© CAMBRIDGE UNIVERSITY PRESS 1999

3. Glimpses of Structure

Stubbornness, stamina, boldness, and luck enabled William Herschel to connect our Universe with Newton's and Kant's speculations. Leaving Hanover in 1757 after the French occupation, he settled in England as an itinerant teacher, copier, and composer of music, becoming organist of the Octagon Chapel at Bath in 1766. But his real interest from childhood was astronomy. He privately built a succession of larger and larger reflecting telescopes and systematically swept the heavens. His sister, Caroline, emigrating in 1772, helped with these nightly observations, to the eventual destruction of her own singing career. In 1781, Herschel had the great luck to find Uranus, the first planet discovered since the days of the ancients, although he originally thought it was just a comet. Fame followed quickly, and fortune soon after when George III granted him a pension for life. He supplemented this by building small telescopes for sale (until his wealthy marriage in 1788) and became a full-time astronomer. Career paths, like the subject itself, have changed considerably since then.

For twenty years, starting in 1783, Herschel searched for nebulae with his 20-foot telescope and its 18 7/10 inch speculum mirror. Messier's catalog, available in 1781,had inspired him first to try to resolve known nebulae with his superior telescope, and then to discover more. Originally, he believed all nebulae were star clusters and claimed to have resolved M31, the Andromeda galaxy, into stars. Nearly three decades later after he had discarded this belief, he also dropped the claim (Hoskin, 1963). This was to be the first of several times that Andromeda would mislead astronomers.

Although the nature of the nebulae was unknown, their projected positions at least were straightforward. Messier (1781) noticed that their distribution on the sky was irregular; 13 of the 103 in his catalog are in the Virgo constellation. As it happens, all 13 are galaxies and so Messier was first to see a cluster of galaxies. The Virgo cluster with thousands of members is one of the largest and most varied of those nearby.

Soon after, in his more extensive catalog, Herschel (1784) found the Coma Cluster with its "many capital nebulae" and noticed other inhomogeneities and voids:

In my late observations on nebulae I soon found, that I generally detected them in certain directions rather than in others: that the spaces preceding them were generally quite deprived of their stars, so as often to afford many fields without a single star in it: that the nebulae generally appeared some time after among stars of a certain considerable size, and but seldom among very small stars, that when I came to one nebula, I generally found several more in the neighborhood; that afterwards a considerable time passed before I came to another parcel; and these events being often repeated in different altitudes of my instrument, and some of them at a considerable distance from each other, it occurred to me that the intermediate spaces between the sweeps might also contain nebulae; and finding this to hold good more than once, I ventured to give notice to my assistant at the clock, "to prepare, since I expected in a few minutes to come at a stratum of the nebulae, finding myself already" (as I then figuratively expressed it) "on nebulous ground." In this I succeeded immediately; so that I now can venture to point out several not far distant places, where I shall soon carry my telescope, in expectation of meeting with many nebulae. But how far these circumstances of vacant places preceding and following the nebulous strata, and their being as it were contained in a bed of stars, sparingly scattered between them, may hold good in more distant portions-of the heavens, and which I have not yet been able to visit in any regular manner, I ought by no means to hazard a conjecture. The subject is new and we must attend to observations and be guided by them, before we form general opinions.

Part of this patchiness, we know now, is caused by interstellar obscuration and part is intrinsic.

Seven months later, Herschel (1785) was ready to announce his conjectures and general opinions:

By continuing to observe the heavens with my last constructed, and since that time much improved instrument, I am now enabled to bring more confirmation to several parts that were before but weakly supported, and also to offer a few still further extended hints, such as they present themselves to my present view. But first let me mention that, if we would hope to make any progress in an investigation of this delicate nature, we ought to avoid two opposite extremes, of which I can hardly say which is the most dangerous. If we indulge a fanciful imagination and build worlds of our own, we must not wonder at our going wide from the path of truth and nature; but these will vanish like the Cartesian vortices, that soon gave way when better theories were offered. On the other hand, if we add observation to observation, without attempting to draw not only certain conclusions, but also conjectural views from them, we offend against the very end for which only observations ought to be made. I will endeavour to keep a proper medium; but if I should deviate from that, I would wish not to fall into the latter error.

