Authors: Kitty Ferguson
Slipher went on designing and improving his own instruments and continued to find that most of the nebulae he was able to study did indeed show red shifts. In early 1921 he reported a nebula that according to his calculations was increasing its distance at a speed of approximately 2,000 kilometres per second. In 1922, he sent Arthur Eddington, an eminent physicist at Cambridge, measurements for 40 spiral nebulae, 36 of which were receding. Eddington was intrigued. He also was a cautious man, but he went so far as to suggest that this discovery about the nebulae, which were widely thought to be the most remote objects yet known, might be a hint about ‘general properties of the world’, by which he meant ‘universe’.
By 1925, astronomers had measured 45 nebular Doppler shifts – Slipher 41 of them, other astronomers the remaining four. The score was now 43 red shifts to two blue shifts. What might have been a coincidence was definitely beginning to look like a trend.
Clearly Slipher had made a discovery of enormous importance, but it wasn’t obvious at first what it signified. Slipher’s own initial interpretation was that the drift of the solar system through space was increasing the distance between it and the nebulae. One problem with interpreting the significance of the red shift was that knowing the nebulae were moving away from
us
, or we from them, still didn’t tell us how far away they were or what they were.
Now it so happened that when Slipher first announced his findings about red shifts to the American Astronomical Society in 1914, a young man named Edwin Hubble was in the audience. Hubble’s background was unusual. His earlier university training hadn’t been in astronomy at all. By the time he came to astronomy full time he was already a highly successful lawyer.
Born in Missouri in 1889, Hubble attended both high school and university in Chicago and then went to Oxford on a Rhodes scholarship. He was a polymath who was ready to tackle just about anything, including tank diving and high-level amateur boxing. But it was astronomy that won him in the end. He practised law for only a few months before going back to the University of Chicago to study astronomy and work as a research assistant at the Yerkes Observatory. ‘All I want is astronomy,’ he said. ‘I would much rather be a second-rate astronomer than a first-rate lawyer.’ In 1917, with a PhD in hand and an offer of a job at Mount Wilson, Hubble first went off to France to fight in World War I, returning in 1919. He came to Mount Wilson just before Shapley left there to take up the post at Harvard.
With the 60-inch reflecting telescope at Mount Wilson, and sometimes the more powerful 100-inch reflector that had just come into service in 1918, Hubble began to investigate the nebulae. By 1922 he had confirmed that nebulae that do not have a spiral structure don’t shine with their own light. Either they shine by light reflected from stars within or near them, or they absorb enough energy from nearby stars to cause the hot gas of which they are composed to glow. Hubble’s study confirmed earlier strong suspicions that these nebulae are part of the Milky Way system, but that didn’t settle the question of the spiral nebulae. Hubble turned his attention to those next.
He was sure that some of the spiral nebulae were made up of
stars
, but for many spirals, even by using a magnifying glass to scrutinize the best photographs he was getting with the 100-inch telescope, he couldn’t produce the sort of evidence that would make him, and others, certain this was so. Hubble decided to investigate a faint patch of light called NGC6822, which
could
be resolved into stars. And there, in 1923, he found some of those stars varying in brightness. At first he failed to recognize their significance. He turned instead to the Andromeda nebula, where other astronomers had been discovering faint novae.
In the autumn of 1923, Hubble was using the 100-inch telescope to photograph Andromeda night after night. His observations were part of a survey to search for novae there that might be used to test out Curtis’s ideas about nebulae. Hubble almost immediately found a couple of novae and another faint object that he at first thought was a third. At this point he decided to delve into the Mount Wilson archives to look for this star on older photographic plates. The comparison showed that it was actually a variable star, a Cepheid with a period of approximately a month, which meant that its absolute magnitude at its brightest was about 7,000 times as bright as the Sun. In order for it to appear as faint as it did, it had to be about 900,000 light years away. Hubble looked again at the photographs he had recently taken of the nebula NGC6822, and this time he recognized the varying stars as Cepheids, allowing him to calculate that NGC6822 was about 700,000 light years away.
