Hi Kent!
Kudos for walking through these sections of the retopo. I was very uncertain about my ability to do a good quality retopo but you helped me through it!
I don't fully understand what the purpose of star junctions are. It was a little hard to follow the times when you decided to implement a star junction around the nose but it started making a little more sense at around the eye. What would you say is your rule of thumb with deciding to implement a star junction?
what you call a star junction is also known as an extrusion pole, it is half of a partnership with the nose pole...
this illustration is in 3D mode but it is an inset if you lay it flat, like around the eyes it allows you to inset a loop (or extrusion)...
it takes 4 of them to make the loop complete.
if you can command these two types of poles you will rule retopology...
hope this is helpful
Another way of looking at this:
a head as we sculpted is topologically equivalent (homeomorph) to a sphere (if you did it well and didn't make holes in the mesh) and you cannot retopologize it without poles. (I can prove this, but it is not trival) The only 2 dimensional object in 3D space that can be made without poles is a (one-holed) torus (or something homeomorph to it, like, for instance a coffeecup (with handle)).
Now we want to subdivide it later to sculpt fine details, so we want a retopology that has (almost) only quads and they should be as similar in size and 'squareness' as possible and as we have non-concentric loops (around the eyes and mouth for instance), we need star junctions between them to avoid stretching like this:
The placement of those poles on an organic model takes into consideration, where they can do the least harm when the mesh deforms.
Mark and Spikey give excellent insight into the technical nature of poles! Poles are generally an unwanted (yet unavoidable) component of polygonal geometry due to the problems they can cause. The most notable being:
The higher the point/edge count in a pole, the worse its effects. 5-pointed stars are typical and we try not to go higher than that. The goal when modeling is to relegate poles to the least problematic areas. Meaning A) the least visible like behind the ear is good to relegate a pole, also hidden under hair or B) areas with the least potential for deformation when posing or animating.
Since we're using such a high polycount for this project AND it's not being animated (at least not yet), poles aren't a terribly consequential issue. Still it's a good habit to be aware of poles and how to relegate them for minimal consequence. Generally animation is where they cause bigger problems.
Hope that helps!