What characteristic is generally observed in stars that have reached the turnoff point?

The turnoff point on a Hertzsprung-Russell diagram indicates the stage in a star's life when it has exhausted the hydrogen in its core and is transitioning away from the main sequence phase of stellar evolution. At this point, stars typically begin to burn hydrogen in a shell surrounding an inert helium core. Therefore, stars that have reached the turnoff point have completed their core hydrogen burning process, marking a critical transition to the next phases of stellar evolution.

This characteristic is used by astronomers to determine the age of star clusters since all stars in a cluster will generally share a similar initial mass and composition. The more massive stars evolve faster, reaching the turnoff point sooner than less massive ones, making it a useful indicator of the cluster's overall age.

The other options present different stages of stellar evolution or characteristics but do not align with the definition of the turnoff point. For instance, if a star is still in the main sequence, it has not yet reached the turnoff point, and newly formed stars have not yet begun any significant nuclear fusion processes beyond the initial phases. Additionally, while some helium may be present, the primary characteristic at the turnoff point is the completion of hydrogen burning, not the composition of helium.