmond Halley discovered one of the nearest and largest globular clusters, best visible from the Southern Hemisphere and known as Omega Centauri (plate XIII). This designation takes the form of the name of a star, which is how it appears to the naked eye, perhaps a slightly fuzzy one. Describing it as a ‘lucid spot or cloud’, Halley could not resolve the cluster into individual stars with his telescope.

  William and Caroline Herschel found many ‘nebulae’ during their ‘sweeps’ of the sky, and with their superior telescopes identified a proportion of the ‘nebulae’ as star clusters. William classified their shapes, labelling some of them as ‘globular’. His son John Herschel noticed that the globular clusters were not uniformly distributed across the sky, but were concentrated towards the constellation Sagittarius. In 1909 the Swedish astronomer Karl Bohlin deduced from this that the globular clusters surround the centre of the Galaxy, which also lies in the direction of Sagittarius. The American astronomer Harlow Shapley used Henrietta Leavitt’s method for determining the distance of variable stars in the clusters to map their three-dimensional distribution. In 1918 he estimated the distance to the centre of the globular cluster system as some 60,000 light years, and the diameter of the whole system of globular clusters as 300,000 light years. Although these values are twice the currently accepted estimates, Shapley had discovered that our Galaxy is astoundingly large – much larger than Galileo and the Herschels had ever imagined.

  The origin of the globular clusters is enigmatic. They seem to be a mixture of star clusters that are native to our own Galaxy and others that have fallen into our Galaxy when it absorbed galaxies that it encountered at random in intergalactic space.

  By contrast to the neat spherical look of the globular clusters that orbit high above the plane of our Galaxy, straggly, irregularlooking star clusters congregate in the plane of the Galaxy near its spiral arms. The early observers could not know that these clusters would be the keys that unlocked the mystery of how stars evolve. Star clusters allow astronomers to develop techniques for comparing different stars, because they eliminate many of the usual uncertainties and differences from star to star. All the stars in a given cluster are at the same distance from the Earth, so the light of each is dimmed to the same extent by distance. All the stars were formed at the same time from a single gas cloud. They are of the same age and composition, but have different masses.

  On the basis of these assumptions, the Danish astronomer Ejnar Hertzsprung was able to discover between 1907 and 1911 that the brightnesses of most of the stars in a cluster correlate to their temperatures. He found that in both the Pleiades and the Hyades the brighter stars were hotter (blue dwarfs) and the fainter ones were cooler (red dwarfs). Henry Norris Russell noticed the same correspondence in nearby stars. It transpired that the hot, bright stars were the more massive ones and the cool, dim stars less massive. In old clusters the bright blue stars were missing, and there were bright red stars instead (red giants and supergiants): the blue stars became bright red stars as they aged. In even older clusters many of the red giants had died and become white dwarfs, neutron stars or black holes

  Putting all the different clusters in order of age, the situation became clear. The bright, massive stars aged quicker than the dim, less massive ones, becoming red giants and then dying faster than the others. Massive stars have more hydrogen fuel than the less massive ones, but they burn it much faster, and consequently run out of energy and die earlier, just as a profligate millionaire may go bankrupt more quickly than a poor miser.

  The internal structures of the different kinds of stars were first calculated by Arthur Stanley Eddington and James Jeans between 1916 and 1924. In the 1950s and 1960s, with the advent of the first electronic computers, it became possible to link these calculations together to track how stars changed from one kind to another as they aged. The results could then be used to date stars in different star clusters, each cluster consisting of an array of stars of all possible masses, but each star having reached a different stage in their life history.

  For decades, star clusters were the main way of verifying theoretical calculations about the size, composition and life cycles of stars. There was no way to see past the surface of a star in order to study what was really happening inside it. This changed in the 1970s with the discovery of helioseismology and solar neutrinos, which proved that these calculations had been remarkably accurate. Star clusters were the ancient keys to a modern problem, keys with which astronomers were first able to unlock the opaque outer layers of the stars in order to discover their secrets.