These measurements to Andromeda and NGC6822 would later prove to be underestimates. Nevertheless they settled the question whether the spiral nebulae are in the Galaxy or are remote independent ‘island universes’ – other galaxies. By Hubble’s measurement the Andromeda nebula was much further away than any star in the Milky Way. That indistinct, oval blur that we see from the northern hemisphere was definitely another galaxy – a collection of millions of stars. Astronomers
now
measure its distance as about two and a quarter million light years. The Andromeda galaxy is the furthest object visible from Earth with the naked eye. At that distance, the nova of 1885 in Andromeda had to have been much brighter than any ordinary nova. It was in fact something rarer, a supernova, the explosion of a star at the end of its life, as bright as nearly a billion Suns.
Hubble rushed news of his discovery about Andromeda and NGC6822 to Harlow Shapley. On first reading Hubble’s message, Shapley turned to his colleague Cecilia Payne-Gaposhkin and commented, ‘Here is the letter that has destroyed my Universe.’
Hubble christened Andromeda and other similar independent systems ‘extragalactic nebulae’. By the end of the year he had resolved the outer part of the Andromeda nebula into stars, and a year later he was confident enough about the nature of the spiral nebulae in general to present his findings to the American Astronomical Society. His paper won an award donated by the American Association for the Advancement of Science to the two most outstanding and important papers presented at the meeting. The other winner was a paper on the digestive tracts of termites.
Over the next five years Hubble went on accumulating evidence and began developing techniques for estimating the distances to galaxies out beyond the range in which individual stars could be seen and identified as Cepheids. One technique resembled the one Shapley had employed when estimating the distances to the globular clusters: assuming that the brightest stars in all galaxies were approximately the same absolute magnitude and using these stars as distance indicators. That allowed Hubble to measure galaxies four times as remote as the furthest galaxy in which he could find a Cepheid. He estimated this range as about 10 million light years.
Hubble didn’t stop there. He began hammering new rungs into the cosmic distance ladder at a furious pace. He decided
that
globular clusters could be used as a standard, assuming that the brightest globular clusters in all galaxies are approximately the same absolute magnitude. To go further yet, Hubble decided to assume that all galaxies have approximately the same absolute magnitude, or at least all fall within a narrow range. Recognizing that there would be some amount of error in this method, he nevertheless calculated that his distances would be no more than three times too large or three times too small. Hubble estimated that his techniques took him out to about 500 million light years – a volume of space containing about 100 million galaxies. Others would later refine his method and measure even further by assuming that the brightest galaxy in a galaxy cluster has approximately the same absolute magnitude as the brightest galaxy in every other cluster.
The accuracy of all this measurement stood or fell on the reliability of the measurement of the distance to Cepheid variables, using a variation of the statistical parallax technique. The whole ladder stood on that footing. The Cepheid yardstick has been revised several times over the years. Nevertheless, however rough the initial measurements were, Hubble did establish once and for all that the universe extends for billions of light years. His results seemed to indicate that the distribution of galaxies and galaxy clusters is fairly uniform throughout space. Hubble’s investigations and those of his successors into the nature of Andromeda and other ‘extragalactic nebulae’ also turned a mirror on our own Galaxy, helping astronomers get a better idea of what it must be like as a whole, for of course they couldn’t see it from a distance in space as they could Andromeda.
In the years following Hubble’s discovery that at least some of the nebulae are well outside the Milky Way, and the coinciding realization that our own system is one galaxy among many others, some astronomers were still reluctant to trust Cepheids as reliable distance calibrators. There was one particularly nagging problem. As more and more galaxies were measured,
most
of them turned out to be quite a bit smaller than ours. Andromeda was only one sixth the size of the Milky Way. Others were smaller still. Was our Galaxy really exceptionally large? Perhaps by far the largest? That seemed suspicious enough for astronomers to have some doubts about the measurements.
By the 1940s the bright lights of the rapidly growing city of Los Angeles were making Mount Wilson a less-than-ideal location for a telescope. However, in the middle of the decade, at the height of World War II, these lights were often blacked out because of the threat of bombing raids. Though the citizens of Los Angeles undoubtedly found this unpleasant, it was a stroke of luck for astronomer Walter Baade, who because he was a German national had not been allowed to join the war effort in military research, but instead was left in almost solitary splendour at Mount Wilson, with the 100-inch telescope virtually all to himself.
