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Chapter 19: Problem 12

Why does a star's luminosity decrease after helium fusion begins in its core?

Short Answer

Expert verified
A star's luminosity decreases after helium fusion begins in its core because the core expands, causing the star's outer layers to cool and decrease in size. The rate of fusion reactions also decreases due to this expansion, and while each helium fusion reaction releases more energy, the overall output decreases as there are fewer reactions. This results in a reduction in the star's total luminosity.

Step by step solution

01

Understanding Fusion in Stars

Stars produce energy through a process called nuclear fusion. In this process, atomic nuclei combine to form a heavier nucleus, releasing energy in the process. In the early stages, this happens through hydrogen fusion, where hydrogen atoms fuse to create helium.
02

Transition to Helium Fusion

As the star exhausts its hydrogen supply, the core contracts under gravity, but the rest of the star expands. So the star becomes a red giant. The core gets hotter and denser until it's hot enough for helium to start fusing into carbon. This process releases even more energy per reaction than the initial hydrogen fusion.
03

Luminosity Decrease After Helium Fusion

After helium fusion begins, the core expands, causing the outer layers of the star to cool and decrease in size - this is why the star's overall luminosity decreases. The energy per reaction might be higher for helium fusion, but the rate of reactions decreases because of the expanded core. Consequently, although the energy produced in each fusion reaction increases, the total output (and thus luminosity) decreases because the overall number of reactions decreases.

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Key Concepts

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

Star Luminosity
Star luminosity is a measure of the total amount of energy emitted by a star in a given period. It is a crucial aspect of a star's characteristics, as it provides insights into the star's size, temperature, and energy production processes. Understanding luminosity in stars is essential because it helps astronomers determine the distance to stars and the conditions within their cores, where nuclear fusion occurs. When helium fusion begins in a star's core, the luminosity decreases because the core expands, and the outer layers cool and shrink, leading to a reduction in the total amount of light and energy the star releases.
Nuclear Fusion in Stars
Nuclear fusion is the powerhouse of stars, a process where atomic nuclei merge to form a heavier nucleus. This reaction releases a significant amount of energy, primarily in the form of light and heat, which is why stars shine. In the cores of stars, the fusion of hydrogen atoms into helium makes up the initial stage of a star's life cycle, known as the main sequence. As stars run out of hydrogen, they evolve to fuse heavier elements. The energy released during helium fusion is greater per reaction than during hydrogen fusion, yet the star's luminosity decreases because the number of reactions reduces due to the expansion of the core.
Red Giant Phase
When a star enters the red giant phase, it marks a significant change in its life cycle. This phase occurs after a star has exhausted the hydrogen in its core, and the core contracts while the outer layers expand. The star increases in size, often swelling to many times its original diameter. Despite the increase in size and temperature in the core, allowing helium to fuse into heavier elements like carbon, the star's overall luminosity can decline. This is because the outer layers spread out, cool down, and become less dense, diminishing the star's brightness as seen from afar.
Stellar Evolution
Stellar evolution explains the changes a star undergoes during its lifetime, from formation to its ultimate fate, which could be as a white dwarf, neutron star, or black hole, depending on its initial mass. The journey includes numerous stages, such as the main sequence, red giant or supergiant phase, and eventual collapse. Helium fusion is a pivotal point in this evolution, leading to the red giant phase and contributing to a diverse cosmos. Although helium fusion results in a temporary decrease in luminosity, it is a step towards creating complex elements that are the building blocks for planets and life itself.

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Most popular questions from this chapter

(a) The main-sequence stars Sirius (spectral type A1), Vega (A0), Spica (B1), Fomalhaut (A3), and Regulus (B7) are among the 20 brightest stars in the sky. Explain how you can tell that all these stars are younger than the Sun. (b) The third-brightest star in the sky, although it can be seen only south of \(29^{\circ}\) north latitude, is \(\alpha\) (alpha) Centauri A. It is a main- sequence star of spectral type G2, the same as the Sun. Can you tell from this whether \(\alpha\) Centauri A is younger than the Sun, the same age, or older? Explain your reasoning.

What will happen inside the Sun 7 billion years from now, when it begins to mature into a red giant?

Explain why the Earth is expected to become inhospitable to life long before the Sun becomes a red giant.

What is the difference between Population I and Population II stars? In what sense can the stars of one population be regarded as the "children" of the other population?

What is a red dwarf? How are thermonuclear reactions in the core of a red dwarf able to consume hydrogen from the star's outer layers?
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