Chapter 20, Problem 1CC

To determine

The reason due to which the helium fusion in a star’s core, that releases energy, actually lead to a decrease in its luminosity. Also, mention the name of these stars in the HR diagram.

Answer to Problem 1CC

Solution:

The energy released from core helium fusion enlarges the star's core, which decreases the rate of reaction for both the hydrogen fusion in shell and the helium fusion in core. With a less rate of energy liberated in and around the star’s core, the luminosity of star also decreases. These stars are called horizontal branch stars because the changes in luminosity are very less.

Explanation of Solution

Introduction:

Most stars have the same chemical composition but the difference in stars depend on the two primary properties like their surface temperature and luminosities. These two properties vary from one star to another. H-R diagrams or hertz sprung- Russell diagrams are a graphical tool used by astronomers for the classification of stars according to their spectral type, luminosity, temperature, color, evolutionary track, and temperature.

Explanation:

During helium fusion in core, the surrounding hydrogen fusion in the shell of the star provides most of the red giant’s luminosity and helium flash is not observed. The core expands when core helium fusion begins as a result of which the temperature of the core decreases. The cooling of the core indirectly cools the surrounding hydrogen fusing shell, due to which the shell liberates energy slowly. Hence, the luminosity decreases a bit after core helium fusion begins.

The contraction of star’s external layer is because of the slow energy release. While shrinking, there is an increase in the star’s surface temperature. So, the evolutionary track of the star moves to the left in the H-R diagram. In this stage, the changes in luminosity are very low due to which a horizontal shift can be seen in the evolutionary track, along the path which is referred as the horizontal branch.

Conclusion:

The fusion of helium enlarges the core which decreases the energy of both the hydrogen fusion in the shell and the helium fusion in core. This decrease in rate of energy decreases the luminosity of the stars. These stars are called horizontal branch stars.