B00B7H7M2E EBOK, page 17
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.
Not long after Leavitt’s discovery, the Danish astronomer Ejnar Hertzsprung sought to remedy this situation. He applied a variation of the statistical parallax method and estimated the distance to two Cepheids. Using the relationships between period and absolute magnitude that Leavitt had established, he went on to calculate that the Small Magellanic Cloud is 30,000 light years away. That distance, though far short of the nearly 169,000 light years astronomers currently measure it to be, was much greater than anyone had been expecting.
Leavitt’s ‘standard candles’, as she dubbed the Cepheid variables, almost immediately drew the attention of other astronomers, and one of them was Harlow Shapley. Shapley, born in Missouri in 1885, had come to astronomy by a serendipitous path. As a teenager he’d had a job as a crime reporter for a newspaper in Kansas, and he’d thought his formal education would be in journalism. Shapley arrived at the University of Missouri in 1907, but found that the school of journalism hadn’t been built yet. When he returned a year later, nothing had changed, and Shapley was out of money and patience. As he later described it, he was ‘all dressed up for a university education and nowhere to go’. He opened the university catalogue and began reading about the courses, starting at the front with the letter A. Archaeology didn’t appeal. On the next page was Astronomy.
Four years later, with both a BA and an MA from Missouri, Shapley moved on to Princeton, and it was there that he began to investigate a different kind of variable from the Cepheids Leavitt had been studying. He concentrated on systems called ‘eclipsing binaries’ in which a pair of stars orbit one another (or, to be more exact, orbit their common centre of mass) with one star periodically eclipsing the other. The effect is that of a variable, for the light output changes as the stars eclipse one another. Shapley wanted to find out whether Cepheid variables might actually be binaries.
In 1914 Shapley’s work came to the attention of George Ellery Hale, at the Mount Wilson Observatory in California. Hale had been the force behind the construction of the observatory, and at his invitation Shapley came to Mount Wilson to work with the 60-inch reflecting telescope there (‘60-inch’ refers to the size of the mirror), the largest telescope in the world at the time, for Lord Rosse’s had fallen into disrepair. Soon Shapley was able to satisfy himself that Cepheid variables were not binaries.
Astronomers now know that Cepheids are elderly stars that have got past their ‘main sequence’ phase (the lengthy phase during which a star steadily converts its hydrogen to helium) and become ‘red giants’. When the main sequence phase ends, the star starts to shrink. As it does, it heats up, and the heat flows into the outer layers of the star, energizing atoms of ‘singly ionized’ helium. ‘Singly ionized’ means there is an electron missing from these atoms. The increase in energy knocks yet another electron off, leaving the atoms ‘doubly ionized’. Doubly ionized atoms readily absorb light. As a result of that absorption, the atmosphere of the star becomes opaque, holds in heat like a thermal blanket, gets hotter still, and expands. The star’s outer layers swell, billowing out to about a hundred times the star’s earlier size. As the star expands, it cools, for the energy has more room to spread out, and as the helium atoms cool they once again change from a doubly ionized state to a singly ionized state. The atmosphere becomes transparent again and begins to shrink, and the cycle starts all over again. That is the mechanism behind the pulsation of a Cepheid. It swells and shrinks and changes brightness repeatedly at a steady rate.
Shapley, like Hertzsprung, decided to try to measure actual distances to Cepheids, using a method of his own. Our Sun is not a Cepheid variable, but knowing something about the reasons for Cepheids’ pulsation (though not the full explanation known today) gave Shapley a new way to compare a Cepheid with the Sun. Having the distance and size of the Sun, he found a way to calculate a Cepheid’s absolute magnitude. Cepheids turned out to be some of the brightest of all stars.
With his new understanding and the finest telescope in the world, Shapley began looking for a rich source of Cepheids and found it – the unattractively named ‘globular clusters’. These are dense jewel-like spherical clusters of stars. Using the Cepheids in them as standard candles, Shapley began calculating their distances.
He made three discoveries about the globular clusters that he thought significant. First, in all the globular clusters to which he could calculate the distances, the brightest stars were always approximately the same absolute magnitude. Shapley had a new yardstick. Even if he didn’t see a Cepheid in a cluster, he could assume that the absolute magnitude of the brightest stars there was the same as that for the brightest stars in clusters he had already been able to measure. Shapley’s second discovery about the globular clusters was that they appeared to be spread equally above and below the plane of the Milky Way, and this seemed to indicate that they were part of the same system. Thirdly, some of them were distributed much further out than the stars of the Milky Way – so far out, in fact, that later in the century there would be confusion as to whether some groupings of stars were globular clusters in the Galaxy’s halo or separate dwarf galaxies.
It occurred to Shapley that globular clusters might form a skeletal outline to the Milky Way. Relative to the solar system, the distribution of the clusters appeared to be lopsided. A great many of them were gathered in an enormous sphere far from the solar system, a cluster of clusters centred on a point in the direction of the constellation Sagittarius. Shapley published his results in 1918 and 1919, interpreting them to mean that the centre of this spherical group of globular clusters was the centre of the Milky Way system. Our Sun was nowhere near that centre.
We hold our breath waiting for the clamour of opposition and controversy that surely must follow such an announcement. Humanity evicted once again – forced to move down yet another notch from our original exalted position in the centre of the universe! It’s surprising to learn that there was little public or scholarly outcry about Shapley’s discovery, though there were conflicting theories at the time.
