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Chapter 13: Problem 25

The reason most stars are main-sequence stars is that the main-sequence phase is the a. longest part of a star's lifetime. b. brightest part of a star's lifetime. c. hottest part of a star's lifetime. d. time when stars die.

Short Answer

Expert verified
The main-sequence phase is the longest part of a star's lifetime, making it the correct answer (a).

Step by step solution

01

Title - Understand the Question

Determine why the majority of stars are categorized as main-sequence stars by examining the characteristics of the main-sequence phase.
02

Title - Review Main-Sequence Phase Characteristics

The main-sequence phase is where stars spend most of their lifetime. During this phase, stars fuse hydrogen into helium in their cores, which provides a stable energy output.
03

Title - Compare Answer Choices

Analyze each of the provided options: a. longest part of a star's lifetime, b. brightest part of a star's lifetime, c. hottest part of a star's lifetime, d. time when stars die. We need to identify which attribute makes the main-sequence phase prevalent among stars.
04

Title - Identify the Correct Answer

The main-sequence phase is indeed the longest part of a star's lifetime. This is why most stars observed are in this stage. Therefore, the correct choice is a.

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Key Concepts

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

Star Lifecycle
Stars go through several distinct stages during their lifetime. They start as a cloud of gas and dust, which contracts under gravity to form a protostar.
As the protostar heats up, it eventually becomes hot enough to start the process of nuclear fusion, where hydrogen turns into helium. This is when it enters the main-sequence phase.

During their lifecycle, stars may go through phases such as:
  • Main-sequence, where hydrogen fusion happens in the core.
  • Red giant, where the star expands and cools after hydrogen in the core is exhausted.
  • White dwarf, neutron star, or black hole, depending on the star’s mass. These are the final stages.
Each stage has unique characteristics and lifespans, with the main-sequence phase being the longest.
Stellar Evolution
Stellar evolution describes the changes a star undergoes over time. It begins when a star forms from a collapsing cloud of gas and dust.
During the main-sequence phase, stars remain stable because the pressure from hydrogen fusion balances gravitational forces.

Key factors influencing stellar evolution include:
  • Mass: Higher mass stars burn fuel faster and have shorter lifespans.
  • Fusion Processes: Stars fuse different elements at different stages, starting with hydrogen and progressing to heavier elements.
  • End States: Depending on initial mass, stars can become white dwarfs, neutron stars, or black holes after exhausting their nuclear fuel.
Understanding stellar evolution helps astronomers predict the future of stars and the dynamics of galaxies.
Hydrogen Fusion
Hydrogen fusion is the process that powers main-sequence stars. In this phase, hydrogen nuclei combine under high temperature and pressure to form helium.
Fusion releases a tremendous amount of energy, which creates outward pressure, balancing the star's gravity, and maintaining stability during the main-sequence phase.

Important points about hydrogen fusion include:
  • Temperature Required: Fusion occurs at core temperatures exceeding millions of degrees Kelvin.
  • Energy Output: The energy from fusion powers the star and emits light and heat, making stars visible.
  • Stability: During the main-sequence, hydrogen fusion provides a consistent energy output, making this phase the longest in a star’s life.
Hydrogen fusion is crucial for the stability and lifespan of stars, being the key process during the main-sequence phase.

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Most popular questions from this chapter

Which quantities are plotted on an H-R diagram? a. brightness on the horizontal axis; luminosity on the vertical axis b. brightness on the horizontal axis; temperature on the vertical axis c. luminosity on the vertical axis; temperature on the horizontal axis d. main sequence on the vertical axis; spectral type on the horizontal axis

Other than the Sun, the only stars whose mass astronomers can measure directly are those in eclipsing or visual binary systems. Why?

Star A is more massive than star B. Both are main-sequence stars. Therefore, star A is than star B. (Choose all that apply.) a. brighter b. fainter c. hotter d. colder e. larger f. smaller

Sirius, the brightest star in the sky, has a parallax of 0.379 arcsec. What is its distance in parsecs? In light-years?

T/F: The mass of a star is the primary indicator of its observable properties.
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