B00B7H7M2E EBOK (25 page)

‘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.

By the end of the century, it was generally agreed that the Magellanic Clouds were made up of stars. There was less consensus about whether they were part of the Milky Way system or distinct from it but closely related. It was Edward Pickering’s cataloguing, plus some of the techniques for measuring distances to groups of stars, that began to provide a clearer understanding of the Magellanic Clouds as the new century began. Today astronomers measure their distance from
us
at about 169,000 light years and consider them satellite galaxies of the Milky Way, but the earlier name has stuck – the Large and Small Magellanic
Clouds
. When Leavitt examined them, their distance was still unknown.

The reasoning behind Henrietta Leavitt’s study was that if stars in the Magellanic Clouds were all approximately the same distance from Earth, then it was not differences in distance that caused some of them to look brighter than others. It seemed safe to conclude that stars that looked bright there really did have greater absolute magnitude than stars that looked dim there, and meaningful comparisons could be made between their apparent magnitudes. The Magellanic Clouds were close enough for individual stars to be identified and studied, though not close enough for their distances to be measured by direct parallax.

In the closing years of the 19th century and the early years of the 20th, the Harvard College Observatory had an outpost known as its Southern Station in Arequipa, Peru. By then photography had come into wide use in astronomy, not only allowing researchers to compare observations much more systematically than before but also making it possible for important discoveries to be made away from the telescope itself. From plates taken in Arequipa, Leavitt, in Boston, was able to identify 2,400 variable stars in the Small Magellanic Cloud.

Leavitt found that some of these variables had a remarkably dependable pattern to their variation: a steep increase to maximum brightness, and then a more gradual fall-off in brightness. Some took much longer than others to complete the pattern, and their range of brightness was also different. Nevertheless, the overall pattern was recognizable enough to set them apart. Leavitt noticed that among her sample in the Small Magellanic Cloud, the brighter a star of this type was, the longer it took to complete the pattern. The brightest ones took almost a month (some are now known to take over three months), the faintest only a day or so.

This relationship between a star’s period (the length of time it takes a star to complete the cycle) and its brightness was the sort of clue for which Leavitt and others had been searching. The period of pulsation was a characteristic that wouldn’t change with distance. Leavitt found 25 stars of this distinctive family in the Small Magellanic Cloud, all of which showed a clear relationship between brightness and period. She compared these ‘light curves’ with those of previously discovered variable stars nearer to Earth, in the Milky Way Galaxy, and found a match in the star Delta Cephei. ‘Cepheids’ is hence the name given to these variable stars.

Leavitt published her initial findings in 1908. Four years later, in 1912, she had compiled enough evidence to show that Cepheids could potentially provide a much more reliable way than any known before to pin down distances both in the Galaxy and far beyond it.

Wasn’t it working backwards to have a discovery about stars so far away lead to a method to measure distances closer to home? One reason things happened this way is that studying the stars in the Galaxy and comparing their distances can be an extremely confusing undertaking. They are certainly not all the same distance from Earth, and brightness is no gauge of their distance. Take two hypothetical stars, Star A and Star B. Let’s say that Star A’s absolute magnitude is twice as great as Star B’s – Star A is a
much
brighter star. Nevertheless, if Star A is twice as far away from us as Star B, Star A will actually look the fainter of the two to us on the Earth. (Recall the inverse square law and the discussion of the two lightbulbs.) The key to Leavitt’s discovery was finding a family of stars with a good sample of its members in an area where she knew all the stars were approximately the same distance from us. The Magellanic Clouds were such an area, and there was no area like that within the Milky Way.

It might seem, however, that the Magellanic Clouds are surely large enough so that differences in stars’ apparent
magnitudes
might deceive us there, just as they do in the Milky Way Galaxy. However, referring to our hypothetical situation above,
no
star in a Magellanic Cloud is twice as far away from us as another in the same Cloud. Think of it this way: if I look out of my window here in the eastern United States, I can say that the fence is twice as far away as the garage. Looking at them from Tokyo, I could not say that. For all intents and purposes, from Tokyo this fence and garage are the same distance away. So it is with the Magellanic Clouds. The stars there are so far away that we can treat them as being all the same distance from us.

Leavitt had found in the Small Magellanic Cloud that Cepheids with the same range of absolute magnitude had the same period of variation, which meant that if she knew how one Cepheid’s period related to another Cepheid’s period, she also knew the relationship between their absolute magnitude. For example, Leavitt found in her sample that if one Cepheid had a period of three days, and another had a period of 30 days, the second was six times brighter than the first. This meant that anywhere else in the sky she discovered a Cepheid variable star, she could measure its period of variation and be fairly sure that that would tell her how bright that star would appear
if
it were sitting among the stars in the Small Magellanic Cloud, and how its actual distance compared with stars there. This was definitely an advance in measuring stellar distances, but, again, as with Kepler’s third law and Herschel’s siriometers, what Leavitt had was a system of
relationships
, not absolute distance measurements. No one then knew the exact distance to the Magellanic Clouds, or the distance or absolute magnitude of any Cepheid in the Milky Way Galaxy. No Cepheid – not even Polaris, the North Star, the nearest Cepheid – was close enough to be measured by the parallax method. The cosmic distance ladder had been extended considerably, but it didn’t reach the ground.