Galaxies: Our Cosmic Neighborhood Unveiled
The Great Galaxy Debate of 1920
● In 1920, astronomers Harlow Shapley and Heber Curtis
debated the nature of the Universe
● Competing theories:
● Shapley: Milky Way is the entire Universe
● Curtis: Spiral nebulae are separate "island Universes"
● Neither side had complete evidence at the time
The Breakthrough: Hubble's Discovery
● Edwin Hubble observed the Andromeda Nebula (M31) in 1923
● Discovered Cepheid variable stars in Andromeda
● Calculated distance of 900,000 light-years
● Proved Andromeda was a separate galaxy beyond the Milky
Way
What is a Galaxy?
● Massive collection of stars, gas, and dust
● Typically contains hundreds of millions to trillions of stars
● Ranges from 10,000 to 100,000 light-years across
● Our home: The Milky Way Galaxy
Galaxy Classification: Spiral Galaxies
● Characterized by flat, rotating disk
● Central bulge of older, redder stars
● Often have a bar of stars through the center
● Types include:
● Grand Design spirals
● Flocculent spirals
● Tightly or loosely wound arms
Viewing Spiral Galaxies
● Appearance changes based on viewing angle
● Face-on view reveals most detail
● Edge-on view shows flat disk structure
● Star-forming regions visible as pink hydrogen clouds
● Young blue stars trace spiral arms
Elliptical Galaxies
● Shaped like cotton balls or footballs
● Lack distinct structure
● Minimal gas and dust
● Populated by older, redder stars
● Believed to form from galactic collisions
Galactic Collisions: Cosmic Train Wrecks
● Galaxies can collide at hundreds of kilometers per second
● Collision process takes hundreds of millions of years
● Surprisingly, stars rarely physically collide
● Gas clouds slam together, forming new stars
Galactic Cannibalism
● Large galaxies grow by consuming smaller galaxies
● Our Milky Way is currently eating smaller galaxies
● Evidence:
● Terzan 5 star cluster
● Streams of stars from Sagittarius and Canis Majoris dwarf
galaxies
Peculiar Galaxies
● Unusual shapes caused by galactic interactions
● Often result from collisions
● Examples:
● Cartwheel Galaxy
● Hoag's Object
● Can form ring-like structures
Irregular Galaxies
● Shapeless or chaotic structure
● Typically smaller galaxies
● May be:
● Too small to form a defined shape
● Damaged by previous collisions
● Examples: Magellanic Clouds
Satellite Galaxies
● Smaller galaxies orbiting larger ones
● Milky Way has about two dozen satellites
● Notable examples:
● Large Magellanic Cloud
● Small Magellanic Cloud
● Contains active star-forming regions
The Expanding Universe Perspective
● Astronomy continually challenges our understanding
● Humans are not the center of the Universe
● Each discovery reveals:
● We are smaller than we thought
● The Universe is more vast and complex
● Humility in scientific exploration
Active Galaxies and Galactic Clusters
● Some galaxies emit extraordinary amounts of energy
● Galaxies are not evenly distributed
● Form clusters containing thousands of galaxies
● Ongoing research continues to expand our understanding
Reflection: Our Cosmic Context
● We are part of a vast, dynamic Universe
● Galaxies are living, changing structures
● Continuous scientific exploration reveals:
● Complexity of cosmic structures
● Interconnectedness of celestial bodies
● Astronomy helps us understand our place in the Universe
Key Takeaways
● Galaxies come in multiple types and shapes
● Galaxies are dynamic, evolving systems
● Collisions and interactions are common
● Our understanding of the Universe is constantly expanding
● The Milky Way is just one of billions of galaxies
