Authors: Kitty Ferguson
Experts have studied the physics of stars, the rate at which they convert hydrogen to helium and burn up their fuel, their masses, their temperatures, their life cycles, their spectra, the way they affect one another in binary and ternary groupings, the idiosyncrasies that set some of them apart, and the wobbling that indicates there are planets orbiting a star. These days, the child of astronomer parents who sings the nursery song ‘Twinkle, twinkle, little star. How I wonder what you are!’ is asking for an extraordinarily long lecture! But none of this burgeoning knowledge about stars has diminished the significance of Cepheids. They are the best hope for establishing absolute distance measurements far into the universe. The Cepheids Leavitt first studied were in the Magellanic Clouds, about 169,000 light years away. With modern ground-based telescopes astronomers detect Cepheids in galaxies 15 million light years distant. With the Hubble Space Telescope, NASA’s orbiting observatory that came into full use after repairs in December 1993, researchers study them in galaxies near to 60 million light years away. The Hubble telescope’s mirror is smaller than many ground-based telescopes, but because it’s based outside the distorting atmosphere of the Earth, it concentrates the light from stars into images that are many times sharper.
Cepheids were only one of the identifiable families of stars that astronomers found among the growing sample whose distances they could measure or estimate using various parallax techniques. One other valuable category are the RR Lyrae stars, which all have the same absolute magnitude averaged over each star’s varying brightness.
Looking at stars’ spectra has proved to be among the most effective ways to gain knowledge about them. In the past few decades astronomers have continued to study the spectra of thousands of them whose distances are known from various methods. Working from a discovery of Hertzsprung’s that for stars of any particular spectral type there is a correlation between the width of the spectral lines and a star’s absolute magnitude, they have succeeded in compiling tables giving the absolute magnitude for stars with
any
combination of spectral type and line width. Check a star’s spectral lines, look up the absolute magnitude it should have, compare this with its apparent magnitude, and you know its distance! The method is called spectroscopic parallax. Its precise accuracy depends on how correct the measured distances were for the stars used to compile the table, but astronomers believe it is, for the most part, highly reliable. Not only does this method allow one to measure the distance to any star for which a spectrum can be obtained, it also makes it possible to find the distance to a cloud of gas or dust by finding a star in it and measuring that star’s distance.
Another tool that, like spectroscopy, was inherited from the 19th century and put to optimum use in the 20th is the Doppler shift. When Slipher measured the red shifts of the nebulae in the 1920s, he used the 24-inch telescope at the Lowell Observatory, which was one of the finest of that day. But Slipher had to spend night after night in the unheated telescope dome, exposing each photograph for 20 to 40 hours to obtain spectra from which he then could measure the shifts. The equipment used by Edwin Hubble was better, but he and his contemporaries still had to expose a single photographic plate to hours of light coming through a telescope to record the tiny spectrum that they could then study with a magnifier to find the spectral lines. Today the job of finding the red shifts of galaxies and quasars is done in minutes by telescopes equipped with charge-coupled devices (CCDs) – silicon chips that convert light from the night
sky
into digitized images. Computer-run arrays make it possible to take the spectra of many galaxies at once.
Red shift has become the tool of choice, and often also the tool of necessity, for measuring the distant universe, and most of the mapping on the largest scales has been based on red-shift measurements only. By mid-century, astronomers had measured red shifts for about 100 galaxies. By 1970 the number was around 2,000. Today, red shifts have been catalogued for more than 100,000 galaxies and counting. There are ongoing projects such as the Sloan Digital Sky Survey which alone is expected to collect the red shifts of one million galaxies.
Red-shift measurement is not, however, infallible. How much the light from an object is red-shifted gives an indication of how rapidly the object is increasing its distance from Earth, providing a basis for comparing the distances of far-off objects. The method would work flawlessly in an ideal situation where everything was moving directly away from everything else with the expansion of the universe (like the raisins in the rising loaf of bread) and there was no other motion going on. Unfortunately, the situation is not that simple.
Galaxies do more than recede from one another with the expansion of the universe. Some of them are locked in binary systems, with two galaxies orbiting a common centre of mass, so that at any one time, one of the pair is likely to be moving away from us at a speed greater than the simple expansion of the universe and the other at a speed less than that expansion rate, though they are really about the same distance away. Spiral galaxies also spin like giant Catherine wheels, and if a binary system is made up of two spiral galaxies, they probably spin in opposite directions, causing yet more complication in the overall motion. Pairs and singles among galaxies are almost all grouped together in galaxy clusters, orbiting the gravitational centre of the cluster. The clusters orbit the gravitational centres of superclusters, and all of these – galaxies, clusters and superclusters – are pulling and tugging at themselves and one
another
by means of their gravitational attraction. Since it’s extremely difficult to take all of this motion into account and interpret it correctly, measurements derived from red shift are continually subject to fine-tuning.
Halton C. Arp, who has spent many years at Mount Wilson Observatory studying galaxies, has cast a minority vote on the question of how much red shift should be trusted as a way of measuring distance to far-off galaxies and quasars. With Geoffrey Burbidge of the University of California, Arp has studied the brightest quasar, 3C273. Its red shift indicates that it should be about two billion light years away. Arp has found, however, that 3C273 appears to be interacting with a giant elliptical cloud of hydrogen gas no more than 65 million light years away in the constellation Virgo. There are other mysterious cases where objects at dramatically different red shifts appear to be linked. Arp believes that quasars are not as remote as most astronomers have been measuring them, but much closer to home. Most of Arp’s colleagues pass off his evidence as coincidence.
While astronomy was probing deeper and deeper into space as the 20th century progressed, at a rate that makes earlier centuries look somnolent by comparison, it was also meeting some frustrating obstacles. Within the Milky Way Galaxy, visibility with the naked eye and optical telescopes is obscured by blotchy clouds of interstellar dust, especially in the direction of the Galactic centre. Of course, as Jansky discovered, the clouds of dust that block visible light are transparent to other types of radiation, but, except for radio waves and visible light, no other radiation can pass through the Earth’s atmosphere. The revelations of early radio astronomy led many people to suspect that there might be much more to the universe than any earth-bound telescope would ever see. World War II radar had helped usher in the age of radio astronomy. It took another impetus from the arena of international politics to boost telescopes above the Earth’s atmosphere.
Space-based astronomy began as a distant by-product of the Treaty of Versailles, which ended World War I. That treaty limited Germany to the production of artillery of small calibre. Germany turned research efforts and funds to rocketry instead of artillery and made significant progress in this new area. One result was the V-2 rockets used to attack southern England in 1944 and 1945, during World War II. A captured stock of V-2 rockets ended up in the United States, and 25 of them were set aside for scientific use. Projects were soon underway to develop controls that could position these rockets with sufficient accuracy and stability for astronomical observations to be made from them.
By the late 1950s, Western technology had improved steadily, and scientists were becoming accustomed to large budgets justified by the Cold War arms race and national security interests, even for projects that had no immediately obvious practical applications. A partnership of unprecedented scope between government and big-ticket science had begun. However, it was a surprise from the Soviet Union that jump-started Western space-age astronomy in earnest. On 4 October 1957, the Soviet Union launched Sputnik 1, the first human-made satellite to orbit the Earth. The perception was that the Soviet Union had pulled ahead in the race into space and, by implication, in the arms race as well. To catch up, the Western nations, particularly the United States, began to pour money into the development of technology for spacecraft and satellites, and also into science education. I was a teenager then, and I must admit I can’t recall there
being
a race into space until suddenly ‘they’ were winning. The panic filtered down to the level of my high school in Texas which wasn’t, evidently, as good at producing scientists as Soviet schools were. Better physics and maths books and lab equipment were purchased, teachers went to workshops for retraining, and we were all urged to take more classes in these subjects. My younger brother’s stock, as a fledgling physicist and computer whiz, went up. Mine, as a
classical
musician, went down. At the University of Texas, my later-to-be husband was called unpatriotic by his maths professor for choosing not to major in maths.
