<Li> A supermassive black hole at the very center of the central bulge </Li> <Li> A near - spherical dark matter halo </Li> <P> The relative importance, in terms of mass, brightness and size, of the different components varies from galaxy to galaxy . </P> <P> Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies . These long, thin regions resemble a spiral and thus give spiral galaxies their name . Naturally, different classifications of spiral galaxies have distinct arm - structures . Sc and SBc galaxies, for instance, have very "loose" arms, whereas Sa and SBa galaxies have tightly wrapped arms (with reference to the Hubble sequence). Either way, spiral arms contain many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so bright . </P>

What is the dilemma about the spiral arm structure of a typical spiral galaxy