Secrets of the Universe

by Paul Murdin

  Of course, astronomy moved on and clearer examples of black holes were discovered, in particular through techniques involving the detection of gravitational waves.

  Distances of the Stars

  The radiance of that star was shining years ago

  Were a star quenched on high,

  For ages would its light,

  Still travelling downward from the sky,

  Shine on our mortal sight.

  Henry Wadsworth Longfellow, ‘Ode to Charles Sumner’, 1875

  When we look at distant stars in the night sky, we are actually looking into the past. Since antiquity, astronomers knew that if a star was observed regularly from the same position, it ought to appear to move very slightly each year. They sought to discover these small movements to measure the distances of the stars from Earth. Most stars are so far away that their light takes years to reach the Earth.

  Efforts to calculate the distances of the stars from Earth date back more than 2,000 years. The Greek astronomer Aristarchus of Samos in the fourth century BCE and Copernicus in 1543 CE both realized that if the Earth moves around the Sun, the fixed stars should move in a reflection of the Earth’s motion. This is manifestly not the case – the stars move very slightly, but nowhere near as much as the Sun’s rising and setting. Both men came to the same conclusion: the stars are much further away than the Sun, and consequently the radius of the Earth’s orbit is negligible compared with the distance of the stars.

  The apparent movement of a star due to the Earth’s motion around the Sun is called the star’s annual parallax. ‘Parallax’ means the apparent shift of something due to the motion of the observer. Hold your finger up at arm’s length, and keep it still, but move your head from side to side. The finger moves against the background. The angle by which it moves is its parallax.

  In 1580 the Danish astronomer Tycho Brahe built a vast pre-telescopic sighting instrument, called the Great Mural Quadrant, at his Uraniborg observatory on the island of Hven to measure star positions. It was mounted on a wall built precisely north–south to measure the altitude of stars as they passed due south. It had a brass measuring scale with a 2-metre radius, and was at the time the most accurate instrument ever built to measure star positions, but even Brahe could not determine the parallaxes of stars, as they were so far away. He concluded that the stars were more than 700 times more distant than the Sun.

  Dutch scientist Christiaan Huygens took a different approach. If the Sun is a star and all stars are the same brightness, then the reason why the stars are so much fainter than the Sun is that they are further away and their light is diminished by distance – in fact by the amount of their distance squared. Huygens thought that if you could measure the relative brightness of the Sun and a star such as Sirius, then by finding the square root of the difference you could calculate the relative distances of the Sun and the star. Huygens tried to measure the brightness of Sirius relative to the Sun by covering the Sun’s face with a card pricked with different-sized holes. He matched the appearance of sunlight through the smaller holes with Sirius. His estimate, published posthumously in 1698, was that the distance of Sirius is 27,664 times the distance of the Sun.

  This is a difficult measurement to make because the contrast in brightness between the Sun and Sirius is so great. The Scottish mathematician James Gregory tried a variation on Huygens’s technique in 1668 by comparing the brightness of Sirius to that of a planet. He chose a time when a given planet was at its greatest distance from the Earth and roughly the same brightness as the star. He then waited until the planet was much nearer the Earth, and bright enough to compare with the Sun. He used his knowledge of the planet’s distance to link together all his measurements. Using this approach, Gregory estimated the distance of Sirius at 83,190 times the distance to the Sun. Isaac Newton likewise calculated the distance to a typical bright star as 1 million times the distance of the Sun, but regarded this number (which was in fact much closer to modern estimates) as controversial and did not pursue the topic to further conclusions.

  Unfortunately, this method for measuring the distances of the stars was fatally flawed: it incorrectly assumed that all stars are of the same brightness. With the rise of instrumental technology in the eighteenth and nineteenth centuries – telescopes that produced sharp images over large areas of the sky, and with finely calibrated scales for measuring angles – the geometrical method became the most promising technique. In 1669 Robert Hooke built a telescope on the side of a wall of his London house, intending to measure the parallax of Gamma Draconis, a star that passes directly overhead in London, but gave up after a few inconclusive measurements, illness and an accident that damaged the telescope lens. Samuel Molyneux and James Gregory tried again in 1725 and established that the parallax of Gamma Draconis was less than 1 arc second (1⁄3600 of a degree). On this basis they concluded only that the star was at a distance from the Earth further than 200,000 times the Sun’s distance. They were right – the Gamma Draconis is in fact at 5 million times the Sun’s distance.

