Island universes from Wright to Hubble.
In 1924 Edwin Hubble used Cepheid variable stars to measure the distance to M31. He determined once and for all that the famous "nebula" in Andromeda was too distant to belong to the Milky Way. By proving that this and other, spiral nebulae are external galaxies - distant Milky Ways in their own right - Hubble expanded our view of the universe immensely and ended one of the great astronomical debates of the 20th century. This discovery, goes the usual story, marked the start of our modern view of the universe.
But did it? The idea that the nebulae are other galaxies - immense "island universes" of stars in an inconceivably vast cosmos - was a very old one by the early 20th century. It began in the 1750s with the speculations by British astronomer Thomas Wright and: German philosopher Immanuel Kant that the Milky Way was not infinite but a bounded, flattened disk. And right from the beginning, they believed that irresolvable nebulae visible in telescopes were Milky Way-like collections of stars seen at great distances.
Modern astronomy textbooks generally squeeze the entire 175, years from Wright to Hubble into a few paragraphs focused on the beginning and the end of the story. This abridgement implies that Wright's views on external galaxies weren't in widespread favor until Hubble proved them. But was it really that simple? If you like your stories unabridged, you may wonder what happened in those intervening years.
A PRESCIENT NOTION.
A few years ago I came across this curious passage while reading Richard Anthony Proctor's book Other Worlds Than Ours (1870): "Yet once more, according to accepted views, thousands and thousands of galaxies, external to the sidereal system, can be seen with powerful telescopes."
Is this possible? Were nebulae widely considered to be external galaxies more than 50 years before Hubble proved it? I found supporting evidence in a short article titled "A new theory of the universe" in the London Popular Science Review (1869). The article began: "Mr. Proctor has recently put forward, a new theory respecting the arrangement of the stars and nebulae. Instead of looking upon the nebulae as for the most part external galaxies of stars, he considers that they belong to our own sidereal system.
Furthermore, in astronomy books published during most of the 1800s authors referred to nebulae as external galaxies. For example, Thomas the following lyrical remarks in his book Sidereal Heavens (1840): "Winging our flight from the Milky Way over unknown and immeasurable regions, regions where infinitude appears opening upon us in awful grandeur, we approach some of those immense starry clusters called the Nebulae, every one of which may be considered as another Milky Way, with its ten thousands and millions of suns."
The influence of this view can be found in popular writings of the time as well. Here is a passage from Edgar Allan Poe's lengthy essay "Eureka; published in a popular magazine in 1848: "The Galaxy, let me repeat, is but one of the clusters which I have been describing; but one of the miscalled 'nebulae' revealed to us - by the telescope alone, sometimes - as faint hazy spots in various quarters of the sky. We have no reason to suppose the Milky Way really more extensive than the least of these 'nebulae.' Its vast superiority in size is but an apparent superiority arising from our position in regard to it - that is to say, from our position in its midst." Poe went on to predict, correctly, that there are clusters of galaxies and even superclusters of clusters.
In books published after 1870, comments show that opinions had changed but that astronomers had indeed once regarded nebulae as external galaxies. For example, in his treatise Popular Astronomy (1884), Simon Newcomb stated "This idea that the nebulae were other galaxies was more or less in vogue among popular writers until a quite recent period."
RIGHT ANSWER, WRONG REASON.
Since many sources confirm that before 1870 nebulae were considered external galaxies, why was the "great debate" of the early 20th century so great? What happened between 1870 and 1924 that caused a correct idea to fall out of favor? The answer is ironic: new observations and better maps of the sky led to the notion's rejection.
To make sense of this paradox we must understand early ideas about the universe's form. In papers published in 1784 and 1785, William Herschel outlined a strategy for investigating our galaxy's structure. His method was based on two assumptions, that stars are distributed fairly uniformly through space and that they are also uniform in absolute magnitude. Herschel thus counted the stars in his telescope's field to estimate the relative distance to the edge of the galaxy in that direction. The more stars, the farther the edge. If stars were uniformly distributed, clumpy internal nebulae simply didn't fit well into the picture. Herschel's influence no doubt fostered the acceptance of nebulae as external galaxies.
However, as Herschel continued his studies he realized that the assumptions of uniformity were wrong and that many nebulae were probably gaseous and part of the Milky Way. By 1811 he had abandoned his earlier views. "When we examine the Milky Way, or the closely compressed clusters of stars, of which my catalogues have recorded so many instances," he wrote, "this supposed equality of scattering must be given up. We may have also surmised nebulae to be no other than clusters of stars disguised by their very great distance; but a longer experience and a better acquaintance with the nature of nebulae will not allow a general admission of such a principle."