Baade had been born in Schröttinghausen, Germany, and got his PhD from the University of Göttingen in 1919. In 1931, with the political climate in Germany changing, he had come to the United States to work at the Mount Wilson and Palomar Observatories, where he spent 27 years before returning eventually to his native country.
Under ideal viewing conditions during the blackouts, Baade studied the stars in the Andromeda galaxy through the 100-inch telescope. He found that those in the centre of that galaxy and in what he called its ‘outer skeleton’ or halo tended to be red and yellow, while those in the spiral arms were white and intense blue. Baade concluded that the stars must belong to two different ‘populations’. He christened the white and blue stars ‘Population I’ stars. These turn out to be hot young and middle-aged stars. The red and yellow ones he called ‘Population II’ stars. These are much older stars.
Beginning in 1948, Baade was one of the first to use the 200-inch reflecting telescope (known as the Hale Telescope) on Mount Palomar, not far from Mount Wilson. He was puzzled to
find
Cepheid variables, with a difference, in the Andromeda galaxy halo. They were four times fainter than the Cepheids in the rest of the galaxy, the Cepheids Hubble had used to make his measurement. Baade, putting this finding together with what he had earlier discovered about the two different star populations, concluded that each population must have its own kind of Cepheid variable.
All Hubble and his associates had known were Population I Cepheids, the ones in the spiral arms of the Andromeda galaxy. The result was that they had been comparing apples and oranges, for the Cepheids they’d used for comparison were actually fainter Population II Cepheids. For this reason, Hubble’s measurements of the distance and size of the Andromeda galaxy had been too small. Baade’s discovery doubled the size and age of the universe. This increase helped clear up an embarrassing conundrum, for Hubble’s measurements had made the universe younger than geologists knew the Earth to be.
Baade’s discovery also led to a far better understanding of spiral galaxies, including our own. As astronomers recalculated their measurements, they found that spirals are all much larger than Hubble had estimated, and far more remote. While our mental picture of the universe swelled in size and distance, our picture of our own Galaxy shrank accordingly. Clearly, Shapley’s 300,000 light year measurement for the diameter of the Galaxy was an overestimate. The diameter was closer to 100,000 light years. The Milky Way was not 10 times larger than most spirals and six times larger than the Andromeda galaxy. It was fairly average.
In 1952, Baade’s former PhD student Allan Sandage joined the full-time staff at the Hale Observatories. Sandage, born in Oxford, Ohio, had known by the age of 11 that he wanted to be an astronomer when he grew up. When he looked through a friend’s telescope, as he remembers it, ‘a firestorm took place in my brain’. He spent two years at Miami University in Ohio
and
then two in the Navy as an electronics specialist. When World War II ended, Sandage finished his undergraduate degree at the University of Illinois, then headed west and enrolled in 1948 as one of Cal Tech’s (the California Institute of Technology) first PhD candidates in astronomy. While he was still working on his degree he began collecting data at Mount Wilson for Baade, who was his thesis adviser. Then Edwin Hubble, who was observing with the 200-inch telescope, suffered a heart attack and needed a graduate student assistant to help him continue his work. Sandage was summoned to Mount Palomar. After a short sojourn at Princeton as a post-doctoral student, he came back as a member of the staff.
In 1958, five years after Hubble’s death, Sandage discovered that some of what Hubble had thought were bright stars in distant galaxies and had used as measuring rods were instead glowing nebulae lit by many stars. That discovery more than tripled the size of the universe and increased its estimated age to about 13 billion years.
Human beings have a long history of underestimating the distance to objects in the heavens and the size of the universe as a whole. Though there have been a few overestimates, the scientific and popular image of the universe through the years has had to be revised upwards, by alarming increments, again and again. We have no intuitive feel for a distance of 13 billion light years, the size Sandage settled on in the late 1950s. But was that still too small? Or had he actually taken things too far? Could it be we would never know the answer? Nearly everyone who read the newspapers had been aware since the early 1930s that the universe wasn’t cooperating as much as one might wish in this measuring venture. In more than one sense, it wasn’t holding still to have its measurements taken!