This longest division of geologic time doesn't have a fixed period; in astronomy, it's one billion years
This is called parentless argon-40 because its parent potassium is 
not in the... [DOC] [TLE] Extreme Planets | NASA [PAR] For decades, astronomers have known there is a dense, compact source at the 
center ... years old, which is not enough time for the pulsar to slow down to a 
period of .... Hertz, Astrophysics Division Director at NASA Headquarters, 
Washington.