Answer:
It is called often times Top
The metaphor in this statement is "the great millstone" it kind of qualification word it express how big the time was.
But in reality, millstone doesn't even have a relationship with the time.
So "the great millstone" is a metaphor.
Population I stars are consisting of younger stars, clusters and associations that formed aroung 1,000,000 to 100,000,000 years ago, which such stars classified as type O and B are called extreme Population I stars. The members of this population occur near and in the arms of our own galaxy, the Milky Way.
