A bit of mould is a pleiad of flowers; a nebula is an ant-hill of stars.

An observer situated in a nebula and moving with the nebula will observe the same properties of the universe as any other similarly situated observer at any time.

But when we face the great questions about gravitation Does it require time? Is it polar to the 'outside of the universe' or to anything? Has it any reference to electricity? or does it stand on the very foundation of matter–mass or inertia? then we feel the need of tests, whether they be comets or nebulae or laboratory experiments or bold questions as to the truth of received opinions.

I think that the event which, more than anything else, led me to the search for ways of making more powerful radio telescopes, was the recognition, in 1952, that the intense source in the constellation of Cygnus was a distant galaxy—1000 million light years away. This discovery showed that some galaxies were capable of producing radio emission about a million times more intense than that from our own Galaxy or the Andromeda nebula, and the mechanisms responsible were quite unknown. ... [T]he possibilities were so exciting even in 1952 that my colleagues and I set about the task of designing instruments capable of extending the observations to weaker and weaker sources, and of exploring their internal structure.

It seems to me that the evidence ... is opposed to the view that the spirals are individual galaxies comparable with our own. In fact, there appears as yet no reason for modifying the tentative hypothesis that the spirals are not composed of typical stars at all, but are truly nebulous objects.
[Contradicting the view of Heber Curtis during the Shapley-Curtis debate on 26 Apr 1920 to the National Academy of Sciences.]

It will be found that those contained in one article [class of nebulae], are so closely allied to those in the next, that there is perhaps not so much difference between them, if I may use the comparison, as there would be in an annual description of the human figure, were it given from the birth of a child till he comes to be a man in his prime.

Let us ... consider the ovum [egg] as a physical system. Its potentialities are prodigious and one's first impulse is to expect that such vast potentialities would find expression in complexity of structure. But what do we find? The substance is clouded with particles, but these can be centrifuged away leaving it optically structureless but still capable of development.... On the surface of the egg there is a fine membrane, below it fluid of high viscosity, next fluid of relatively low viscosity, and within this the nucleus, which in the resting stage is simply a bag of fluid enclosed in a delicate membrane.... The egg's simplicity is not that of a machine or a crystal, but that of a nebula. Gathered into it are units relatively simple but capable by their combinations of forming a vast number of dynamical systems...

On the most usual assumption, the universe is homogeneous on the large scale, i.e. down to regions containing each an appreciable number of nebulae. The homogeneity assumption may then be put in the form: An observer situated in a nebula and moving with the nebula will observe the same properties of the universe as any other similarly situated observer at any time.

One of the most striking results of modern investigation has been the way in which several different and quite independent lines of evidence indicate that a very great event occurred about two thousand million years ago. The radio-active evidence for the age of meteorites; and the estimated time for the tidal evolution of the Moon's orbit (though this is much rougher), all agree in their testimony, and, what is far more important, the red-shift in the nebulae indicates that this date is fundamental, not merely in the history of our system, but in that of the material universe as a whole.

Our Milky Way is the dwelling; the nebulae are the city.

Picture yourself during the early 1920's inside the dome of the [Mount Wilson Observatory]. … [Milton] Humason is showing [Harlow] Shapley stars he had found in the Andromeda Nebula that appeared and disappeared on photographs of that object. The famous astronomer very patiently explains that these objects could not be stars because the Nebula was a nearby gaseous cloud within our own Milky Way system. Shapley takes his handkerchief from his pocket and wipes the identifying marks off the back of the photographic plate.
Of course, Hubble came along in 1924 and showed that it was just these Cepheid variable stars in the Andromeda Nebula which proved it was a separate galaxy system.

So numerous are the objects which meet our view in the heavens, that we cannot imagine a point of space where some light would not strike the eye;—innumerable stars, thousands of double and multiple systems, clusters in one blaze with their tens of thousands of stars, and the nebulae amazing us by the strangeness of their forms and the incomprehensibility of their nature, till at last, from the limit of our senses, even these thin and airy phantoms vanish in the distance.

The evidence at present available points strongly to the conclusion that the spirals are individual galaxies, or island universes, comparable with our own galaxy in dimension and in number of component units.
[Stating his conviction on the nature of nebulae during the Shapley-Curtis debate on 26 Apr 1920 to the National Academy of Sciences.]

The interpretation of messages from the earth’s interior demands all the resources of ordinary physics and of extraordinary mathematics. The geophysicist is of a noble company, all of whom are reading messages from the untouchable reality of things. The inwardness of things—atoms, crystals, mountains, planets, stars, nebulas, universes—is the quarry of these hunters of genius and Promethean boldness.

The self-same atoms which, chaotically dispersed, made the nebula, now, jammed and temporarily caught in peculiar positions, form our brains; and the “evolution” of brains, if understood, would be simply the account of how the atoms came to be so caught and jammed.

What caused me to undertake the catalog was the nebula I discovered above the southern horn of Taurus on September 12, 1758, while observing the comet of that year. ... This nebula had such a resemblance to a comet in its form and brightness that I endeavored to find others, so that astronomers would not confuse these same nebulae with comets just beginning to shine. I observed further with suitable refractors for the discovery of comets, and this is the purpose I had in mind in compiling the catalog.
After me, the celebrated Herschel published a catalog of 2000 which he has observed. This unveiling the sky, made with instruments of great aperture, does not help in the perusal of the sky for faint comets. Thus my object is different from his, and I need only nebulae visible in a telescope of two feet [focal length].