  Molyneux and Gregory’s experiments made it clear that a nearby star needed to be selected as the target if the geometric method was to have any chance of success. But without knowing the relative distances of the stars beforehand, how would astronomers select a nearby star on which to try the measurement technique? Using a much larger telescope than was available to Molyneux and Gregory, but a similar technique, Wilhelm Struve, a German astronomer in Dorpat (now Tartu, Estonia), chose to measure the parallax of the bright star Vega in 1837 on the assumption that bright means close. Struve first measured a parallax of ⅛ of an arc second, but got ¼ of an arc second when he repeated the experiment. This discredited his methods. A Scottish astronomer, Thomas Henderson, working in South Africa in 1832–33, measured the parallax of the star Alpha Centauri and would have got a reasonably accurate result, but did not analyse his data until he returned to Britain, and even then could not quite believe what he had found and kept reworking his analysis.

  The German astronomer Friedrich Bessel had a better result when he chose the star 61 Cygni because it tracked quickly across the sky. This indicated that it was close to Earth, much as a close-passing bee will move more quickly across your field of vision than a high-flying aircraft. He also used a new technique. Essentially, he measured the angle between 61 Cygni and a star that was very nearby, and watched how this angle varied throughout the year. Bessel’s measurement of 61 Cygni in 1838 from the observatory in Königsberg, Prussia (now Kaliningrad, Russia), with a parallax of ⅛ of an arc second was believed. Bessel’s work boosted Henderson’s confidence in his own result for Alpha Centauri and he published his measurement in 1839. Historians rightly credit Bessel as the astronomer who discovered the true distances of the stars.

  The most accurate stellar parallaxes have been measured from space by the Hipparcos satellite, led by its project scientist Mike Perryman of the European Space Agency. Hipparcos was an acronym for ‘High precision parallax collecting satellite’, with a deliberate echo of the name of the ancient Greek astronomer Hipparchus of Nicaea. From 1989 to 1993 the satellite precisely measured the distances of 120,000 stars and of 2 million stars to a lower degree of accuracy. The data generated by this satellite will be superseded in the 2020s by the Gaia satellite, launched in 2013, which is gathering data on 1 billion stars.

  As we measure the distances to the stars, we actually study the past. Expressed in terms of the time that light takes to travel, the distance of the Sun is 8 light minutes, but the distance of 61 Cygni is 9 light years. That is to say, the light that glimmers down at us from 61 Cygni is actually the light that was emitted by the star nine years ago. The lights that we see when we look up at the night sky originated at many different times long ago, and some show the memory of stars that have since reached the end of their lives and no longer exist. Our view of the sky is thus a view that is peculiar to us, a mosaic of various distances and epochs that meld into a single view for us here and now. We see the
stars in Orion (2,000 light years distant) as they were when Christ walked in Jerusalem. To us they are new-born stars; but to anyone in the Andromeda galaxy (2.5 million light years away), these stars haven’t formed yet. On the other hand, there are stars in Andromeda that have already exploded and died in their own galaxy, but still shine on us in ours.

  The Discovery of our Galaxy

  Stars in an island universe

  Our present evidence, so far as it goes, leads to the belief that the spirals are composed of great clouds of stars so infinitely distant that we cannot make out the individual stars.

  H. D. Curtis, The Nebulae, 1917

  The discovery of nebulae in the seventeenth century opened the doors onto the rest of the Universe. Viewed through the lenses of the newly invented telescopes, the Milky Way resolved into individual stars and astronomers realized that we were not alone. Beyond our Sun there were other suns, and beyond our Galaxy, other galaxies.

