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Chapter 16: Problem 7

Stars begin burning helium to carbon when the temperature rises in the core. This temperature increase is caused by a. gravitational collapse. b. fusing hydrogen into helium in the core. c. fusing hydrogen into helium in a shell around the core. d. degenerate-electron pressure.

Short Answer

Expert verified
c. fusing hydrogen into helium in a shell around the core.

Step by step solution

01

- Understand the Question

Identify what causes the increase in temperature that allows stars to begin burning helium into carbon.
02

- Review the Potential Causes

Evaluate the given options (a, b, c, d). Consider which of these processes are active in a star's life cycle and how they affect core temperature.
03

- Gravitational Collapse

Understand that gravitational collapse refers to the star's core contracting, which increases temperature due to gravitational energy converting to thermal energy.
04

- Fusing Hydrogen into Helium

Consider that fusing hydrogen into helium in the core (b) sustains the core temperature but is not directly responsible for the significant rise needed to burn helium.
05

- Fusing Hydrogen in a Shell

Note that fusing hydrogen into helium in a shell around the core (c) can lead to increased temperature in the core because of the additional energy input into the core.
06

- Degenerate-Electron Pressure

Degenerate-electron pressure (d) supports the core against gravitational collapse in white dwarfs but does not directly cause a temperature increase for helium burning.
07

- Correct Answer Identification

After analyzing the information, the most likely cause for the temperature rise is the fusion of hydrogen into helium in a shell around the core (c).

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Key Concepts

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

Gravitational Collapse
To understand why stars begin burning helium, we need to talk about gravitational collapse. This is a stage in a star's life where its core contracts under its own gravity. As the core shrinks, gravitational energy is converted into heat, leading to a rise in temperature. This increase in temperature is crucial for the star to begin the helium-burning phase.
During gravitational collapse:
  • Gravity pulls the core's material inward.
  • Collapsing material's potential energy turns into heat.
The energy produced heats the core significantly. This heating is necessary for the star to eventually reach the temperatures needed for new types of nuclear fusion, like helium burning.
Helium Burning
Helium burning is when a star fuses helium into heavier elements like carbon and oxygen. This process marks a new phase in the star's life cycle.
Here's what happens during helium burning:
  • The core temperature has to reach about 100 million Kelvin.
  • Helium nuclei collide in conditions of extreme pressure and temperature.
The fusion of helium happens after the star has exhausted its hydrogen supplies in the core.
The energy released during helium burning stabilizes the star, balancing the inward pull of gravity with the outward pressure from nuclear reactions. However, helium burning is a sign that the star is aging and moving toward the later stages of its evolution.
Shell Fusion
Shell fusion occurs after the core has burned its hydrogen. The core contracts and heats up, causing hydrogen in the surrounding shell to start fusing into helium.
This process sets off several changes:
  • The shell around the core undergoes hydrogen fusion.
  • Energy from the shell heats the core even more.
The additional heat supplied by shell fusion is substantial enough to trigger the helium burning in the core.
So, the core is now hot enough for helium fusion, even though the helium itself is not burning in the shell. This shell fusion phase is essential for the progression to the next stages in stellar evolution.

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Most popular questions from this chapter

Recall from Chapter 4 that the luminosity of a spherical object at temperature \(T\) is given by \(L=4 \pi R^{2} \sigma T^{4},\) where \(R\) is the object's radius. If the Sun became a white dwarf with a radius of \(10^{7}\) meters, what would its luminosity be at the following temperatures: (a) \(10^{8} \mathrm{K} ;\) (b) \(10^{6} \mathrm{K} ;(\mathrm{c}) 10^{4} \mathrm{K} ;(\mathrm{d}) 10^{2} \mathrm{K} ?\)

Why are the paths along the H-R diagram that a star follows as it forms (from a protostar) and as it leaves the main sequence (climbing the red giant branch) so similar?

When the Sun runs out of fuel in its core, the core will be a. empty. b. filled with hydrogen. c. filled with helium. d. filled with carbon.

What will the escape velocity be when the Sun becomes an AGB star with a radius 200 times greater and a mass only 0.7 times that of today? How will these changes in escape velocity affect mass loss from the surface of the Sun as an AGB star?

If a main-sequence star suddenly started burning hydrogen at a faster rate in its core, it would become. a. larger, hotter, and more luminous. b. larger, cooler, and more luminous. c. smaller, hotter, and more luminous. d. smaller, cooler, and more luminous.
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