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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

Identify and explain two important ways in which supernovae influence the formation and evolution of new stars.

How does a high-mass star begin fusing helium in its core? How is this process different from what happens in low-mass stars?

Why does the interior of an evolved high-mass star have layers like an onion? a. Heavier atoms sink to the bottom because stars are not solid. b. Before the star formed, heavier atoms accumulated in the centers of clouds because of gravity. c. Heavier atoms fuse closer to the center because the temperature and pressure are higher there. d. Different energy transport mechanisms occur at different densities.

The layers in a high-mass star occur roughly in order of a. atomic number. b. decay rate. c. magnetic field strength. d. spin state.

What mechanism provides the internal pressure inside a neutron star? a. ordinary pressure from hydrogen and helium gas b. degeneracy pressure from neutrons c. degeneracy pressure from electrons d. rapid rotation
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