Radial arms of the SBa, Sbb and Sbc galaxies - why are they radial?

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When looking at the classifications of galaxies in the attached image,

galaxy types.JPG


how do we explain the SB types - especially the SBc which has long arms that reach out radially before beginning to spiral?

If these arms are spinning, then regardless of the speeds we measure, along the straight portion of the arms, the speeds appear to double as distance doubles (like the speeds of sections of a rope spinning around you). It's almost as if they are held together by gravity and are pulled outward by centrifugal force (like a rope spinning around you).

How do we explain these arms that reach 'straight' out?

Thanks!
 

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