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Chapter 16: Problem 24

Why is a horizontal branch star (which burns helium at a high temperature) less luminous than a red giant branch star (which burns hydrogen at a lower temperature)?

Short Answer

Expert verified
Red giant branch stars have larger envelopes and greater energy output, making them more luminous than horizontal branch stars despite burning hydrogen at a lower temperature.

Step by step solution

01

Identify the Types of Stars

Understand that a horizontal branch star is burning helium in its core, while a red giant branch star is burning hydrogen in a shell around its core.
02

Compare Core Structures

A horizontal branch star has a smaller core and a lower hydrogen shell-burning radius compared to a red giant branch star, which has an extended and inert helium core.
03

Analyze Energy Production

In a red giant, the hydrogen shell burning produces high energy that causes the star to expand and increase in luminosity. Horizontal branch stars, though hotter, release energy from a smaller volume, making them less luminous overall.
04

Evaluate Envelope Size

The red giant branch star has a substantially larger outer envelope compared to a horizontal branch star, leading to a greater overall luminosity despite the lower core temperature.
05

Conclude Luminosity Comparison

The overall energy output and larger size of the red giant branch star result in a higher luminosity when compared to the horizontal branch star.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Horizontal Branch Stars
Horizontal branch stars are a specific phase in the life of a star. The primary characteristic of these stars is that they are burning helium in their cores. This phase occurs after a star has exhausted the hydrogen in its core and undergone a contraction, raising core temperatures sufficiently to begin helium fusion.

Horizontal branch stars are relatively less luminous than red giant branch stars. This is because, despite the high core temperatures necessary for helium fusion, the energy generated is confined to a much smaller core radius. Consequently, the overall luminosity is lower compared to the more expansive red giant phase.
Red Giant Branch Stars
Red giant branch stars represent a later stage in stellar evolution. At this stage, the star has exhausted the hydrogen in its core, converting it to helium, and hydrogen fusion continues in a shell surrounding the core.

One of the distinctive features of red giant branch stars is their significant increase in size and luminosity. The energy produced by hydrogen shell burning causes the outer layers of the star to expand enormously.
This results in a vast and luminous outer envelope, making red giants visibly brighter compared to other evolutionary stages.
Helium Core Burning
In horizontal branch stars, helium core burning is the dominant process. This involves the fusion of helium nuclei into heavier elements, primarily carbon and oxygen.

This process requires very high temperatures and pressures, much higher compared to hydrogen fusion. The energy released through helium core burning contributes to the star's overall output, but due to the relatively small core, the star's brightness isn't as high as one might expect.
While the core itself is extremely hot, the reduced volume results in less total luminosity as the energy is radiated away.
Hydrogen Shell Burning
Hydrogen shell burning occurs outside the helium core in red giant branch stars. Here, hydrogen fusion continues in a shell around the inert helium core.

This shell produces substantial amounts of energy because the fusion process in the shell is highly efficient. As the core contracts, temperatures increase in the shell, intensifying the fusion reactions.
The resulting energy from this process causes the star's outer layers to expand dramatically, contributing to the star's increased size and luminosity during the red giant phase.
Stellar Luminosity
Stellar luminosity refers to the total amount of energy emitted by a star per unit of time. It is one of the fundamental properties astronomers use to understand a star's physical characteristics and evolutionary stage.

In the context of horizontal branch and red giant branch stars, luminosity varies significantly due to differences in core burning processes and the structure of the star's outer layers.
Red giant branch stars display higher luminosity due to their expansive outer envelopes and efficient hydrogen shell burning, despite a lower core temperature compared to horizontal branch stars.

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Most popular questions from this chapter

In the Hubble telescope news archive, look up press releases on planetary nebulae (http://hubblesite.org/newscenter /archive/releases/nebula/planetary) and white dwarf stars (http://hubblesite.org/newscenter/archive/releases/star / white-dwarf). Pick a story for each. What observations were reported, and why were they important?

A planetary nebula has an expansion rate of \(20 \mathrm{km} / \mathrm{s}\) and a lifetime of 50,000 years. Roughly how large will this planetary nebula grow before it disperses?

Post-main-sequence stars lose up to 50 percent of their mass because a. jets from the poles release material at an increasing rate. b. the mass of the star drops because of mass loss from fusion. c. the magnetic field causes increasing numbers of coronal mass ejections. d. the star swells until the surface gravity is too weak to hold material

A white dwarf is located in the lower left of the H-R diagram. From this information alone, you can determine that a. it is very massive. b. it is very dense. c. it is very hot. d. it is very bright.

Go to the Hubble Space Telescope's planetary nebula gallery (http://hubblesite.org/gallery/album/nebula/planetary). For each of the three types of symmetry, find an example of a nebula that shows clearly the type of symmetry: spherical (being symmetric in every direction, like a circle), bipolar (having an axis about which they are symmetric, like a person's face, and point-symmetric (being symmetric about a point, like the letter \(S\) ). Print each of the three images you chose, and label the type of symmetry each one represents. For all three nebulae, identify the location of the central star. For bipolar symmetry, draw a line that shows the axis about which the nebula is symmetric. For point symmetry, identify several features that are symmetric across the location of the central star.
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