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Herschel’s map turned out to resemble the late 20th-century model of the Galaxy, which is remarkable because he, as Newton had done when he measured his star distances, worked from unsound assumptions. Herschel allowed himself to assume that all stars have essentially the same absolute magnitude. He chose the star Sirius, the brightest star in the night sky, as his standard. That is, he proceeded on the assumption that if all stars were the same distance as Sirius, they would all look just as bright, so the extent to which they appear to differ in brightness from Sirius can be used as a gauge of their distances. Herschel can’t be as easily excused as Newton for this error, because he must have been aware of a strong argument coming from his contemporary John Michell. Michell, a natural philosopher best remembered as the first to suggest the existence of ‘dark stars’ – what we now call black holes – pointed out that the stars of the Pleiades definitely do not all appear equally bright, and yet, grouped as they are, they must surely be about equal in distance from us.
Herschel also decided to assume that all stars are evenly distributed and that through his telescope he was seeing all the way to the outermost regions of the star-filled universe, so that his star counts really were accurate indications of the total number of stars.
Herschel’s map had the arrangement of stars as a spiky, flat, elongated blob, a model that got the nickname of the ‘grindstone’, though it is difficult to imagine that any grindstone could be so ill-designed and jagged. (See Figure 5.1.)The spiky edges were the result of dark rifts that Herschel observed in the Milky Way. He thought these were probably holes in space and that through them he was seeing emptiness beyond.
Herschel was so bold as to estimate the size of the grindstone. He had used the star Sirius as his standard, so he decided to call the distance to Sirius, whatever it might turn out to be in miles or kilometres, one ‘siriometer’. Herschel calculated that the grindstone measured 1,000 siriometers across and was 100 siriometers thick. When he made his estimate, the first detection of stellar parallax was still 50 years in the future, but it’s now possible to attach definite numbers to his scheme, because Sirius turns out to be not quite nine light years away. That would make Herschel’s grindstone about 9,000 light years from end to end and 900 light years thick. Modern estimates of the dimensions of the Galaxy have it more than 10 times that large.
Figure 5.1
William Herschel’s map – the ‘grindstone’ – was surprisingly like the modern picture of the Milky Way Galaxy.
William and Caroline Herschel also scrutinized the nebulae. The big question in the 1780s was, are they clusters of many stars? The Herschels had the best equipment in the world for finding out. William soon reported with delight that many nebulae were resolvable into stars. He even thought that he could almost make out individual stars in the Andromeda nebula, though astronomers now are sure that would have been impossible with his telescopes. In 1790 the Herschels confirmed the existence of another kind of nebula in which a cloud of luminous gas surrounded a single central star. Herschel thought this might be a planetary system in the making but not an independent cluster of stars similar to our own ‘grindstone’. He also found that even with the best of his instruments he could not resolve all the nebulae into stars or find stars in them. Some must be clouds of gas.
Herschel gave up on his grindstone model later, when he discovered that many double stars are true binaries (two stars orbiting the same centre of mass) with the pair of stars clearly the same distance from Earth yet differing in brightness. He was forced to conclude that stars do not all have the same absolute magnitude. At the same time, his larger telescope was revealing that beyond what he had previously thought were the furthest limits of the universe the stars go on and on. He could find no end to them. Such was Herschel’s influence that other astronomers also abandoned his grindstone model. The question of structure on this scale didn’t resurface in a significant way again until the middle of the 19th century, when another great amateur revived it.
William Parsons, the third Earl of Rosse, who resided at Birr Castle in Ireland, was the feudal lord of the village of Parson-town. He had been educated at Dublin and Oxford and served as a Member of Parliament while still an undergraduate. In 1841, at the age of 41, he succeeded to the earldom, which gave him free time and an ample independent income to pursue his passion for astronomy. Lord Rosse also had a good knowledge of engineering and plenty of space to build a foundry and workshops. The lack of a skilled workforce in Parsontown he soon remedied by training the labourers on his estate. He did not intend to buy a telescope and install it at Birr Castle. He was going to design the thing, build it, and cast the mirror himself.
