Spiral Arms

I am still interested in how much information galactic sf imparts about galactic structures and would welcome input concerning the contributions of any sf writers with whose works I am less familiar. For simplicity at this time of night, I will merely summarize what has been said about one structure of our spiral galaxy, the arms, by two authors, Poul Anderson and James Blish.

Anderson describes two stages of interstellar colonization whereas Blish, I think, made two mutually inconsistent statements. In the Terran Empire period of Anderson's History of Technic Civilization, human beings have colonized a four hundred light year diameter volume of space toward the end of one galactic arm whereas, in the later Commonalty period, they have spread through two or three of the arms. (This does not tell us a great deal about the arms but does emphasize that they are significant structures within the galaxy.)

Blish said in conversation that his "Rift" might have been a gap between two spiral arms whereas he definitely stated in a novel that there were no such gaps, just areas where dust concealed the stars. I hoped by a quick google search to confirm or disconfirm this latter statement but have not been able to do so as yet. Another detail in a Blish text is a division of the galaxy into clusters, each of which is part of a "condensation," each of which is part of an arm. This remains vague but does give us a picture of something more out there than just individual stars with their planetary systems.

Of course, if we want to read popular science accounts of galaxies, we can easily google them but it remains an interesting question how much of the scientific information is incorporated into, and communicated by, sf novels that are written on a galactic scale.