Stellar Rotation Along the HR-Diagram

The HR-diagram (Hertzsprung-Russell diagram, HR-diagram) has already been discussed in the introductory chapter. It is a plot of stellar temperatures (or spectral types, or color indices) against stellar brightness. From such a diagram we can also derive stellar ages and stellar evolution. In this chapter we will show the relationship between stellar age and rotation. Stars rotate because they are formed by a contraction of an interstellar gas cloud. The particles of such a gas cloud have randomly distributed momentum, but there is always a small excess of angular momentum in a particular direction. When the cloud contracts, the rotational velocity of such a cloud is enhanced by conservation of angular momentum, and the contracting cloud begins to accelerate its rotation.