Though improvements in refracting telescopes had by then made them temporarily far more popular than reflectors for both observatories and private use, Lord Rosse’s intention was to build a reflector larger than anything Herschel had used. Did he consider the weather in Ireland and wonder whether this was the most desirable location for the world’s largest telescope? Later he wrote to his wife: ‘The weather here is still vexatious: but not absolutely repulsive.’
Lord Rosse began experimenting with the construction of smaller telescopes and worked his way up. Eventually he achieved his goal – a tube 56 feet long and eight feet in diameter, with a six-foot mirror weighing four tons, set up to protrude like a giant cannon aimed at the sky from an amazing castle-like structure. This time there is no record of a concert. Though professionals in the field of astronomy scoffed that Lord Rosse was more interested in designing and constructing telescopes than using them, he began observing with his ‘Leviathan of Parsontown’ in 1845, even before the supporting structure was completed. He aimed his celestial cannon at the nebulae.
Lord Rosse knew that these faint, fuzzy patches in the sky had both intrigued and frustrated William Herschel. Herschel, his sister Caroline, and his son John, a fine astronomer in his own right who spent some years at the Cape of Good Hope surveying far southern skies, had catalogued thousands of nebulae. Nevertheless at the time Lord Rosse looked at them in the mid-19th century, both with his smaller telescopes and with his ‘Leviathan’, the nebulae were still one of the great enigmas of astronomy. Controversy continued over whether some were conglomerations of gas, perhaps not far away, which might be the birthplace of new stars and planets, or whether they were instead incredibly enormous clusters of stars, too distant to be resolved by an earthly telescope.
Lord Rosse saw the nebulae as no one had before. They were not mere clouds. By 1848 he had resolved 50 of them into stars. Some had complex structure and, as Lord Rosse went on observing and drawing what he observed, more and more of them turned out to be spiral, lens-shaped formations. It became impossible not to suspect that the then out-of-date idea that these were star formations similar to our own, and extremely distant, might be correct after all. It also seemed likely that our own star system was, like them, spiral and lens-shaped, which was remarkably close to the way Herschel had pictured it in his ‘grindstone’ model.
William Huggins, who like Lord Rosse had the wherewithal to build his own private observatory, and who had been one of the first to discover that light from the Sun and from other stars has similar spectral lines, also turned his attention to the nebulae. He analysed the light coming from the Orion nebula, the Crab nebula and others similar to them and found their spectra were like the spectra of hot, luminous gases, not the same sort of spectrum as light coming from the Sun and the stars. But he also discovered that light from other nebulae, the great Andromeda nebula for one, gave a continuous spectrum of the sort one would expect if it were made up of stars.
In 1885, the distant heavens gave earthly astronomers a spectacular opportunity, or at least many of them thought that was what it was. They had already judged that the Andromeda nebula, one of the largest of the spirals, was probably the closest. In this nebula a new star suddenly appeared and became bright enough to be just visible to the naked eye. Astronomers knew of only one kind of exploding star – a nova. Comparison of this star’s brightness with the brightness of previous novae, and later with a nova in 1901, indicated that the Andromeda nova was relatively close to us. That meant of course that the whole Andromeda nebula was close, by some estimates the nearest thing outside the solar system – certainly not a distant formation as large as the Milky Way. All of which added to the confusion of what seemed to be conflicting spectral analyses.
While this study and speculation was going on, in the last quarter of the century, astronomers were beginning to realize the potential value of a fabulous new tool – photography. Back in 1839, Daguerre in France and Fox-Talbot in England had almost simultaneously announced their discoveries of the photographic process. That same year, William Herschel’s son John Herschel took one of the earliest photographs, a view of his father’s 40-foot telescope through the window of his house at Slough. (See illustration 6 in the plate section.)
