How do we determine the age of stars? Can we determine the age using a H-R diagram?

Question

How do we determine the age of stars? Can we determine the age using a H-R diagram?

Answer 1

The H-R diagram can be used to determine the relative ages of stars, however, since a star's movement on the diagram is based on its initial mass, it is not a reliable source for age.  We can guesstimate a star's age by its color (blue would be young, red would be old) and luminosity (assuming the star would burn its fuel to become dimmer before hitting the giant stage, if that is in its fate), and thus, the H-R diagram (since it uses both of those facets) can be used in that manner.  But again, it is guesstimate.

Answer 2

Color is one way.  Red stars are cooler and older.  Blue and white stars are hotter and younger.  

The H-R diagram charts the relationships between temperature and brightness, but with an understanding of stellar evolution you can guess a star's age by it's position on the diagram.