  In 1609 and 1610 Galileo turned his telescope on the night sky, and was amazed when he looked at the Milky Way. The milky luminescence, visible to the naked eye and thought at the time to be a seam, a road or a cloud, split into thousands of stars. Galileo announced his discovery in his 1610 treatise Sidereus Nuncius (‘Starry Messenger’): ‘We are at last free from earthly debates about the nature of the Milky Way. It is, in fact, nothing but a collection of innumerable stars grouped together in clusters. Upon whatever part of it the telescope was directed, a vast cloud of stars is immediately presented to view. Many of the clouds are rather large and quite bright, while the number of smaller ones is quite beyond calculation.’ Excited by Galileo’s report, other astronomers followed him in inspecting the sky, chancing on interesting clouds or ‘nebulae’. The Bavarian mathematician Simon Marius discovered the Andromeda Nebula in 1612, describing it as looking like a ‘flame seen through horn’ (lanterns in Marius’s day had windows made from thin sheets of horn) as the nebula had a characteristic elliptical shape. The Andromeda Nebula is just visible to the naked eye and had actually been noted by the Persian astronomer ‘Abd al-Rahman al-Sufi in 964. In pre-Islamic cultures the constellation of Andromeda was depicted as a fish. At the fish’s mouth lay a fuzzy patch, the first known representation of the Andromeda Nebula, which, in his Book of Fixed Stars, Al-Sufi called ‘the Little Cloud’. It was rediscovered again in 1654 by the Sicilian priest Giovanni Battista Hodierna, who compiled a list of up to forty similar nebulae.

  The list of ‘nebulous stars’ grew longer as astronomers started observing with telescopes systematically, measuring the positions of all the stars in the sky and noting what they looked like. The astronomer Johannes Hevelius from Danzig measured 1,564 stars, listing sixteen as nebulous in his posthumous catalogue of 1690. John Flamsteed, the first British Astronomer Royal, catalogued 2,935 stars and mentioned several that were nebulae; his successor Edmond Halley added six more, including the ‘star’ Omega Centauri that he observed from Saint Helena, where it was possible to see more of the southern part of the sky. In an expedition to the Cape of Good Hope, South Africa, to survey the whole of the southern sky, Abbé Nicolas-Louis de Lacaille discovered dozens of nebulae while he was measuring the positions of uncharted stars and inventing new southern constellations.

  Between 1764 and 1781 the comet-hunter Charles Messier compiled all these discoveries, along with some of his own, into a catalogue of nebulae that eventually totalled over one hundred entries. The catalogue became an essential tool for astronomers, a source list of objects on which to test new theories and instruments.

  Messier’s catalogue was sent to William Herschel for review on the same day that he was elected to the Royal Society for his discovery of Uranus, which inspired him to examine all the entries with his new telescopes. He then began to search for and classify other fainter nebulae. His method was to sweep the telescope over the sky in a raster pattern (a scanning pattern of parallel lines looking like a rake), noting double stars and nebulae from his perch high on the telescope, shouting down the details to his sister Caroline, who took notes at a table below. Herschel described this process (in eighteenth-century spelling) as ‘star gaging’. He had expected to add only a few nebulae to Messier’s list, but, in the end, he found more than two thousand. With his telescope, Herschel was able to resolve many of the nebulae he found into individual stars, and, like Galileo, thought that all would eventually succumb to his increasingly powerful instruments.

  Meanwhile, in 1750, the English mathematics teacher Thomas Wright had developed his theory of the Milky Way. He modelled it as a band of light that arises from a flattened slab of stars. Look from within the slab, in the plane, and you see many stars and much starlight; look across the slab and you see fewer stars, and thus less starlight. An account of Wright’s theory was published in a newspaper in Königsberg where it was read by the philosopher Immanuel Kant, who was inspired to work on the problem. He proposed in 1755 that the Milky Way star system was flattened because it was rotating, and was held together by its own gravity. Kant also proposed that other nebulous patches like the Andromeda Nebula were similarly rotating masses of stars like the Milky Way, held together in the same way.