Though there were some fine astronomical photographs in the mid-19th century, exposure times were not yet fast enough for photography to be of practical, routine use to astronomers. John Herschel’s exposure time for the photo of the telescope was two hours. When Lord Rosse recorded his observations he did it with drawings, not photographs. But when in the 1870s the use of dry gelatin plates reduced the exposure time required in terrestrial photography to about second, a new epoch in astronomy began. It was no longer necessary to rely on words or drawings to share observations, or on memory to compare what a portion of sky had looked like on one night with its appearance on another. Photographs taken on successive nights or over a span of days, weeks and years allowed astronomers to study how the sky changed. Photographic records took the place of such descriptions as Galileo’s of Jupiter’s moons, or John Herschel’s of the star Alpha Hydrae, in 1838:
21 March Alpha Hydrae inferior to Delta Canis Majoris, brighter than Delta Argus and Gamma Leonis.
7 May Alpha Hydrae fainter than Beta Aurigae, very obviously fainter than Gamma Leonis, Polaris or Beta Ursae Minoris.
10 May Alpha Hydrae much inferior to Gamma Leonis, rather inferior to Beta Aurigae. It is still about its minimum.
11 May Alpha Hydrae brighter than Beta Aurigae no doubt.
12 May Castor and Alpha Hydrae nearly equal.
‘Very obviously fainter’ . . . ‘rather inferior’ . . . ‘much inferior’ . . .‘brighter, no doubt’ . . . ‘nearly equal’. What a difference photography was to make in the precision with which observations could be reported and compared!
The English astronomer Isaac Roberts pioneered long-exposure photographs, allowing more light to enter the camera through the telescope than would happen in a quicker exposure. It was he, in 1888, who took the first photographs of Andromeda. Roberts’ photographs confirmed that Andromeda is spiral in shape and clearly revealed the spiral arms in the galaxy’s outer regions. However, even those photographs couldn’t settle the question of what Andromeda actually is. Eleven years later, photography was first put to the task of recording a spectrogram of Andromeda, which indicated that it was a ‘cluster of sun-like stars’. Yet Huggins had just previously seen mixed dark and light bands from Andromeda!
The confusion about the nebulae was part of a continuing debate about the larger picture. Friedrich von Struve, who had first measured the parallax of Vega, believed that the Milky Way’s disc’s edges extended to infinity, with interstellar matter absorbing the light from remote regions so that they remain eternally hidden from us. Others argued the pros and cons of a proposal that the Milky Way consisted of concentric rings of stars.
Partly because of the advent of photography, increasing attention was given not only to the position of stars but to their motions. It turned out that this motion was not, as previously thought, random. Stars were more likely to move in the plane of the Milky Way. In 1904, J.C. Kapteyn discovered that the majority of those stars which are easiest to observe move in two streams towards different parts of the sky. Like William Herschel he counted stars and found that Herschel had not been far off in his conclusions about their distribution.
Kapteyn thought the Sun was near the centre of the galaxy; while American astronomer Harlow Shapley would soon argue, based on study of globular clusters, that it was not. In 1913, the Dutch astronomer C. Easton concluded that the whole universe was one large spiral, shaped like the spiral nebulae. He thought that these nebulae were only miniatures of the greater spiral, within its boundaries.
The definitive answer was still almost quarter of a century away. However, at the turn of the twentieth century, astronomy was not far from a tremendous breakthrough when it came to establishing a foothold beyond the range of parallax measurement.
Two ingredients were necessary for any significant advance: first, the ability to identify a class or family of stars by some characteristic other than brightness – some characteristic certain not to change with distance; second, the ability to measure the distance to at least a few of the stars in that family or, failing that, at least to decide that a number of stars in the family were all approximately the same distance from us.
