This script creates spirals which match those of Galaxies.
They are not logarithmic spirals.

http://mnras.oxfordjournals.org/content/397/1/164.short

The reference also has an equation for Ring Galaxies, with tanh.
(A quick option to add tanh to the formula did not work.)

The reference also has a formula using angle of pitch.
(Not tried out yet.)

Blend makes a nice curve between the centers of two spirals at 180 degrees.
Also joined edges of a few pairs of spirals and ran planar.
Rebuild can be used.

- Brian




Edit: Correcting typo's. 9/18/2014