I'm very glad to see Mark Hanson and Mike Selby get an APOD! They are great astrophotographers!
NGC 1566 is, in my opinion, the best example in the relatively nearby sky of a "perfect" spiral. The winner in that category is often said to be M74, but I think that NGC 1566 is better.
As you can see, the arms of NGC 1566 are much more "dramatic" than the arms of M74. The inner arms of NGC 1566 are bright, extremely symmetrical and peppered with pink nebulas and star formation. M74, by contrast, is known as the "Ghost galaxy", because it is so hard to spot in an amateur telescope due to the faintness of its arms and disk.
Mark Hanson and Mike Selby has not got access to a really professional telescope. Therefore, I can't resist showing you an "official" Hubble image of M74, and a picture of NGC 1566 based on Hubble Legacy Archive:
NGC 1566. Andrealuna Pizzetti/Hubble Legacy Archive
The "dramatic" nature of the arms of NGC 1566 is obvious, as is the "soft" nature of the arms of M74. The arms of NGC 1566 stand out starkly because of the brilliant star formation along the length of the inner arms (and the apparent "emptiness" just outside them), whereas in M74 the arms stand out much more weakly from the disk.
Another difference is that NGC 1566 is a barred galaxy. NGC 1566 does not have a long or strong bar, but you can clearly see that NGC 1566 appears to have a "skewed" center. Mark Hanson and Mike Selby's image shows you that NGC 1566 does not have a "filled", perfectly "round" outer disk, but that its outer arms give it a "dancing" shape.
M74, by contrast, is "all round".
It is interesting to compare NGC 1566 with NGC 1365:
The shape of NGC 1365 is obviously even more dramatic than the shape of NGC 1566. NGC 1365 seems to be made up exclusively of a long bar and two long arms that start at impossible angles at the ends of the bar. I'd say that NGC 1566 is "intermediate" between M74 and NGC 1365!
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