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Chapter 16: Problem 14

\(\mathbf{T} / \mathbf{F}:\) When a star first runs out of fuel in the core, it becomes brighter.

Short Answer

Expert verified
True. The star becomes brighter as a result of increased fusion in the hydrogen shell.

Step by step solution

01

Understanding Star Fuel

Stars generate energy by fusing hydrogen into helium in their cores. This fuel sustains nuclear reactions that produce light and heat.
02

Fuel Depletion and Core Collapse

When a star exhausts its hydrogen fuel in the core, the core contracts due to gravity. This contraction increases the core temperature and pressure.
03

Hydrogen Shell Burning

The increased temperature from the core contraction heats the surrounding shell of hydrogen. This shell ignites and starts fusing hydrogen at a faster rate than before.
04

Increase in Brightness

The faster hydrogen fusion in the shell releases more energy, causing the outer layers of the star to expand and the star to become more luminous. Hence, the star becomes brighter.

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Key Concepts

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

Hydrogen Fusion
Hydrogen fusion is the process that powers stars. In the core of a star, extreme pressure and temperature enable hydrogen atoms to combine and form helium, releasing a tremendous amount of energy. This energy is what keeps the star shining brightly. The core's conditions are so intense that hydrogen nuclei can overcome their natural repulsion, allowing fusion to occur. This process is essential for the stability and longevity of stars since it provides both energy and the pressure needed to counteract gravity. As long as hydrogen fusion continues, a star remains in the stable main sequence phase of its life.
Core Collapse
When a star consumes all the hydrogen in its core, it enters a new phase called core collapse. Without hydrogen to fuse into helium, the core can no longer support itself against gravitational forces. This leads to the core contracting and heating up. The increase in temperature and pressure is significant and sets the stage for the next phase in the star’s lifecycle. The collapse happens because the core’s balance between outward pressure from nuclear fusion and the inward pull of gravity is disrupted. The result is a much denser and hotter core, leading to further evolutionary steps.
Shell Burning
Once the core collapses and heats up, it indirectly affects the surrounding layers of the star. The layer of hydrogen just outside the core becomes hot enough to ignite, entering a phase known as shell burning. In the shell-burning phase, hydrogen fusion resumes, but now in a shell around the core rather than in the core itself. This shell burning is more intense and releases more energy than core fusion, causing vast amounts of energy to be generated. Consequently, the outer layers of the star expand, marking a significant change in the star's structure and energy output.
Star Brightness
As the shell burning process releases more energy, the outer layers of the star expand and the star becomes significantly brighter. The increase in the star's luminosity is a direct result of the heightened rate of hydrogen fusion in the shell around the core. The immense energy output from the shell burning causes the star's outer layers to inflate, making the star larger and brighter. This phase marks a critical point in stellar evolution, where changes in energy dynamics lead to noticeable differences in the star’s appearance and brightness as seen from Earth. Thus, a star indeed becomes brighter when it first runs out of hydrogen fuel in its core.

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Most popular questions from this chapter

Go to the American Association of Variable Star Observers (AAVSO) website (www.aavso.org). What does this 100 -yearold organization do? Read about the types of intrinsic variable stars. Click on "Getting Started." If you have access to dark skies, you can contribute to the study of variable stars. Go to the page for observers (www.aavso.org/observers) and click on each item in the "For New Observers" list, including the list of stars that are easy to observe. Assemble a group and observe a variable star from this list. (Another option is to join AAVSO's project searching for novae, at www.aavso .org/nova- search-section.

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.

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.

Is it possible for a star to skip the main sequence and immediately begin burning helium in its core? Explain your answer.

T/F: All stars eventually die.
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