You might also appreciate W. W. Morgan's account of his discovery of the Perseus Spiral Arm of our Galaxy in his observational data on OB stars:
"This was in the fall of 1951, and I was walking between the observatory and home, which is only 100 yards away. I was looking up in the sky ... just looking up in the region of the Double Cluster [in Perseus], and I realized I had been getting distance moduli corrected the best way I could with the colors that were available, for numbers of stars in the general region ... Anyway, I was walking. I was looking up at the sky, and it suddenly occurred to me that the double cluster in Perseus, and then a number of stars in Cassiopeia, these are not the bright stars but the distant stars, and even Cepheus, that along there I was getting distance moduli, of between 11 and 12, corrected distance moduli. Well, 11.5 is two kiloparsecs ... and so, I couldn’t wait to get over here and really plot them up. It looked like they were at the same distance ... It looked like a concentration ... And so, as soon as I began plotting this out, the first thing that showed up was that there was a concentration, a long narrow concentration of young stars ... There are HII regions along there too ... And that was the thing that broke [the problem] down."
"This was in the fall of 1951, and I was walking between the observatory and home, which is only 100 yards away. I was looking up in the sky ... just looking up in the region of the Double Cluster [in Perseus], and I realized I had been getting distance moduli corrected the best way I could with the colors that were available, for numbers of stars in the general region ... Anyway, I was walking. I was looking up at the sky, and it suddenly occurred to me that the double cluster in Perseus, and then a number of stars in Cassiopeia, these are not the bright stars but the distant stars, and even Cepheus, that along there I was getting distance moduli, of between 11 and 12, corrected distance moduli. Well, 11.5 is two kiloparsecs ... and so, I couldn’t wait to get over here and really plot them up. It looked like they were at the same distance ... It looked like a concentration ... And so, as soon as I began plotting this out, the first thing that showed up was that there was a concentration, a long narrow concentration of young stars ... There are HII regions along there too ... And that was the thing that broke [the problem] down."
Full article here: https://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_que...