What is "what do we mean by the main-sequence turnoff point of a star cluster?

The main-sequence turnoff point of a star cluster is a critical concept in stellar evolution and provides a powerful tool for estimating the cluster's age. It refers to the point on the <a href="https://www.wikiwhat.page/kavramlar/Hertzsprung-Russell%20diagram">Hertzsprung-Russell diagram</a> (HR diagram) where the stars in the cluster begin to deviate, or "turn off," from the <a href="https://www.wikiwhat.page/kavramlar/main%20sequence">main sequence</a>.

Here's a breakdown:

• Main Sequence: Stars on the main sequence are fusing hydrogen into helium in their cores. A star's position on the main sequence is primarily determined by its mass; more massive stars are hotter, more luminous, and reside at the upper left of the HR diagram.

• Star Cluster Aging: Stars in a cluster are born at roughly the same time. As the cluster ages, the most massive stars, which have the shortest lifespans, exhaust their core hydrogen fuel first. They then evolve off the main sequence, becoming red giants or supergiants.

• The Turnoff Point: The location of the turnoff point indicates the mass of the stars that are just beginning to leave the main sequence. Stars above the turnoff point have already evolved off the main sequence, while stars below the turnoff point are still fusing hydrogen in their cores.

• Age Determination: Because the lifespan of a star is directly related to its mass (more massive = shorter lifespan), the turnoff point provides a direct estimate of the cluster's age. A cluster with a turnoff point at higher luminosities (massive, hot stars) is younger than a cluster with a turnoff point at lower luminosities (less massive, cooler stars). By comparing the observed turnoff point of a cluster to theoretical stellar evolution models, astronomers can determine the cluster's age with reasonable accuracy.

In essence, the main-sequence turnoff point acts as a cosmic clock, allowing astronomers to measure the ages of <a href="https://www.wikiwhat.page/kavramlar/star%20cluster">star clusters</a>.