What happens at the turnoff point in stellar evolution?

The turnoff point is a significant stage in a star's evolution, particularly for those in a cluster with a uniform age. At this stage, stars begin to deviate from the main sequence, which is the stable phase during which they fuse hydrogen into helium in their cores.

As stars exhaust the hydrogen fuel in their cores, the balance between the gravitational forces pulling inward and the radiation pressure pushing outward is disrupted, causing stars to leave the main sequence. This transition marks the beginning of changes in their structure and energy production processes. Stars that reach the turnoff point no longer maintain hydrogen fusion at their core and subsequently evolve into different types of stars, entering phases like the red giant phase or other later stages of evolution depending on their initial mass.

The other scenarios mentioned do occur at different stages of stellar evolution, but they do not accurately describe the process happening at the turnoff point. For example, entering the red giant phase typically occurs after a star leaves the main sequence, and supernovae occur in massive stars toward the end of their lives much later than the turnoff point.