Ironically, Proctor suggested that few astronomers noticed Herschel's change of view. So the idea that nebulae are external galaxies continued despite its rejection by Herschel himself.
A NEW IDEA EMERGES
In his book The Universe and the Coming Transits (1874), Proctor outlined many of the observations that caused astronomers to change their ideas about galaxies. These observations fell into two categories: those that contradicted assumptions of stellar uniformity and those that indicated nebulae are a physical part of the Milky Way.
Proctor himself compared the proper motions of bright stars with those of faint stars and found an equal average proper motion for each group. This indicated that stars of very different apparent magnitudes often lie at the same distance from us, and thus stars did not all have the same true brightness. Proctor also discovered what he called "star-drift" - groups of stars having the same proper motion - confirming the clustering of stars in a way other than by visual impressions.
A variety of mid-19th-century observations suggested nebulae could be considered part of the Milky Way. For example, in 1864 William Huggins found spectroscopic proof that some nebulae are indeed gaseous and thus aren't star clusters or galaxies too distant to be resolved into stars.
Perhaps the most fascinating argument was based upon a distribution pattern familiar to amateur astronomers. Star clusters, planetary nebulae, and emission nebulae are primarily found in the plane of the Milky Way. In contrast, galaxies seem most concentrated where stars are thinly spread - in the general region of the galactic poles. Today this distribution makes sense because we know that star clusters and nebulae are generally concentrated toward the midplane of the Milky Way's disk, while dust in the disk blocks our view of other galaxies. But this pattern made a different kind of sense to mid-19th-century astronomers. To them it indicated that the fuzzy objects must be nebulae within our sidereal system. In Other Worlds Than Ours Proctor wrote: "We have, then, no other conclusion to form but that the association thus observed between starless regions and richness of nebular distribution indicates a very close relation indeed between the stars and nebulae; that in fact, the nebulae in a sense represent the missing stars, that the region where those nebulae appear has been drained of star material, so to speak, in order to form them."
Proctor explicitly rejected extinction in the galactic plane as an explanation for the nebulae's distribution. The largest reflectors of the time seemed capable of nearly resolving the farthest edges of the Milky Way. If light was being blocked the edges should not be resolvable.
These arguments influenced the thinking of astronomers and helped to set the stage for the great debate of the early 20th century. Consider the following two passages:
"On the other hand, nebulae as a class show a distinct aversion for the galaxy, and are found most abundantly in those parts of the sky farthest from it, much as if they represented the raw material which was lacking along the Milky Way, because [it is] already worked up to make the stars which are there so numerous." (George Comstock, 1901)
"If however, there are other stellar systems of the same order as our own, they are neither the nebulae nor the clusters which the telescope reveals, but are far beyond the reach of any instrument at present existing." (Charles Young, 1902)
The fall of the view that nebulae are external galaxies left astronomers wondering if stars formed systems at all. Proctor believed that the universe was probably fairly uniform to begin with, and that through "processes of aggregation" it developed stars of differing sizes, clusters of stars, and gaseous masses gathered into annular or spiral shapes. He speculated that some force, perhaps centrifugal, caused matter to aggregate near the galactic poles, forming irresolvable nebulae but few stars.
In Popular Astronomy Simon Newcomb was less willing to assign a structure to the universe, confirming only that it was a disk or perhaps a ring with the Sun near the center and nebular regions on either side.
History is contingent. There was no one path destined to connect Wright to Hubble. If nebulae had been considered external galaxies throughout the late 19th century, would the debate of the early 20th century have been monumental? Or would Hubble's observations merely have been a ho-hum confirmation of a long-accepted idea, in much the same way that Friedrich Wilhelm Bessel's observation of stellar parallax in 1838 proved that Copernicus was right and Earth orbits the Sun? Perhaps Hubble never would have taken an interest in the nature of nebulae, and today the "tuning-fork" classification of galaxies, the eagerly sought expansion rate of the universe, and even the Space Telescope would carry a different name.
DAVID RUSSELL teaches earth science, chemistry, and astronomy at Owego Free Academy in upstate New York. Readers interested in astronomical history can reach him at 22 Ithaca Rd., Candor, NY 13743.
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|Title Annotation:||early examinations of nebulae|
|Publication:||Sky & Telescope|
|Date:||Jan 1, 1999|
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