Stellar Classification of White Dwarfs

piareround

OK recently I found a really old table stars based on partally on Yerkes Classification system with a small section on white dwarf stars. It has the stars organized into groups labeled DO, DA, DB, DF, DG, DK, and DM. However, I remember that the current classification system for white dwarf stars as primarily DO, DA, DB, DQ, DZ, DC, DX.

First, Has anyone heard of the first classification system? If so why where the letters changed? What happened?

Second, I really have trouble visualizing DQ, DZ, and DC on the HR diagram compared to the other white dwarfs. Does anyone have pictures of DQ, DZ, or DC stars on the HR Diagram?