Our Galaxy:  The Milky Way
a.
b.
c.
d.
e.

                            f. (Mouse-over image below to see annotated version.  Click on image to play Monty Python's Galaxy Song.)

 
a.    Thomas Wright (1750):  from theological considerations and the shape of the
       Milky Way in the sky:  an infinite "slab" of stars
 
b.   William Herschel (1790):  using star counts ("star gauging") the galaxy is a
       disk with the Sun in the center
 
c.    J. C. Kapteyn (1910):  more detailed star counts show edge "structure":
       unknown to Kapteyn this "structure" is due to interstellar extinction which obscures
       the starlight over the vast interstellar distances

d.    Harlow Shapley (1920):  using pulsating variable stars, Shapley determines the
       positions of many globular clusters and finds that they are distributed around
       another "center" rather than the Sun:  the center of the Milky Way Galaxy
       (Shapley's value - approximately 10 kpc from the Sun)

e.   Present schematic model of the Milky Way Galaxy.  Note that the disk is sometimes
      called the "disk component" and the bulge-halo (which includes individual halo stars
      and the globular clusters) are all parts of the "spheroidal component".  The members
      of the spheroidal component are distributed in a spheroidal volume around the center
      of the galaxy which extends beyond the limits of the diagram above.

f.    Artist's conception of the Milky Way Galaxy seen face on, incorporating the latest
      information about the structure of the Galaxy.