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Chapter 17: Problem 8

In a high-mass star, hydrogen fusion occurs via a. the proton-proton chain. b. the CNO cycle. c. gravitational collapse. d. spin-spin interaction.

Short Answer

Expert verified
b. the CNO cycle.

Step by step solution

01

- Understand Hydrogen Fusion Methods

There are various methods by which hydrogen fusion occurs in stars, primarily through the proton-proton chain and the CNO cycle.
02

- Compare Proton-Proton Chain and CNO Cycle

The proton-proton chain primarily occurs in low-mass stars (like the Sun), and it involves the fusion of hydrogen nuclei (protons) to form helium.
03

- Identify CNO Cycle

In high-mass stars, the CNO cycle (carbon-nitrogen-oxygen cycle) is the dominant fusion process, where carbon acts as a catalyst to convert hydrogen into helium.
04

- Eliminate Incorrect Options

Gravitational collapse is not a fusion process but a precursor to star formation. Spin-spin interactions are related to quantum mechanics and not relevant to hydrogen fusion in stars.
05

- Select the Correct Answer

Based on these considerations, the correct fusion process in high-mass stars is the CNO cycle.

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Key Concepts

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

The CNO Cycle
In high-mass stars, the primary process of hydrogen fusion is the CNO cycle. This stands for Carbon-Nitrogen-Oxygen cycle. Here, carbon, nitrogen, and oxygen nuclei act as catalysts to convert hydrogen into helium. Unlike the proton-proton chain, which occurs in low-mass stars, the CNO cycle is favored in high-mass stars due to their higher core temperatures.
  • The cycle begins with a carbon nucleus.
  • This nucleus captures a proton and transforms through a series of reactions involving nitrogen and oxygen.
  • Eventually, the process releases a helium nucleus and returns the carbon nucleus to the start.
This cyclical process is highly efficient in high-mass stars, generating significant energy and playing a crucial role in the star's life cycle.
The Proton-Proton Chain
The proton-proton chain is the dominant fusion process in low-mass stars, including our Sun. In this process:
  • Two protons (hydrogen nuclei) combine to form a deuterium nucleus, a positron, and a neutrino.
  • The deuterium nucleus then fuses with another proton to create helium-3.
  • Finally, two helium-3 nuclei combine to form helium-4 and release two protons.
This chain reaction, although less temperature-dependent than the CNO cycle, is efficient in the relatively cooler cores of low-mass stars. The energy released through the proton-proton chain is essential for the star's stability and longevity, helping it avoid gravitational collapse.
Gravitational Collapse
Gravitational collapse is a crucial phase in the formation of a star but not a fusion process. It occurs when a gas cloud, primarily consisting of hydrogen, contracts under its own gravity.
  • As the cloud contracts, its temperature and pressure increase.
  • Once the core temperature becomes high enough, nuclear fusion ignites.
  • The initial fusion process in low-mass stars is the proton-proton chain, while in high-mass stars, the CNO cycle dominates.
Gravitational collapse leads to the birth of a star, providing the necessary conditions for hydrogen fusion to commence, maintaining the star's equilibrium against further collapse.

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Most popular questions from this chapter

Explain the differences between the ways that hydrogen is converted to helium in a low-mass star (proton-proton chain) and in a high-mass star (CNO cycle). What is the catalyst in the CNO cycle, and how does it take part in the reaction?

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