1917 – Heber Curtis, using novae found in a spiral, found their distance to be far outside the MW. He further suggested, based on mag., that they are “of and order of 100 times as far away…”
1917 – Ritchey also found faint novae in spiral nebulae, suggesting distances in the millions of light years. He also noted that they number over 1 million.
1917 -- Slipher at the Am. Philosophical Society gave his report on 25 velocities, after getting his math checked by computer Elizabeth Williams in Boston. Slipher announced these observations were favorable to the “island universe” theory. Slipher also suggested the spirals might be “’scattering’ a precocious intimation of the cosmic expansion.”
1920 – Wolf (at Heidelberg) found a SN in NGC2608 in 1920, showing that it contained stars, thus extragalactic.
1920 – Cosmologies “Great Debate”. Curtis presented his work that the spirals were “island universes”. Shapley presented his view. [They never actually debated.

] And where was Hubble in this great presentation moment to favor a universe comprised of galaxies? *cough* [Hubble and Curtis did work with one another.]
Curtis stated…
1) Spirals displayed the spectra typical for collections of stars, not gas.
2) No spiral had ever been found in the MW.
3) Spirals seen away from the MW due to MW gas/dust blocking views.
4) Novae (not SN) were too faint to not be very far away.
5) Slipher great redshift velocities.
But Shapley also argued that the MW was large and Curtis that it wasn’t. So both had some good arguments.
1922 -- – Opik found, using radial redshifts for his assumed central mass, the distance to Andromeda to be 450 kpc.
1923 – Eddington published his
Mathematical Theory of Relativity, 2nd edition in 1924, which included Slipher’s 41 radial velocities. It was reprinted many times and became a textbook. He included Slipher’s redshifts, thus entering mainstream science.
1923, Oct 4th – Hubble discovers a Cepheid in Andromeda. He estimates its distance to be 900k lyrs. [He was unaware that Cepheid come in two main flavors, and he used the wrong ones, hence is low distance estimate.]
1923 Dec – Hubble, using the brightest star method, measured distance of elliptical (like the LMC) to be 1 million lyrs.