Book cover for Astronomy

Astronomy

Andrew Fraknoi, David Morrison, Sidney C. Wolff

ISBN #9781938168284

1st Edition

1,010 Questions

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36,741 Students Helped

Homework Questions

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

This chapter section highlights the diverse endpoints of stellar evolution, emphasizing that low-mass stars evolve into white dwarfs, which are supported by electron degeneracy pressure, while massive stars end their lives in energetic core-collapse supernovae leading to neutron stars or pulsars. Additionally, the dynamics in binary systems can initiate novae or type Ia supernovae, and gamma-ray bursts represent the most energetic events, offering insights into cosmic history.

Learning Objectives

1

Explain the different ultimate fates of stars based on their mass.

2

Describe the process by which low-mass stars shed mass and form white dwarfs, including the role of electron degeneracy pressure.

3

Differentiate between the outcomes of stellar evolution in isolated stars versus binary systems.

4

Analyze the triggers and characteristics of various stellar explosions such as type Ia and type II supernovae, as well as gamma-ray bursts.

Key Concepts

CONCEPT

DEFINITION

Low-Mass Stars

Stars with masses typically below about 8 solar masses that end their lives by shedding their outer layers to form white dwarfs.

White Dwarf

A dense, compact stellar remnant supported by electron degeneracy pressure, resulting from the mass loss of a low-mass star.

Electron Degeneracy Pressure

A quantum mechanical pressure arising from the Pauli exclusion principle that prevents electrons from occupying the same energy state, thus supporting white dwarfs against further collapse.

Core Collapse

A catastrophic implosion of a massive star's core, leading to type II supernovae and the formation of neutron stars or pulsars.

Type II Supernova

An explosive event marking the end of a massive star’s life, characterized by the collapse of its iron core and a subsequent explosion.

Neutron Star / Pulsar

The dense remnants left behind after a core-collapse supernova; pulsars are rapidly rotating neutron stars emitting beams of radiation.

Binary System

A system of two stars orbiting a common center of mass, where mass transfer episodes can lead to dramatic events such as novae and type Ia supernovae.

Novae

A sudden brightening of a star due to a thermonuclear explosion on the surface of a white dwarf in a binary system, typically triggered by mass transfer.

Type Ia Supernova

An explosive event in a binary system where a white dwarf accretes enough mass from its companion to reach a critical limit, leading to a thermonuclear explosion.

Gamma-Ray Burst

A highly energetic and beamed emission event, believed to be associated with massive stellar explosions, that provides insights into the early universe.

Example Problems

Example 1

How does a white dwarf differ from a neutron star? How does each form? What keeps each from collapsing under its own weight?

Example 2

Describe the evolution of a star with a mass like that of the Sun, from the main-sequence phase of its evolution until it becomes a white dwarf.

Example 3

Describe the evolution of a massive star (say, 20 times the mass of the Sun) up to the point at which it becomes a supernova. How does the evolution of a massive star differ from that of the Sun? Why?

Example 4

How do the two types of supernovae discussed in this chapter differ? What kind of star gives rise to each type?

Example 5

A star begins its life with a mass of $5 \mathrm{M}_{\text {sun. List }}$ but ends its life as a white dwarf with a mass of $0.8 \mathrm{M}_{\text {sun. List }}$ the stages in the star's life during which it most likely lost some of the mass it started with. How did mass loss occur in each stage?
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Step-by-Step Explanations

QUESTION

How does a low-mass star evolve to become a white dwarf?

STEP-BY-STEP ANSWER:

Step 1: The low-mass star exhausts its nuclear fuel, first using hydrogen and then helium in its core.
Step 2: As the nuclear reactions cease, the star sheds its outer layers, creating a planetary nebula and leaving behind the core.
Step 3: The remaining core contracts under gravity and is halted from collapsing further by electron degeneracy pressure.
Step 4: The final, stable remnant is a white dwarf, which slowly cools over time.
Final Answer:

White Dwarf Formation

QUESTION

What triggers a type Ia supernova in a binary star system?

STEP-BY-STEP ANSWER:

Step 1: In a binary star system, a white dwarf accretes matter from its companion star.
Step 2: As the white dwarf’s mass increases, it approaches the Chandrasekhar limit (approximately 1.4 solar masses).
Step 3: Exceeding this limit causes the white dwarf to undergo runaway thermonuclear reactions.
Step 4: The result is a powerful and uniform explosion known as a type Ia supernova.
Final Answer:

Type Ia Supernova in Binary Systems

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Common Mistakes

  • Confusing the roles of electron degeneracy pressure with thermal pressure in white dwarfs.
  • Misunderstanding the difference between type Ia and type II supernovae, particularly regarding their progenitor systems.
  • Assuming that all supernovae result in the formation of black holes, rather than recognizing the formation of neutron stars or white dwarfs in certain mass ranges.
  • Overlooking the influence of binary interactions in triggering novae and type Ia supernovae.