BSG 4.13 Notion

Sometimes a Great Notion (2006-01-16)

Earth is revealed to have been populated by Cylons until a nuclear war, 2000 years in the past. Starbuck tracks the homing signal and finds her own remains on the planet. The identity of the fifth of the Final Five Cylons is revealed.

So is the fifth Cylon Starbuck or Ellen ?
If it is Ellen, then what is Starbuck ?