The telescopes of the 17th and 18th century challenged all traditional views of the cosmos, and posed a flurry of even more challenging questions. One was the nature of the fuzzy "nebulae", a few of which were known from ancient times, but in telescopes presented a profusion of forms and appearance.
Many explanations for nebulae were proposed — many of them were very fanciful. A few were at least more mechanistic in nature. Two of them presaged the present understanding of these phenomena.
Immanuel Kant was not the first to propose that the elliptical (spiral) nebulae might consist of stars, but he published a stunningly correct line of reasoning, based on the best scientific principles. His explanation was rejected by the scientific community for a hundred and fifty years, until E. Hubble proved conclusively that some nebulae were in fact made of stars, and lay outside our own Milky Way galaxy.
The most common reason given for rejecting Kant's reasoning was that the distance scales required were "unthinkable". (A funny complaint, considering that no astronomical scale is humanly "thinkable".)
It should be emphasized that even today and even in the nearest of these galaxies, only a few extremely bright stars, whose brightness somehow varies in time, can be resolved individually. And in the 18th century, the equipment for observing the starlike properties of the galaxies did not yet exist.
Today, unfortunately, many textbooks record that Kant was right, but do not present his reasons — or worse, dismiss his reasoning as unscientific speculation. This is a sorry statement of the current state of philosophy of science — the rank and file haven't progressed at all since Kant's time. (I saw the argument in English only in the famous popular book The Realm of the Nebulae by Hubble himself — who had the perception expected of a scientist.)
Kant's theory is usually referred to as the "island universe" theory, and an alternative theory of Laplace is called the "nebular hypothesis". The latter held that these spiral shapes were indeed whirlpools of material, and were solar systems in the making. Since reversing its stance in the 1920s, the scientific community now tends to portray Kant as being right, but for somehow the wrong reasons, and Laplace as being silly and wrong. But of course... Laplace proposed a mechanism that is very similar to the current understanding of how both solar systems and galaxies form — from a swirl of gas and dust. He had a viable and correct picture, and was wrong only in proposing that the spiral nebulae were examples of solar system formation.
The passage is in Immanuel Kant, in "Allgemeine Natürgeschichte und Theorie des Himmels", published first in 1755.
This translation is from Edwin Hubble‘s old book The Realm of the Nebulae, which my parents had bought me.
Kant credits Thomas Wright (1711–86), with first proposing that (some) nebulae were assemblages of stars, in An Original Theory or New Hypothesis of the Universe (London, 1750).
I come now to another part of my system, and because it suggests a lofty idea of the plan of creation, it appears to me as the most seductive. The sequence of ideas that led us to it is very simple and natural. They are as follows : let us imagine a system of stars gathered together in a common plane, like those of the Milky Way, but situated so far away from us that even with the telescope we cannot distinguish the stars composing it ; let us assume that its distance, compared to that separating us from the stars of the Milky Way, is in the same proportion as the distance of the Milky Way is to the distance from the earth to the sun ; such a stellar world will appear to the observer, who contemplates it at so enormous a distance, only as a little spot feebly illumined and subtending a very small angle ; its shape will be circular, if its plane is perpendicular to the line of sight, elliptical, if it is seen obliquely. The faintness of its light, its form, and its appreciable diameter will obviously distinguish such a phenomenon from the isolated stars around it.
We do not need to seek far in the observations of astronomers to meet with such phenomena. They have been seen by various observers, who have wondered at their strange appearance, have speculated about them, and have suggested sometimes the most amazing explanations, sometimes theories which were more rational, but which had no more foundation than the former. We refer to the nebulae, or, more precisely, to a particular kind of celestial body which M. de Maupertius[] describes as follows : “These are small luminous patches, only slightly more brilliant than the dark background of the sky ; they have this in common, that their shapes are more or less open ellipses ; and their light is far more feeble than that of any other objects to be perceived in the heavens.”
It is much more natural and reasonable to assume that a nebula is not a unique and solitary sun, but a system of numerous suns, which appear crowded, because of their distance, into a space so limited that their light, which would be imperceptible were each of them isolated, suffices, owing to their enormous numbers, to give a pale and uniform luster. Their analogy with our own system of stars ; their form, which is precisely what it should be according to our theory ; the faintness of their light, which denotes an infinite distance ; all are in admirable accord and lead us to consider these elliptical spots as systems of the same order as our own—in a word, to be Milky Ways similar to the one whose constitution we have explained. And if these hypotheses, in which analogy and observation consistently lend mutual support, have the same merit as formal demonstrations, we must consider the existence of such systems as demonstrated. … We see that scattered through space out to infinite distances, there exist similar systems of stars [nebulous stars, nebulae], and that creation, in the whole extent of its infinite grandeur, is everywhere organized into systems whose members are in relation with one another. … A vast field lies open to discoveries, and observation alone will give the key.
My notes:
"unermeßlich" is translated as "infinite", but perhaps a more direct
and correct translation would be "immense".
Kant is often credited with coining the term "island universe", but
that term does not appear in this publication, and I have been unable
to track its source down.
From Wright's An original theory or new hypothesis of the universe
"...the many cloudy spots, just perceivable by us, as far without our Starry
regions, in which tho' visibly luminous spaces, no one star or particular
constituent body can possibly be distinguished; those in all likelihood may
be external creation, bordering upon the known one, too remote for even our
telescopes to reach."
"... the Milky Way is formed of an infinite Number of small Stars, ... a vast infinite Gulph, or Medium, every Way extended like a Plane, and inclosed between two Surfaces"
Wright speaks of the individual galaxies as "Creations", and goes on to propose an "unlimited Plenum of Creations".
I think what he has in mind is something like the more modern notion of "parallel universes" -- they are individual "Creations".
His use of the word "infinite" and "unlimited" is also interesting. I wonder if in his time these English words were being used more loosely than today, or whether he really intended the mathematical abstraction of infinity.
Descartes had built a world system, largely on the idea of vortices. Whittaker, in A History of the Theories of Aether and Electricity muses: " It is curious to speculate on the impression which would have been produced had the spirality of nebulae been discovered before the overthrow of the Cartesian theory of vortices. "