What did shock astronomers and provoke a great deal of controversy was Shapley’s measurement of the size of the Milky Way system. It was so huge that Shapley surmised that the system and its outline of globular clusters – in all, by his calculation, 300,000 light years across – must indeed be all there was to the whole universe. Nebulae such as Andromeda were not independent systems. At most they were minor satellites of the Milky Way. We now know that Shapley overestimated the size of the Galaxy because he failed to take into account the way intervening dust affects light coming to us from the globular clusters. The dust dims the light, and the sources looks fainter and further away than they really are. Astronomers now measure the Galaxy at approximately 100,000 light years, only a third as large as Shapley thought it was.
Opposition to Shapley’s estimate came from one Heber Curtis of the Lick Observatory, whose road to astronomy had been almost as odd as Shapley’s. Curtis had been a professor of classics and Latin, but when the college at which he taught merged with the University of the Pacific, he changed hats and became a professor of astronomy and mathematics.
Curtis wouldn’t accept that Cepheids could be used as reliable standard candles. He insisted that the spiral nebulae were other systems far outside our own, that our system was much smaller than Shapley’s estimate, and that the solar system was the centre of the Milky Way. Curtis suggested that the 1885 nova in Andromeda, whose apparent magnitude had been taken as indication that the Andromeda nebula was well within the Milky Way system, might actually have been of much greater absolute magnitude than the nova in 1901 to which it was compared – and hence much further away than most astronomers thought.
Shapley scoffed at this suggestion. If the Andromeda nebula was another star system like our own, not nearby at all, the 1885 nova there would have had to have had an absolute magnitude equal to a billion ordinary stars. It was absurd to imagine it could have been that bright, or that the universe could be large enough to contain a great many independent systems as big as ours. Furthermore, if nebulae like Andromeda were rotating at the rate some observations indicated, and were as far away as Curtis was claiming, they had to be rotating at a speed faster than the speed of light. That was impossible.
Curtis, undiscouraged, proceeded to search for other novae in Andromeda, so as to have more to compare with the one there in 1885 and the nova of 1901. He discovered several, all much dimmer than the nova of 1885, and that added weight to his argument that the 1885 nova was indeed exceptionally bright. He insisted that it was the dimmer and much more common novae, not the uncharacteristically bright one, that should be compared with novae elsewhere in order to calculate the distance to Andromeda. He decided that Andromeda was hundreds of thousands of light years away, far outside the Milky Way. Shapley still disagreed.
In 1920, the arguments between Shapley and Curtis culminated in a debate arranged by the National Academy of Sciences in Washington DC – attended by Albert Einstein, for one. The debate settled nothing. Hindsight shows that each man was right about some things and wrong about others. Together they could have put together a good picture of the universe. Most thought that Shapley had come off the worse in the debate, though astronomers were soon agreeing that his location of the centre of the Milky Way Galaxy was correct. Shapley left Mount Wilson not long after the debate to become the director of the Harvard College Observatory. Pickering, Henrietta Leavitt’s mentor there, had died in 1919.
The first two decades of the 20th century ended without a resolution to the question of how large our star system is and whether there is anything else besides it in the universe. The nebulae were still a puzzle. Even as late as the 1920s, there continued to be opposition to the ‘island universe’ theory that argued that some of the nebulae were other systems on the scale of the Milky Way. This opposition had some observational data weighing in on its side.
There was evidence that the nebulae were insignificant in size compared to the Milky Way – not its equals at all. Earlier indications that some nebulae had the spectra of stars (and must not therefore be made up only of gas) were called into question by the discovery that nebulae sometimes reflect light from elsewhere, and what is seen from them isn’t all their own light. There were also Shapley’s arguments: the enormous size of the Galaxy made it highly improbable that there should be others like it, and the speed of rotation in a spiral galaxy, were it sufficiently distant to put it outside the Milky Way, would be greater than light speed.
Waiting in part on the unravelling of these mysteries, there was an upheaval in the making that would rival Copernicus’s De revolutionibus as a watershed in the history of astronomy.
Back near the beginning of the century, Vesto Melvin Slipher, a young Midwesterner with only an undergraduate degree from the University of Indiana, came to work at the Lowell Observatory in Flagstaff, Arizona. Slipher was a quiet man, methodical and meticulous. When it came to insisting on tying up loose ends before announcing a discovery, he rivalled Copernicus. Slipher spent his entire career at Lowell. While working there he earned his MA and PhD from Indiana. In 1916 he became the observatory’s acting director; and in 1926, its director.
The wealthy astronomer Percival Lowell, who had built the observatory that bore his name and, at the time he hired Slipher, was its director, was one of those who believed the nebulae could be other solar systems in an earlier stage of formation. He set Slipher to work measuring spectra of spiral nebulae, looking for Doppler shifts in their light.
This was no easy assignment. Slipher couldn’t merely point the telescope at a nebula and snap the camera. Exposures lasted for 20 to 40 hours, over several nights. A researcher couldn’t stray far from the unheated telescope dome – purposely left cold because heat could mar the image – for he had to be certain that the nebula remained at the centre of the field of vision. The result at best was a faint, diffuse image of the nebula, not a concentrated point of light like a star. Using the spectroscope to spread it out further made it even fainter. The spectral lines were hard to identify, sometimes too faint if the spread-out image was too large. On the other hand, an image sufficiently bright for the lines to stand out clearly was apt to be too small for the shift in them to be measured.
