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Lecture on Stellar Evolution

Figure 22.1: Ant Nebula

  • During later phases of stellar evolution, stars expel mass that returns to the interstellar medium to form new stars.
  • Hubble Space Telescope image shows a star (Menzel 3, or Ant Nebula) losing mass.
  • Distance from the Sun: approximately 3000 light-years.
  • Central star has ejected mass in two opposite directions.
  • The Ant Nebula is about 1.6 light-years long.
  • Image color coding:
    • Red = emission line of sulfur
    • Green = emission line of nitrogen
    • Blue = emission line of hydrogen
    • Blue/Violet = emission line of oxygen
  • Credit: NASA, ESA, Hubble Heritage Team (STScI/AURA)

Chapter Outline

  • 22.1 Evolution from the Main Sequence to Red Giants
  • 22.2 Star Clusters
  • 22.3 Checking Out the Theory
  • 22.4 Further Evolution of Stars
  • 22.5 The Evolution of More Massive Stars

Thinking Ahead

  • The Sun and other stars cannot last forever; they will exhaust their nuclear fuel and cease to shine.
  • Question: How do stars change during their long lifetimes?
  • Importance: Understanding these changes is crucial for the future of Earth.
  • Stars have different characteristics based on mass, temperatures, luminosities, and aging processes.
  • Observational studies combined with theories help piece together the life story of stars.

22.1 Evolution from the Main Sequence to Red Giants

Learning Objectives

  • Understand the H–R diagram as a tool for plotting star properties and tracking their evolution beyond the main sequence.

Main Sequence Phase

  • Hydrogen fusion in a star's core is the primary energy source during the main-sequence stage.
  • Duration on the main sequence is primarily influenced by the star's mass.
  • The main-sequence phase is likened to a star's “adolescence” or “adulthood.”
  • The zero-age main sequence marks when a star stops contracting and begins hydrogen fusion.
  • Key terms:
    • Zero-age main sequence: The location in the H–R diagram when stars begin hydrogen fusion.
  • Fusion process: Only 0.7% of hydrogen is converted into energy, resulting in minimal mass change.
  • Chemical composition changes significantly:
    • Hydrogen depletion occurs in the core, and helium accumulation follows.

Changes in Stars

  • As hydrogen depletes in the core and helium accumulates, the star's luminosity, temperature, and size change.
  • Once changes occur, the star's point moves away from the zero-age main sequence on the H–R diagram.
  • Core temperature and density gradually increase, enhancing fusion rates.
  • For example, if temperature doubles, fusion rate increases by a factor of $2^4 = 16$.

Lifetimes on the Main Sequence

  • Star mass influences lifetime in the main-sequence phase; more massive stars consume fuel faster.
  • Comparison:
    • More massive stars consume fuel rapidly due to higher core temperatures.
    • Example:
    • 1 solar mass star: ~10 billion years
    • .4 solar mass star: ~200 billion years
    • Massive stars: spent 1 million years on the main sequence.
  • Table 22.1 summarizes main-sequence lifetimes across various stellar masses.

Star Mass and Color

  • The mass of a star correlates with its luminosity and temperature during main-sequence evolution.

22.1 • Transition from Main Sequence to Red Giant

Evolution Process

  • Once core hydrogen supply is exhausted, the star's core contracts, producing heat through gravitational collapse.
  • This heat allows hydrogen fusion in a shell outside the core, leading to outer layer expansion.
  • The transition represents a critical phase change; the star grows large, redder, and more luminous.
  • The star leaves the main-sequence band on the H–R diagram, moving upward and to the right.
  • For massive stars, the evolution to red supergiant occurs, while lower-mass stars become red giants.

Comparison between Red Giants and Main-Sequence Stars

  • Characteristics of Betelgeuse compared with the Sun:
    • Mass: 1 vs. 16
    • Radius: 700,000 km vs. 500,000,000 km
    • Surface temperature: 5,800 K vs. 3,600 K
    • Luminosity: 1 vs. 46,000
    • Age: 4.5 billion years vs. 10 million years.

22.2 Star Clusters

Characteristics and Types of Clusters

  • Star clusters are groups of stars formed together with similar age and composition.
  • Types:
    • Globular Clusters: Over 150 known; spherical shape, containing hundreds of thousands of stars, typically much older stars.
    • Open Clusters: Found in the disk of the Galaxy; smaller in number and age, usually contains a few dozen to several hundred stars.
    • Stellar Associations: Group of very young stars indicating recent star formation, typically containing hot, bright stars.
  • Table 22.3 summarizes their differences in number, structure, age, and location in the Galaxy.

22.3 Checking Out the Theory

Age Determination through H–R Diagram

  • The H–R diagram changes systematically as clusters age, helping to determine starlife stages.
  • Younger clusters show bright blue stars in their main sequence, while older clusters lack such stars.

Observational Evidence

  • NGC 2264 (Christmas Tree Cluster) serves as an example of young clusters that still contain interstellar matter for star formation.

22.4 Further Evolution of Stars

Helium Fusion and Stellar Changes

  • The ignition of helium fusion (triple-alpha process) marks significant evolutionary changes in low-mass stars:
    • Helium flash results from rapid energy generation in the star’s core, producing a brief burst of helium fusion.
  • Stars can further produce other elements via additional fusion processes.

Creation of Planetary Nebulae

  • Aging stars result in planetary nebula formation after they shed outer layers, revealing hot inner regions.
  • These nebulae glow from UV radiation from the central star.
  • Characteristics of planetary nebulae include distinct shapes and structures formed by mass loss processes during late-stage evolution.

22.5 The Evolution of More Massive Stars

Fast Evolution of Massive Stars

  • High-mass stars evolve much more quickly, undergoing multiple stages of fusion processes through various elements (up to iron).
  • Ultimately, massive stars die largely through supernova explosions, which create and disperse heavy elements into space.

Chemical Composition Differences

  • Stars in globular clusters are chemically poorer in heavy elements compared to those in open clusters, reflecting their age differences.

Key Terms

  • Association: A loose group of young stars.
  • Globular Cluster: A large, spherical cluster of hundreds of thousands of stars around a galaxy center.
  • Helium Flash: The rapid ignition of helium at the core of a red giant star.
  • Main-sequence Turnoff: Point in H–R diagram where stars begin to leave the main sequence.
  • Nucleosynthesis: Process of forming heavy elements from lighter ones.
  • Open Cluster: A relatively loose group of stars within the Galaxy.
  • Planetary Nebula: Ejected gas shell from a dying low-mass star.
  • Triple-alpha Process: Fusion of three helium atoms into carbon.
  • Zero-age Main Sequence: Denotes main sequence for newly formed stars.

Summary

  • Stars evolve through stages defined by mass, changing their structure and fusion processes.
  • Lifetimes on the main sequence vary significantly, with massive stars evolving more rapidly and ultimately leading to different end stages.
  • Star clusters are essential for understanding stellar evolution due to their identical formation histories and varied mass distributions.