See materials at

    http://spiff.rit.edu/classes/phys443/lectures/gal_1/gal_1.html


  Start with simple classes:
       spiral, elliptical, other = irregular

  Then introduce Hubble tuning fork
       explain elliptical sub-class number
       explain meaning of Sa, Sb, Sc, Sd   and barred


  Difference in stellar populations - why are spirals blue, ellipticals red?
       "early" and "late" 
       making models of stellar populations
       https://ui.adsabs.harvard.edu/abs/2005MNRAS.361..725B/abstract
       Show color-color diagrams of stars and of galaxies


  More quantitative ways of classifying galaxies
       concentration index
       overall color
       radial profile of brightness

  Galaxy Zoo training exercise
       and Galaxy Zoo exercise as homework?