26.2 Types of Galaxies

Galaxies come in various shapes and sizes, each with unique characteristics. From elliptical to spiral and irregular, these cosmic structures showcase the diversity of our universe. Understanding galaxy types helps us grasp the broader picture of galactic evolution and formation.

Galaxy shapes can change over time through mergers, interactions, and internal processes. These transformations affect star formation rates, gas content, and overall structure. By studying galaxy dynamics and composition, we gain insights into the complex interplay of visible matter, dark matter, and stellar populations.

Galaxy Types and Characteristics

Types of galaxy structures

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• Elliptical galaxies exhibit smooth, elliptical shape with no distinct features
  • Contain mostly old, red stars and little to no gas or dust for new star formation
  • Classified based on their ellipticity from E0 (nearly spherical) to E7 (highly elongated)
• Spiral galaxies feature distinct spiral arms extending from a central bulge
  • Contain a mix of young, blue stars (in spiral arms) and older, red stars (in bulge)
  • Rich in gas and dust, allowing for ongoing star formation (Milky Way, Andromeda)
  • Classified as barred (SB) with central bar or unbarred (SA) without bar
• Irregular galaxies lack a distinct, organized structure
  • Often smaller than elliptical and spiral galaxies (Large and Small Magellanic Clouds)
  • May have been distorted by gravitational interactions with other galaxies
• Galaxy morphology and the Hubble sequence provide a classification system for these structures

Galaxy shape evolution

• Galaxy mergers and interactions can cause galaxies to collide and merge
  • Resulting in a change of shape and structure (Antennae Galaxies)
  • Mergers can trigger intense star formation and lead to the formation of larger galaxies
• Secular evolution involves gradual changes in a galaxy's structure over time due to internal processes
  • Formation of a central bar or the winding up of spiral arms
• Environmental effects can strip galaxies in clusters of their gas and dust by the intracluster medium
  • Ram pressure stripping can transform spiral galaxies into lenticular (S0) galaxies

Components of spiral galaxies

• Bulge forms the central, spheroidal component of a spiral galaxy
  • Contains mostly older, red stars and little to no gas or dust
  • Can be classical (smooth, elliptical) or pseudobulges (disk-like with younger stars)
• Disk is the flat, rotating component of a spiral galaxy
  • Contains a mix of young, blue stars and older, red stars
  • Rich in gas and dust, allowing for ongoing star formation
  • Divided into the thin disk (younger stars and gas) and the thick disk (older stars)
• Spiral arms are regions of enhanced star formation and density within the disk
  • Contain many young, massive, blue stars and bright HII regions (Orion Nebula)
  • Thought to be caused by density waves that compress gas and trigger star formation
  • Can be grand design (well-defined, symmetric) or flocculent (patchy, less distinct)

Galaxy dynamics and composition

• Dark matter halo surrounds and extends beyond the visible galaxy
  • Influences the galactic rotation curve, which shows how rotation speed varies with distance from the center
• Stellar populations within galaxies can be categorized based on age and composition
  • Population I stars are younger, metal-rich stars found in the disk
  • Population II stars are older, metal-poor stars found in the halo and bulge
• Some galaxies contain active galactic nuclei, extremely luminous central regions powered by supermassive black holes