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In 1781, scanning the heavens with a telescope along with his younger sister Caroline, looking for faint, distant objects, Herschel discovered the planet Uranus. As a result of that, George III appointed him his personal astronomer, with few duties other than to keep the royal family advised on the subject of astronomy and any new wonders in the skies, and occasionally to impress visiting dignitaries. In 1787, the king also granted Caroline a salary.

The Herschels moved to Datchet, near Windsor, so that William could perform his new duties more conveniently. To supplement what was not a large stipend (less than he earned as an organist) he began to make and sell telescopes, and he also set about building bigger and better instruments for his own use. Herschel’s telescopes were reflecting telescopes, a type that had fallen somewhat out of favour. Herschel’s success gave them a new popularity. (Review
Figure 4.5.
) His largest was a 40-foot instrument with a 48-inch mirror. Its tube was five feet in diameter. Herschel, still the musician, held a small concert in the tube to celebrate its dedication.

Although the size of the 40-foot telescope made it a curiosity and a source of pride for Herschel and for the king, it really was something of a liability. So ungainly and unsafe was this behemoth that King George’s workmen were reluctant to help manoeuvre it. What was worse, Herschel found himself wasting precious time and good viewing nights (rare in England) explaining and demonstrating this wonder to sightseers, who included the king and his family and royalty and astronomers
from
all over the world. ‘Come, let me show you the way to heaven,’ the king was overheard murmuring to the Archbishop of Canterbury, as he ushered him towards the telescope.

Herschel ended up preferring his more manageable 20-foot reflector, though the use of that also involved physical risks. His sister Caroline wrote:

My brother began his series of sweeps when the instrument was yet in a very unfinished state, and my feelings were not very comfortable when every moment I was alarmed by a crack or fall, knowing him to be elevated fifteen feet or more on a temporary crossbeam instead of a safe gallery. The ladders had not even their braces at the bottom; and one night, in a very high wind, he had hardly touched the ground before the whole apparatus came down.

Just before Herschel’s 40-foot telescope was dismantled, 17 years after his death, his son Sir John Herschel composed a ballad, and he and his family went into the tube on New Year’s Eve to sing it. The great telescope’s life ended as it had begun, with a concert rather than an astronomical observation.

For William Herschel, building bigger telescopes was not primarily a matter of hubris or impressing the king. He wanted to learn how the universe is constructed on the largest scale. Rather than tackle the whole at once, Herschel chose 700 different regions of the sky on which to concentrate his attention. In each of these regions he proceeded meticulously to count the stars of different brightnesses and to catalogue all the double stars he could find, for he hoped to measure their annual parallaxes. Thus began the first project to map the universe in three dimensions, something astronomers are still doing 300 years later.

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: