Coming of Age in the Milky Way (25 page)

Shapley had found that the globular clusters are distributed across a spherical expanse of space, as if they were part of an enormous metaglobular cluster, and that the center of this sphere is nowhere near the sun, but lies far away to the south, past the stars of Sagittarius. In a superbly daring intuitive leap, Shapley then conjectured—accurately, as it turned out—that the center of the realm of the globular clusters was also the center of the galaxy itself. As Shapley put it, “The globular clusters are a sort of framework—a vague skeleton of the whole Galaxy—the … best indicators of its extent and orientation.” If so, the sun lies far from the center of things: “The solar system can no longer maintain a central position,” Shapley asserted.
¹⁰

Shapley’s triumph was marred only by problems with his calculation
of distances. The diameter of the Milky Way galaxy previously had been reckoned—by various investigators, Shapley among them—at some fifteen to twenty thousand light-years. Now, with his Cepheid variable work in hand, Shapley concluded that the correct figure was three hundred thousand light-years—more than ten times larger than the dimensions entertained by his contemporaries, and three times the most generous estimates we have today.
^*

Various errors contributed to Shapley’s inflated picture of the Milky Way galaxy. Like many of his contemporaries, he underestimated the extent to which clouds of interstellar gas and dust dim the images of distant stars, making them appear farther away than they really are. Moreover, he assumed that the Cepheid variable stars he observed in globular clusters were essentially identical to those Henrietta Leavitt had found in the Magellanic Clouds; actually, as Walter Baade and other astrophysicists were to find, the cluster variables are less massive and intrinsically less bright, and therefore by implication less distant, than a straightforward comparison of their periods with those of their younger cousins would imply. Inaccuracies of this sort are routine on the cutting edge of science, but they had the dolorous effect of misleading Shapley into thinking that the Milky Way, rather than being but one galaxy among many, was a system of unique grandeur. He began to think of the Milky Way as more or less the entire universe, and to regard the spiral nebulae but its subjects or its satellites.

For these and perhaps for subtler psychological reasons as well, Shapley came to take a proprietary interest in defending what he called “the enormous, all-comprehending” dimensions of the galaxy that he had charted. He called this view his “big galaxy” hypothesis. Those who agreed with him tended to think of the word “big” in terms of its Norse etymology, from
bugge
, meaning “important.” Those who disagreed preferred to emphasize the word’s Latin etymology, from
buccae
, for “puffed up.”

Among the dissidents was Heber Curtis of Lick Observatory, an advocate of the “island universe” theory. Shapley reacted to
Curtis’s arguments with the abhorrence of a patient contemplating surgery: Curtis, he noted, “must shrink my galactic system enormously to have much luck with island universes.”
¹¹
The issue was formally debated by Shapley and Curtis under the auspices of the National Academy of Sciences, in Washington, D.C., on April 26, 1920. Shapley was generally judged the loser, but, as is usually the case in science, the debate settled little and the last word belonged not to men but to the sky.

The hypothesis defended by Curtis, that the spiral nebulae were galaxies of stars, would be confirmed if a spiral could be unambiguously resolved into stars. That vital step was accomplished in 1924 by Shapley’s colleague and nemesis, Edwin Hubble. A tall, elegant, and overbearing man with a highly evolved opinion of his potential place in history, Hubble made everything he did look effortless—he had been a track star, a boxer, a Rhodes scholar, and an attorney before turning astronomer—and one of the things he did most effortlessly was to infuriate Shapley. Hubble took scores of photographs of M33 and its neighbor M31, the Andromeda spiral, and found there what he later called “dense swarms of images in no way differing from those of ordinary stars.”
¹²

Whether the pinpoints of light on Hubble’s photographic plate really
were
stars, however, was open to contention; Shapley dismissed them as curds in a Laplacian nebula. Here, again, Henrietta Leavitt’s Cepheid variable stars provided the needed mileposts. Cepheids are bright enough to be discernible across intergalactic distances. Using the new one-hundred-inch telescope at Mount Wilson, Hubble photographed the spirals again and again, comparing the plates to find stars that had varied in brightness. His efforts soon bore fruit, and on February 19, 1924, he wrote Shapley, who by then had left Mount Wilson to become director of Harvard College Observatory, a laconic note containing one of the most momentous findings in the history of science: “You will be interested to hear that I have found a Cepheid variable in the Andromeda Nebula.”
¹³

Hubble deduced that Andromeda lies about one million light-years away, an estimate half the distance of later ones but clearly sufficient to establish that the spiral was well beyond even Shapley’s “big galaxy.” Shapley replied sourly that he found Hubble’s letter to be “the most entertaining piece of literature I have seen for a long time.”
¹⁴
Later he complained that Hubble had given him insufficient
credit for his priority in using Cepheid variables to chart distances. But the game was over. Hubble’s paper announcing that he had found Cepheids in spirals—read (in his Olympian absence) at a joint meeting of the American Astronomical Society and the American Association for the Advancement of Science in Washington, D.C., on New Year’s Day, 1925—initiated the final decline of the nebular hypothesis, the ascendancy of the island universe hypothesis, and humankind’s realization that we live in one among many galaxies.

Hubble went on to identify not only Cepheids but novae and giant stars in Andromeda and other galaxies. These studies helped allay his fear that the laws of physics might break down beyond our home galaxy, rendering his distance measurements invalid. Newton, too, had wondered whether “God is able … to vary the Laws of Nature, and make Worlds of several sorts in several Parts of the Universe.”
¹⁵
Hubble, in his short paper announcing the finding of Cepheids in M31, took time to caution that his results depended upon the assumption that “the nature of Cepheid variation is uniform throughout the observable portion of the universe.” When he found Cepheids and other familiar stars in the galaxy NGC 6822, he wrote with evident relief that “the principle of the uniformity of nature thus seems to rule undisturbed in this remote region of space.”
¹⁶

Some astronomers have a gift for making lovely, sharp photographs of galaxies with large telescopes. Hubble was not one of them, though he was adept at extracting essential data from the generally flawed plates he did obtain. Nor was he especially skillful at taking spectra, but in this he was soon aided by one Milton Humason, a resourceful young man with an inquiring mind who started out on Mount Wilson as a muleteer and observatory janitor, began assisting the astronomers in their work at the telescope, and eventually became an expert observational astronomer in his own right. Throughout the 1930s and 1940s, Hubble and Humason pushed back the frontiers of the observable universe, charting and cataloging ever more distant galaxies. Eventually, Hubble was taking photographs that were strewn with the images of more remote galaxies than foreground stars.

In 1952, the year before Hubble died, Walter Baade announced at a meeting of the International Astronomical Union in Rome that he had discovered an error in the calibration of the Cepheid period-luminosity
value, the correction of which doubled the cosmic distance scale. Further refinements in the distance scale were attained by Hubble’s former assistant Allan Sandage, later in collaboration with the Swiss astronomer Gustav Tammann, and it became possible for astronomers to measure, with some confidence, the distance to galaxies hundreds of millions to billions of light-years away.

At these distances, time commands a significance equal to that of space. Inasmuch as it takes time for the light from a distant galaxy to pass through space, we see the galaxy as it was long ago: The galaxies of the Coma cluster, for instance, appear to us as they looked seven hundred million years ago, when the first jellyfish were just appearing on Earth. Owing to this phenomenon, called lookback time, telescopes probe not only out into space but back into the past. It should, therefore, be possible to determine, by looking far into deep space, whether the universe was once different than it is today. Evidence that this is indeed the case came in the 1960s, when Sandage and radio astronomer Thomas Matthews discovered quasars, and Maarten Schmidt determined that they were extraordinarily far away. Quasars appear to be the nuclei of young galaxies, at distances of a billion light-years and more. There is nothing quite like them in the universe today. And so the exploration of space opened the pages of cosmic history.

The work of charting our place in the universe goes on, and today we can say with some confidence that the sun is a typical yellow star that lies in the disk of a major spiral galaxy, about two thirds of the way out from the galactic center. The disk contains not only stars and their planets but also vast, rarefied lakes of hydrogen and helium gas, denser knots of gas where atoms have been able to find one another and bind together as molecules, and giant thunderheads of soot given off by smoky stars. Waves generated by harmonics in the gravitational interaction of the myriad stars move across the disk in a graceful, spiral pattern, plowing the interstellar material into globules dense enough to collapse under the attraction of their own gravitational force. In this way new stars are formed, and it is the light of the most massive and shortest-lasting of the young stars that illuminates the spiral arms, making them visible. The spiral arms, then, are not objects but processes—as transitory, by the bounteous spatiotemporal standards of the Milky Way, as the back-blowing veils of froth that whitecap the waves of earthly oceans.

Beyond the Milky Way lie more galaxies. Some, like the Large Magellanic Cloud and the Andromeda galaxy, are spirals. Others are ellipticals, their stars hung in pristine, cloudless space. Others are dim dwarfs, some not much larger than globular clusters. Most belong, in turn, to clusters of galaxies. The Milky Way is one of a few dozen galaxies comprizing a gravitationally bound association that astronomers call the Local Group. That group in turn lies near one extremity of a lanky archipelago of galaxies called the Virgo Supercluster. If we could fly the sixty million or so light-years from here to the center of the supercluster, we would encounter many sights worth seeing along our way—the giant cannibal galaxy Centauras A, an elliptical busily gobbling up a spiral that blundered into it; the distended spiral M51, with its one outflung arm stretching after a departing companion galaxy; the furiously glowing spiral M106, with its bright yellow nucleus and its shoals of blue-white stars; and, at the supercluster core, the giant elliptical Virgo A, wreathed in thousands of globular star clusters, harboring some three trillion stars, and adorned by a blue-white plasma jet that has been spat from its core with the velocity of a bolt of lightning.

Beyond Virgo lie the Perseus, Coma, and Hercules clusters, and beyond them so many more clusters and superclusters of galaxies that it takes volumes just to catalog them. There is structure even on these enormous scales; the superclusters appear to be arrayed into gigantic cosmic domains that resemble the cells of a sponge. Beyond
that
, light from faraway galaxies, riding the contours of curved space, becomes as dappled as the moon’s reflection on a pond in a gentle breeze. Out there, awaiting some future Hubble or Herschel, lie many a tale of things past, or passing, or to come.