That the milky way is a most extensive stratum of stars of various sizes admits no longer of the least doubt; and that our sun is actually one of the heavenly bodies belonging to it is as evident. I have now viewed and gaged this shining zone in almost every direction, and find it composed of stars whose number, by the account of these gages. constantly increases and decreases in proportion to its apparent brightness to the naked eye. But in order to develop the ideas of the universe, that have been suggested by my late observations, it will be best to take the subject from a point of view at a considerable distance both of space and of time.

Let us then suppose numberless stars of various sizes, scattered over an infinite portion of space in such a manner as to be almost equally distributed throughout the whole. The laws of attraction, which no doubt extend to the remotest regions of the fixed stars, will operate in such a manner as most probably to produce the following remarkable effects.

Form I. In the first place, since we have supposed the stars to be of various sizes, it will frequently happen that a star, being considerably larger than its neighboring ones, will attract them more than they will be attracted by others that are immediately around them; by which means they will be, in time, as it were, condensed about a center; or, in other words, form themselves into a cluster of stars of almost a globular figure, more or less regularly so, according to the size and original distance of the surrounding stars. The perturbations of these mutual attractions must undoubtedly be very intricate, as we may easily comprehend by considering what Sir Isaac Newton says in the first book of his Principia, in the 38th and following problems; but in order to apply this great author's reasoning of bodies moving in ellipses to such as there are here, for a while, supposed to have no other motion than what their mutual gravity has imparted to them, we must suppose the conjugate axes of these ellipses indefinitely diminished, where the ellipses will become straight lines.

Form II. The next case, which will also happen almost as frequently as the former, is where a few stars, though not superior in size to the rest, may chance to be rather nearer each other than the surrounding ones; for here also will be fonned a prevailing attraction in the combined center of gravity of them all, which will occasion the neighboring stars to draw together; not indeed so as to form a regular globular figure, but however in such a manner as to be condensed towards the common center of gravity of the whole irregular cluster. And this construction admits of the utmost variety of shapes, according to the number and situation of the stars which first gave rise to the condensation of the rest.

Form III. From the composition and repeated conjunction of both the foregoing forms, a third may be derived, when many large stars, or combined small ones, are situated in long extended, regular, or crooked rows, hooks, or branches; for they will also draw the surrounding ones, so as to produce figures of condensed stars coarsely similar to the former which gave rise to these condensations.

Form IV. We may likewise admit of still more extensive combinations; when, at the same time that a cluster of stars is forming in one part of space, there may be another collecting in a different, but perhaps not far distant quarter, which may occasion a mutual approach towards their common center of gravity.

Form V. In the last place, as a natural consequence of the former cases, there will be formed great cavities or vacancies by the retreat of the stars towards the various centers which attract them; so that upon the whole there is evidently a field of the greatest variety for the mutual combined attractions of the heavenly bodies to exert themselves in.

In a paper whose abstract is its title, Herschel (1811) later illustrated some shapes of the nebulae that had led him to these conclusions. Figure 3.1 shows Herschel's drawings based on the visual appearance of these nebulae through his telescopes. Many of our modern classifications are here, apart from the spirals, which could not be resolved; with hindsight we can look for hints of their structure in these sketches. It was this wide range of patterns that prompted Herschel and many subsequent astronomers to propose that we were seeing sequential evolution.

Figure 3.1. W. Herschel's (1811) sketches of some of the nebulae in his catalog

The first great debate to issue from these catalogs was whether nebulae were part of the Milky Way, or formed a separate local system, or were very distant. It was even conceivable - although proponents of "Occam's Razor" might have scoffed - that there were different types of nebulae and all three possibilities were true. John Herschel (1847) began the debate by devising an equal area (isographic) projection of the celestial sphere onto a disk in a plane and counting nebulae in areas of 3° polar distance and 15' right ascension. His statistical distribution confirmed the general inhomogeneity and showed the Virgo Cluster with its southern extension, as well as several superclusters (e.g., Pisces). "The general conclusion which may be drawn from this survey, however, is that the nebulous system is distinct from the sidereal, though involving, and perhaps, to a certain extent, intermixed with the later." A rather ambiguous, but essentially correct, conclusion. Herschel continues on to discuss how the nebulae divide "into two chief strata, separated (apparently) from each other by the galaxy." The following year, Nichol (1848) wrote about "superb groups of galaxies separated from each other by gulfs so awful, that they surpass the distance that divide star from star .... Amid this system of clusters, floats the galaxy whose glories more nearly surround us." The idea that some of the nebulae might be rather special was becoming stronger. Astronomers were beginning to call them "galaxies."

