Chapter 20, Problem 1Q

To determine

To define:

Horizontal branch.

The location of horizontal branch in an H-R diagram.

The difference between stars on the horizontal branch and red giants or main-sequence stars.

Explanation of Solution

Introduction:

An H-R diagram describes the evolutionary stage of a star. Horizontal branch represents the phase that the stars undergo when their luminosity doesn’t change much.

At the red giant phase, the core of the star contracts and its outer layers begin to expand. The outer layers of the star have a lower temperature than the core. However, due to the increase in the temperature inside, the luminosity increases. Star moves to the right in the H-R diagram at this stage. When Helium fusion triggered at the core, it begins to expand again. That process would decrease the temperature of the core a little bit. This also slows down the nuclear fusion a little bit and outer layers would begin to contract again. The contraction allows increasing the temperature and the star moves to the left in the H-R diagram again. Even though the temperature is increasing, since the radius is also decreasing at the same time, the luminosity remains almost the same. Hence, the evolutionary track moves horizontally in the H-R diagram. It is located above the main-sequence phase and between the main-sequence branch and the red giant branch.

However, red giant stars’ luminosity is much higher than the main-sequence stars’ and even horizontal branch stars’. Inside the core of horizontal branch stars Helium fusion can occur and Carbon and Oxygen will be produced. But Helium fusion will never occur in red giants. On the other hand, main sequence stars not even began the Helium fusion process.