  The implications of Kant’s second suggestion were astounding. Although by the eighteenth century astronomers were comfortable with Galileo’s revelation that the Earth was not the centre of the Solar System, and some even had speculated that there might be more than one planetary system, the idea that there could be more than one galaxy required another mighty transformation of perspective. Before they could convince the public, however, astronomers needed to prove that the theory was correct.

  Herschel gave quantitative expression to Kant’s suggestions as he investigated the structure of the Milky Way by counting the density of its stars. He thought that the number of visible stars in a given area of the sky would indicate the extent of the Milky Way in that direction. Using this technique, Herschel discovered that the stars filled a flattened circular structure that he compared to the shape of a grindstone. He suggested that the Andromeda Nebula is another ‘Milky Way’ – a galaxy like our own, seen at an angle; a closely compressed cluster of stars that would eventually be individually resolved from the nebulosity. As time went on, he became less sure of this, and made contradictory statements; when he died in 1822, no one was really sure what Herschel thought he had discovered.

  Herschel became confused because in his day the classification of ‘nebulae’ was complicated. Originally the word meant something that looked amorphous, like a cloud. Some nebulae, such as the Orion Nebula, are indeed true clouds of gas that can never be resolved into stars, although stars may be embedded within the clouds. Modern astronomers still call these gaseous objects ‘nebulae’, but other cloudy patches, originally called nebulae, are in fact clusters of stars, which may be densely packed together or distant. The weakness of the instrumentation at the astronomer’s disposal or the enormous distance of the cluster prevents this second type of ‘nebula’ from being resolved into individual stars.

  At the beginning of the twentieth century, the development of larger telescopes, like the 24-inch telescope at the Boyden Observatory in Arequipa, Peru, and the 60- and 100-inch telescopes at the observatory on Mount Wilson in California, allowed astronomers to see individual stars in some of the closer ‘nebulae’ when the night air was still and clear. Some were variable stars, Cepheids, whose distances could be determined, paving the way for American astronomer Edwin Hubble’s dramatic 1925 discovery that some ‘nebulae’, like the Andromeda Nebula, are indeed distant galaxies of stars like our own Milky Way, separated from it. Our Galaxy is, in the picturesque phrase of the day, an ‘island universe’, one of many.

  Interstellar Nebulae

  Stars, molecules, dust and gas

  An unformed fiery mist, the chaotic material of future suns.

  William Herschel on the Orion Nebula, 1789

  When William Herschel viewed the nebula in the Sword of Orion through his telescope, he de
scribed it as a ‘fiery mist’ that looked like the flame of a candle (plate XXII). What he saw is actually a small dent on the side of a Giant Molecular Cloud, most of it dark. Its shadowy fields of dust and gas conceal thousands of infant stars and new-born planets.

  Nicolas-Claude Fabri de Peiresc was a French aristocrat who trained as a lawyer and became a diplomat. He was also an amateur scientist. In 1610, when a friend acquired a telescope similar to Galileo’s, Peiresc used it to discover a nebula (now known as the Orion Nebula) surrounding a star in the Sword of Orion. Galileo had already discovered three of the four small stars in the little cluster (now known as the Trapezium) that illuminates the nebula, but had not noticed the nebulosity around them. In 1656 the nebula was rediscovered by the Dutch scientist Christiaan Huygens, who published the first sketch of it. The nebula was recorded as number 42 in Charles Messier’s catalogue.

  On receiving a copy of Messier’s catalogue, William Herschel turned his telescope to observe the Orion Nebula, which he described as ‘an unformed fiery mist, the chaotic material of future suns’. Herschel thought that he had detected changes in the overall shape and brightness of the nebula over the decades that he observed it. This claim was controversial, but potentially very significant, as it suggested that the nebula was not a large, distant object, but small and relatively close to the Earth. If the nebula was very large, its individual parts would not be able to communicate with each other quickly enough for their brightnesses to change at the same time. Nor could the nebula alter quickly if it was made of independent stars.