Little could be decided, however, until distinctions between nebulae were better understood. Lord Rosse's (1850) mammoth 53-foot telescope with its 6-foot speculum mirror resolved some of the nebulae into spirals - a new category and a major clue. Another major discovery that differentiated nebulae was William Huggins's (1864) measurement of their spectra. Some such as Orion, the Crab, and the "planetary nebulae" were mainly gaseous; others remained mysterious. It would take some time for the meaning of Huggins's spectra to become clear. Meanwhile Alexander von Humboldt (1860), intrepid explorer of the earth and the heavens, could forecast:

If these nebulous spots be elliptical or spherical sidereal [stellari groups, their very conglomeration calls to mind the idea of a mysterious play of gravitational forces by which they are governed. If they be vapory masses, having one or more nebulous nuclei, the various degrees of their condensation suggest the possibility of a process of gradual star formation from inglobate matter. No other cosmical structure ... is, in like degree, adapted to excite the imagination, not merely as a symbolic image of the infinitude of space, but because the investigation of the different conditions of existing things, and of their presumed connection of sequences, promises to afford us an insight into the laws of genetic development.

Cleveland Abbe (1867), a twenty-nine year old American astronomer who would later turn to meteorology, took the next step. He divided Herschel's 1864 "General Catalog of Nebulae and Clusters of Stars" with about 5,079 entries (a few being redundant) into ordinary clusters, and globular clusters, and resolved and unresolved nebulae. He concluded that the planetary and annular nebulae - primarily gaseous according to Huggins's spectra - had the same distribution as the clusters and were part of the Milky Way. The other resolved and unresolved nebulae, counted in cells of 0.50 in right ascension and 10° in declination, were outside the Milky Way. At first he thought their comparative absence around the Milky Way might be an illusion caused by the glare of starlight. But since it remained true at large distances from the Milky Way, and increasingly powerful telescopes failed to find many new nebulae near the Milky Way, he concluded the effect was real. His explanation was that the plane of the Milky Way "cuts nearly at right angle the axis of a prolate ellipsoid, within whose surface all the visible nebulae are uniformly distributed."

Two years later, R. A. Proctor (1869) joined the debate, concluding from the same data that the nebulae were part of the Milky Way. How could he do this? From Abbe's tables of counts he constructed an isographic (equal area) projection (initially apparently without realizing that J. Herschel had developed this earlier). The resulting maps of counts - not actual positions - looked similar to Herschel's, despite Abbe's larger and more differentiated data set. This similarity led Proctor to the dangerous conclusion "that no extension of telescope observation can appreciably affect our views respecting the distribution of the nebulae." Proctor saw the same zone of avoidance of the nebulae, but he decided that its coincidence with the Milky Way was too unlikely to be accidental. Therefore "the nebular and stellar systems are parts of a single scheme." He considers the possibility "that there is some peculiarity in the galactic stratum preventing us from looking so far out into space along its length than elsewhere." We now recognize this as obscuration by interstellar dust. Proctor dismissed this possibility because he could see both nebulae and stars intermixed in the Magellanic Clouds. He does admit, however, that a few nebulae, perhaps the spirals, might be external galaxies. In correspondence with the seventy-seven year old John Herschel, Proctor elicited Herschel's vision of a hierarchical universe where each kvel contains miniatures of itself (Hoskin, 1987). Unlike his father, John Herschel seldom speculated, but this speculation was a forerunner of the idea of "self-similar" clustering.

It was left to an amateur astronomer, Sidney Waters (1873), to produce the first accurate equal area projection of the actual positions in Herschel's catalog. Figure 3.2 shows this map, which the Royal Astronomical Society published as a three-color lithograph. Objects are again divided into star clusters, resolvable nebulae, and unresolved nebulae. Waters sees the same patterns as his predecessors, though perhaps more clearly, and concludes like Proctor that they "surely prove beyond question that not only are the clusters, which are peculiar to the Milky Way, related to the nebulae, which seem to form a distinct scheme, but that the two schemes are probably subordinate parts of our sidereal system."

Figure 3.2. Sidney Waters's (1873) equal area map of the positions of nebulae in Herschel's catalog.