Before long the Western nations also had sent spacecraft into orbit, and the race continued, to the enormous benefit of all astronomy, not just space-based projects. Cold War competition paid the escalating bills. Much had changed since 1609, when Galileo put together his own perspicillum, and the 19th century, when Lord Rosse financed the construction of his Leviathan and a university community pooled their resources to buy Harvard’s first good telescope. In the early 20th century, a portion of Lowell’s fortune was enough to build a major observatory, and a corporation, AT&T, paid for the telescope at Bell Labs. The cost of astronomy in the late 20th century was on another scale entirely, which strained even the largest national budgets.
Fortunately, those nations that had the wherewithal also had motivation, other than the furthering of human knowledge, for supporting science. The technology that allowed telescopes to fly above the atmosphere, pointing upwards, also made it possible for surveillance cameras to point downwards. The technology that built and guided space probes also built and guided missiles. Moreover, the world image of a modern superpower included, just as it had done on a smaller scale for the Medicis in Renaissance Florence and Louis XIV of France, an image of technological and scientific pre-eminence.
When telescopes and cameras probed for the first time above the atmosphere in the late 1950s and early 1960s, those who put them there found there was a great deal to be seen. The full potential of space-based astronomy took years to come to fruition and is still not fully realized. Nevertheless, a new era had begun. It was possible at last to view the universe without the refraction problem that had frustrated astronomers since Galileo and Tycho Brahe, and to study objects at many different wavelengths, comparing findings in one range with those in
another
. There were indeed things out there that don’t radiate at all at optical or radio wavelengths and others that look very different when viewed in other parts of the spectrum.
Unfortunately for our understanding of the Milky Way Galaxy, even all of this improved vision did not provide an easy way to calculate the distances to hot nebulae and clouds of gas in which stars are born, and it didn’t allow astronomers to see the Galaxy from the outside. Though they would in time find ways to compile maps that are almost as good as seeing the Galaxy from afar, it was still easier to study things beyond the Galaxy.
The most significant progress in that line of enquiry continued for a while to be made not from space but with ground-based telescopes. Hubble had used galaxies themselves as standard candles. Other astronomers had chosen to assume that the brightest galaxy in a major cluster has approximately the same absolute magnitude as the brightest galaxy in every other major cluster. This method turned out to have some pitfalls: if we see two candles at a distance from us at night, and know that the two would have identical brightness if we saw them close up, we can judge their distance relative to one another by how bright they appear to us – unless without realizing it we happen to catch one of the candles just as a moth has flown into it and caused it to flare unusually bright. There is a risk of something like that happening when researchers use the brightest galaxies in galaxy clusters as standard candles. In a crowded cluster, it isn’t uncommon for galaxies’ paths to cross or come near enough so that a large galaxy ‘cannibalizes’ a smaller one, making the cannibal for a time much brighter than normal. The ‘temporary’ change can last a few hundred million years. Observing a distant galaxy cluster, and deciding that what appears to be the brightest galaxy in it can be used as a distance calibrator, involves a risk of choosing a galaxy that has made a meal of another within the last few hundred million years and therefore cannot be fairly compared with the brightest galaxies in other clusters, who may still be hungry.
Addressing this and other problems, Allan Sandage with Gustav Tammann from Switzerland carried cosmic-distance-ladder construction to unprecedented heights. Beginning in the early 1960s, Sandage and Tammann began using the 200-inch Mount Palomar telescope to set in place carefully calculated rungs leading deeper and deeper into the universe. They first utilized Cepheids to estimate distances to galaxies in the ‘Local Group’, the group that includes the Milky Way, Andromeda and other relatively nearby galaxies. Their next step, also with Cepheids, got them to a great spiral galaxy, NGC 2403, in another ‘group’ called M81. With these distances in hand, they proceeded to measure the size and luminosity of huge clouds of ionized hydrogen gas in these galaxies, and to study how the size and luminosity of these clouds is related to the overall luminosity of the galaxies. That relationship gave them a way to calculate the distances of more remote galaxies containing similar gaseous clouds.