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Chapter 20: Problem 21

Why does the evolutionary track of a high-mass star move from left to right and back again in the H-R diagram?

Short Answer

Expert verified
The evolutionary track of a high-mass star on the H-R diagram moves from left to right and back again due to changes in its structure and nuclear burning processes during its stellar evolution. It starts on the far left (Main Sequence), moves right as it becomes a red giant (expanding and cooling outer shell while the core contracts), moves back left during the Horizontal Branch stage (core helium burning), and then again moves right in the advanced stages on the Asymptotic Giant Branch (after helium in the core is exhausted and the star once again expands).

Step by step solution

01

Understand the H-R Diagram

The Hertzsprung-Russell (H-R) Diagram is a graph that plots stars according to their absolute magnitude or luminosity and their type or surface temperature. The diagram separates the effects of temperature and surface area on stellar luminosity, thereby elucidating the causes of different types of stellar evolution.
02

Initial Stage - Main Sequence

A high-mass star begins its life on the Main Sequence (the diagonal from the top left to the bottom right on the H-R diagram) where hydrogen fusion occurs in the star’s core. As the star ages and depletes its hydrogen, its core contracts and the temperature and pressure rise, supporting nuclear burning at a larger shell around the core. This increases the luminosity and the star moves to the right off the Main Sequence.
03

Red Giant Phase - Rightward Movement

The star moves rightward on the H-R diagram when it becomes a red giant. This happens as the core contracts and the outer shell of the star expands and cools. Although the surface temperature decreases, the increased size of the star results in a greater overall brightness, thus it moves to the right side of the diagram.
04

Core Helium Burning and Horizontal Branch - Leftward Movement

Once the star's core temperature has increased enough to ignite helium to carbon fusion, the core expands and the outer layers contract. This is represented by the star moving back to the left to the Horizontal Branch on the H-R Diagram where the star burns helium in its core.
05

Advanced Stages - Asymptotic Giant Branch (AGB)

After the helium in the core is exhausted, the star again expands and becomes a supergiant (Asymptotic Giant Branch) and the star moves back to the right on the H-R Diagram.

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Most popular questions from this chapter

What is the connection between dredge-ups in old stars and life on Earth?

Why does the mass of a star play such an important role in determining the star's evolution?

Consider a high-mass star just prior to a supernova explosion, with a core of diameter \(20 \mathrm{~km}\) and density \(4 \times 10^{17}\) \(\mathrm{kg} / \mathrm{m}^{3}\). (a) Calculate the mass of the core. Give your answer in kilograms and in solar masses. (b) Calculate the force of gravity on a 1-kg object at the surface of the core. How many times larger is this than the gravitational force on such an object at the surface of the Earth, which is about 10 newtons? (c) Calculate the escape speed from the surface of the star's core. Give your answer in \(\mathrm{m} / \mathrm{s}\) and as a fraction of the speed of light. What does this tell you about how powerful a supernova explosion must be in order to blow material away from the star's core?

Imagine that our Sun was somehow replaced by a \(1-\mathrm{M}_{\odot}\) white dwarf star, and that our Earth continued in an orbit of semimajor axis \(1 \mathrm{AU}\) around this star. Discuss what effects this would have on our planet. What would the white dwarf look like as seen from Earth? Could you look at it safely with the unaided eye? Would the Earth's surface temperature remain the same as it is now?

. On an H-R diagram, sketch the evolutionary track that the Sun will follow from when it leaves the main sequence to when it becomes a white dwarf. Approximately how much mass will the Sun have when it becomes a white dwarf? Where will the rest of the mass go?
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