In the closing years of the 19th century, astronomers were having some success on all of these fronts. They were using the parallax method to measure the distances to as many stars as possible within parallax range and making catalogues of these stars and distances. They were continuing the search for distinguishing characteristics that could be depended on not to change with distance – colour perhaps, or some pattern of variation of colour or brightness, or patterns of spectral lines. And they were attempting to identify groupings of stars that are all approximately the same distance from us. With this combination of efforts, researchers were managing to edge themselves further and further into the cosmos.
There was a risk, just as there was in the analogy with the elephants and the giraffes. Discovering a weakness in one rung of the cosmic distance ladder could, and would several times, necessitate recalibrating the entire structure. But this was a problem astronomers had learned to live with, making repeated adjustments, hoping their margins of error were no greater than they estimated and that somewhere along the line there would be independent evidence to show that their measurements hadn’t been far from the mark. Things were going fairly well. They were about to get much better.
In the early-to-mid-19th century, the fashion for observatory building had spread to the United States. At first most of the telescopes there were imported from Europe. In the 1830s there were good refracting telescopes at Yale University and at Wesleyan University in Middletown, Connecticut, but no actual ‘observatories’ at either place. Yale just stuck its telescope out of a window. In 1838, Williams College, in Williamstown, Massachusetts, opened its Hopkins Observatory, housing a 10-foot Herschel reflector bought by Professor Albert Hopkins in England. The Harvard College Observatory was founded in 1839 but didn’t have a building or very much in the way of equipment. Earlier, in 1815, a delegation had gone to England to purchase a telescope for Harvard, found the desirable instruments too expensive, and went back empty-handed. In 1843 the local citizens of Cambridge, Massachusetts, disgruntled that there was no telescope around through which they could view the Great Comet of that year, offered to share with the university the cost of purchasing one. Harvard accepted, and the telescope was acquired from a distinguished firm in Germany.
By the 1890s the Harvard College Observatory had become a world-class institution. It was there that new rungs on the cosmic distance ladder were about to be nailed in place.
Henrietta Swan Leavitt was born in 1868 and studied at what would later become Radcliffe College in Boston, then known as the Society for the Collegiate Instruction of Women. At the nearby Harvard College Observatory, the eminent astronomer Edward Pickering was cataloguing and analysing stars and mentoring younger scholars.
There were few if any women among these budding astronomers, though women were hired to do the painstaking drudgery of writing down in endless rows of figures the positions and brightnesses of stars. However, women employed by Edward Pickering sometimes had a chance to do more creative work, for occasionally he encouraged someone from among his volunteer or underpaid female clerical staff to take on a more challenging assignment. In 1895 Henrietta Leavitt became a member of Pickering’s staff. She started as a volunteer, received a permanent paid position in 1902, and soon became head of a department.
In 1908, Leavitt was looking for stars that varied in brightness, hoping to find a group of them that were all approximately the same distance away. It was logical to assume that all the stars in one of the Magellanic Clouds were, by cosmic standards, approximately the same distance away.
The Magellanic Clouds are two star formations that are not visible at any time of year in most of the northern hemisphere, where they never rise above the horizon. Seen from the southern hemisphere, they are large, misty smudges of light that could be mistaken for thin veil-like clouds faintly lit by the Moon on a fair night. The Australian aborigines believed that the Large Cloud was a part of the Milky Way that had been torn away. Europeans knew of the existence of these clouds before Ferdinand Magellan’s voyage around South America in 1521 and called them the Cape Clouds. But Magellan’s official recorder Antonio Pigafetta suggested that they be renamed the Clouds of Magellan in honour of that great explorer, who died just short of completing his circumnavigation of the globe.
Astronomers didn’t pay the clouds much attention until William Herschel’s son John studied the southern hemisphere skies in the 1830s and hypothesized that these clouds were fragments detached from the Milky Way. This was not plagiarism from the Australian aborigines. Herschel was in South Africa, not Australia. The younger Herschel thought this might mean the Milky Way was breaking up and that his father had been right to speculate that it couldn’t last forever . . . indeed, that the past might not be infinite either.
