Chapter 26

Spiral Galaxy IC5332

• Catalog Number: IC5332
• Distance: Approximately 30 million light-years from Earth.
• Location: Constellation of Sculptor.
• Images:
  • Left: Visible light and ultraviolet image from Hubble Space Telescope.
  • Characteristics: Bright stars organized in a grand spiral shape; dark lanes of dust among stars.
  • Right: Image from James Webb Space Telescope’s Mid-Infrared Instrument.
  • Characteristics: Dust is transparent; shows cooler, older, dimmer stars; greater spiral structure complexity.

Chapter Outline

• 26.1 The Discovery of Galaxies
• 26.2 Types of Galaxies
• 26.3 Properties of Galaxies
• 26.4 The Extragalactic Distance Scale
• 26.5 The Expanding Universe

Introduction

• Previous chapter focused on the Milky Way Galaxy.
• Questions raised:
  1. Are there other galaxies?
  2. Are they similar to the Milky Way?
  3. How far are they?
  4. Can we see them?
• Light from extremely distant galaxies reveals the universe's young state.
• Exploration of galaxies likened to tourists visiting great cities, recognizing both beauty and familiarity.
• Aim to understand properties of galaxies first, with future chapters covering their history and evolution.

26.1 The Discovery of Galaxies

Learning Objectives

• By the end of this section, students should be able to:
  • Describe the discoveries that established the existence of galaxies beyond the Milky Way.
  • Explain historical terminology and classifications of nebulae.

Historical Context

• 1920s: Widespread skepticism regarding the existence of other galaxies; the Milky Way was believed by many to encompass all that exists.
• Early observations: Nebulae appeared as indistinct fuzzy patches indistinguishable from star clusters or gas clouds.
• Nebulae: From Latin "clouds"; all non-point light sources referred to as such, creating confusion.
• Immanuel Kant (1724-1804): Suggested nebulae might be distant star systems but lacked evidence due to technological limitations.

Advancements in Understanding Nebulae

• By early 20th century, some nebulae identified as star clusters (e.g. Orion Nebula) or gaseous nebulae, although many were still poorly understood.
• The Mount Wilson 2.5-meter telescope provided clarifications.
• Edwin Hubble's Contribution: Used the telescope to resolve individual stars in brighter spirals, including M31, the Andromeda Galaxy.
  • Discovered variable stars (Cepheids) that allowed for distance measurements.
• Estimated Andromeda's distance at about 900,000 light-years, later revised to over double that.

Importance of Hubble’s Findings

• Hubble's discoveries revolutionized astronomy and led to the establishment of extragalactic astronomy.
• Publication in 1925 resulted in overwhelming recognition among astronomers, marking a new era in cosmic studies.

26.2 Types of Galaxies

Learning Objectives

• At conclusion, students should be able to:
  • Describe and classify various galaxy types based on observational characteristics.
  • Explain evolution and changes in galaxy appearance.

Historical Observations of Galaxies

• Early observational challenges made it difficult to document galaxy properties; nights spent capturing photographs for analysis.
• Larger telescopes and advanced detection technology facilitate modern observations.

Galaxy Classifications

• Basic Shapes: Notable galaxies are classified as either:
  1. Spiral Galaxies: Flat with spiral arms (e.g. Milky Way).
  2. Elliptical Galaxies: Blimp- or cigar-shaped.
  3. Irregular Galaxies: Lacking clear symmetry.
• Spiral galaxies consist of:
  • Central bulge
  • Halo
  • Disk containing spiral arms
  • Presence of interstellar material
• Bright emission nebulae are found within the spiral arms, indicating ongoing star formation.

Specific Characteristics of Spiral Galaxies

• Example galaxies: Milky Way and Andromeda exhibit features typical of large spirals:
  • Variations in brightness (bluer in the spiral areas due to young stars).
  • Dusty appearance notable when viewed edge-on.
• Barred Spiral Galaxies: About 2/3 of nearby spirals show boxy structures of stars (Hubble's classification). Other designations:
  • Sa: Tight arms with faint structures and a larger central bulge.
  • Sc: Loosely wound arms with prominent emission nebulae.
• Spiral galaxies typically have diameters ranging from 20,000 to over 100,000 light-years, with observable masses spanning from 10^9 to 10^{12} solar masses (M_{ ext{Sun}}).

Elliptical Galaxies

• Characterized by spherical to ellipsoid shapes and predominantly older star populations (population II stars).
• Lack of spiral arms with occasional globular clusters.
• Size ranges from giant ellipticals with luminosities up to 10^{11} L_{ ext{Sun}} to dwarf ellipticals with mass typical of bright globular clusters.

Irregular Galaxies

• Defined by their chaotic structures, low mass, and active star formation activities.
• Examples include the Large Magellanic Cloud and Small Magellanic Cloud, significant for studies in star formation and stellar evolution.

26.3 Properties of Galaxies

Learning Objectives

• By the end of this section, students will be able to:
  • Measure galaxy mass based on rotation and motion analysis.
  • Discuss mass-to-light ratios for different galaxy types.

Measuring Galaxy Mass

• Spiral Galaxies: Mass identified by measuring rotational velocity of stars and gas using the Doppler effect.
• Elliptical Galaxies: Mass determined through average speeds of stars in orbit and implications of stellar age for stability.

Characteristics and Ratios

• Mass and diameter vary widely across galaxy types, as summarized in Table 26.1.

Mass-to-Light Ratio Explained

• Useful for gauging a galaxy's characteristics, indicative of star types and dark matter presence level.
• Active star formation correlates with lower ratios compared to older stellar populations.

26.4 The Extragalactic Distance Scale

Learning Objectives

• At conclusion, students will be able to:
  • Understand methods of estimating galaxy distances.

Distance Measurement Techniques

• Difficult to accurately measure galaxy distances due to their vastness.
• Variable Stars: Utilized as distance indicators; variability observed in Cepheid stars allows distance determinations.
  • Historical confusion regarding Cepheid classifications raised the measured distances considerably in the 1950s.

Standard Bulbs and Other Techniques

• Type Ia Supernovae: Implemented as effective standard bulbs due to consistent luminosity.
• Tully-Fisher Relation: Links galaxy luminosity to rotational velocity through observations of cold hydrogen gas.

Summary of Distance Methods

26.5 The Expanding Universe

Learning Objectives

• At conclusion, students will understand concepts surrounding the expanding universe and associated laws.

Discovery of Universal Expansion

• Began with Vesto Slipher's redshift observations shortly after 1900, noting most galaxy spectral lines showed redshift, indicating movement away from Earth.

Hubble’s Law

• Formulated relationship between recession velocity and distance.
• Expressed as: v = H imes d where:
  • v: recessional velocity
  • d: distance
  • H: Hubble constant
• Current estimation of Hubble constant around 22 kilometers per second per million light-years.

Implications of Hubble’s Law

• Expansion of the universe indicates galaxies are receding from one another, leading to an understanding that we reside in a uniformly expanding universe, free from a central point.
• The notion of stretching space mirrors the mechanics behind the observed recession, not intrinsic velocity of galaxies.