Finnish word kirkkausaste comes from Finnish aste, Finnish kirkkaus (Brightness.)

Detailed word origin of kirkkausaste

Dictionary entry	Language	Definition
aste	Finnish (fin)	(everyday usage) degree, degree Celsius (short for celsiusaste). Degree. Grade. Order.
kirkkaus	Finnish (fin)	Brightness.
kirkkausaste	Finnish (fin)	(astronomy) magnitude (apparent brighness of a star).