It is worth remembering that we are barely 100 years from the time in which the notion of galaxies was not common. The textbook itself discusses Nebulae near th

It is worth remembering that we are barely 100 years from the time in which the notion of galaxies was not common. The textbook itself discusses Nebulae near the end, in all of 2--3 pages.[1] From p. 199: It was consequently thought, at once, that all nebulae might be shown to be star clusters under sufficiently high powers of the telescope. But the spectrascope has shown that such an inference is untenable, as the spectra of the two things are quite different, and that the spectrum of a nebula contains a line which is found nowhere else, and which is attributed to some substance called nebulum, and which is totally unknown anywhere except in a nebula. I propose we adopt neo-nebulum as a more neutral term than dark matter!! [1] https://archive.org/details/cu31924031322203/page/n213/mode/2up https://archive.org/details/cu31924031322203/page/n213/mode/...