  Supernovae

  Origins of the stardust from which we are made

  All these stupendous objects are daily around us; but because they are constantly exposed to our view, they never affect our minds, so natural is it for us to admire new, rather than grand objects. Therefore, the vast multitude of stars which diversify the beauty of this immense body does not call the people together; but when any change happens therein, the eyes of all are fixed upon the heavens.

  Saint Basil the Great (Bishop of Caesarea), fourth century CE

  Before the sixteenth century, stars were thought to be fixed and eternal. We now know that stars are in constant flux, undergoing a cycle of birth, death and rebirth. The roadside observations of a sixteenth-century nobleman and the striking pictures beamed to earth by the Hubble Space Telescope since 1990 bear vivid witness to supernovae, the explosive collapses of dying stars. This process generates the basic elements that make up all of the matter in the universe, including the building blocks of our own bodies.

  One winter’s evening in 1572, the Danish nobleman Tycho Brahe was returning home in his carriage when he was struck by something unusual in the night sky. It looked like a new star. Brahe halted his carriage and asked passing peasants to confirm what he had seen:

  On the 11th day of November in the evening after sunset, I was contemplating the stars in a clear sky. I noticed that a new and unusual star, surpassing the other stars in brilliancy, was shining almost directly above my head; and since I had, from boyhood, known all the stars of the heavens perfectly, it was quite evident to me that there had never been any star in that place of the sky, even the smallest, to say nothing of a star so conspicuous and bright as this. I was so astonished at this sight that I was not ashamed to doubt the trustworthiness of my own eyes. But when I observed that others, on having the place pointed out to them, could see that there was really a star there, I had no further doubts.

  The ‘new star’, or ‘nova’ (short for nova stella), had in fact been noticed as early as 6 November 1572 by others, including Francesco Maurolico, a star-gazing Sicilian mathematician, and Hieronymus Muñoz, a Spanish philosopher, who saw the nova while giving an open-air evening class. Astronomers Michael Mästlin of Tübingen, Thomas Digges of Kent and Brahe himself all measured the position of the nova by noting that it lay on the intersection of the lines joining certain pairs of stars. They showed that the new star did not move through space (as a comet would), nor did its position in the sky change when the Earth’s rotation changed the position of the person observing the new star. This proved that the nova was a long way from Earth, among the fixed stars. Brahe wrote: ‘I conclude, therefore, that this star is not some ki
nd of comet or a fiery meteor, whether these be generated below the Moon or above the Moon, but that it is a star shining in the firmament itself – one that has never been seen since the beginning of the world.’

  This momentous discovery challenged the ancient Christian and classical concept that the stars were eternal and unchanging. In the cosmology of philosophers such as Aristotle, the orbits of the Moon and Sun marked the boundary between the changeable and permanent parts of the universe. The Moon had phases, the Sun had spots, but the stars were the same forever. The Earth was imperfect and harboured misery, disease and sin, but the stars were perfect and pure, the home in heaven for the blessed saints. Brahe’s new star was irrefutable evidence that this worldview was wrong.

  The star was in fact a supernova. This word was coined by the Swiss-American astronomer Fritz Zwicky in 1931 when he discovered that there were some new stars that were much brighter than others. These new stars were releasing colossal amounts of energy, their brightness making the surrounding galaxy appear dim in comparison.

  A supernova does not represent the birth of a new star, but the destruction of an old one. Stars are in constant balance between two opposing forces: the downward force of gravity and the upward force of pressure generated by the heat and density within the star. Nothing turns off the downward force of gravity. However, in ordinary stars, the amount of upward pressure depends on the energy generated by nuclear processes within the star. Stars contain a finite amount of nuclear fuel, and eventually the fuel gives out. The exhausted star may stabilize as a white dwarf, but some stars are too massive to stabilize. If the internal pressure can no longer support the structure of the massive star, the star collapses and releases a lot of energy in freefall. It is this energy that causes the star to explode and become visible as a ‘new star’ – where the old star had been too dim to be noticed.