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Chapter 20: Problem 1

What is the horizontal branch? Where is it located on an H-R diagram? How do stars on the horizontal branch differ from red giants or main-sequence stars?

Short Answer

Expert verified
The horizontal branch is a stage in the evolution of stars, located to the right of the main sequence on the H-R diagram, signifying they are quite bright but cooler. These stars, typically older and smaller, are more luminous than red giants or main-sequence stars, indicating they have exhausted their core hydrogen and are no longer in the hydrogen fusion phase.

Step by step solution

01

Defining the horizontal branch

The horizontal branch on the Hertzsprung-Russell (H-R) diagram is a stage in the evolution of some stars. It is a phase which occurs after a star has exhausted the nuclear fuel in its core and has expanded into a red giant. The term 'horizontal branch' is derived from the fact that stars in this stage lie along a roughly horizontal line on the H-R diagram.
02

Location on the H-R Diagram

The H-R Diagram plots luminosity against surface temperature for stars. On this diagram, the horizontal branch appears to the right of the main sequence line (where most stars lie) and slightly lower. It's located near the top of the diagram, denoting that the stars in this stage are quite bright, but cooler than Main-Sequence stars.
03

Distinguishing from red giants or main-sequence stars

In terms of differences, stars on the horizontal branch are typically older and have exhausted the hydrogen in their cores. They are smaller in size, moderately cool with lower temperature, but they are more luminous than the red giants or main-sequence stars. Red giants and main-sequence stars have not exhausted their core hydrogen and thus still remain in the hydrogen fusion phase.

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