The Milky Way Galaxy-Chapter 25
Introduction to the Milky Way Galaxy
- The Milky Way is a barred spiral galaxy containing billions of stars, including our Sun.
- It presents a spectacular visual phenomenon, particularly noticeable in areas with minimal light pollution.
- Historical interpretations of the Milky Way include varying cultural myths, from spilled milk in ancient Greece to pathways of sacred animals among Native Americans.
Chapter Outline
25.1 The Architecture of the Galaxy
25.2 Spiral Structure
25.3 The Mass of the Galaxy
25.4 The Center of the Galaxy
25.5 Stellar Populations in the Galaxy
25.1 The Architecture of the Galaxy
Learning Objectives
- Understand the architecture of the Milky Way Galaxy and be able to outline its structure.
Key Concepts
- Mapping Challenges
- Difficulty in studying the Milky Way as we are positioned within it.
- Analogy: Mapping New York City from Times Square vs. a helicopter.
- William Herschel's Measurements (1785)
- First discovery of the Milky Way's architecture using a reflecting telescope.
- Counting stars across the sky revealed a flattened disk structure, leading to the conclusion of its wheel-like shape with the Sun near the center.
- Visual analogy to a halftime band formation illustrating stellar distribution.
Herschel's Methodology
- Observed stars and concluded the Galaxy's disk shape based on the uniform distribution of stars in varying directions.
- Importance of star density measurements led to early mapping efforts in understanding Galactic structure.
Progress Beyond Herschel
- Harlow Shapley's Contributions
- In contrast to Herschel, Shapley established the true center of the Milky Way through his study of RR Lyrae variable stars in globular clusters in 1917.
- Utilized the inverse square law of brightness, allowing distance calculations and the mapping of globular clusters, revealing they are centered around a point in Sagittarius, not the Sun.
- He identified the Milky Way was not special, being one of hundreds of billions of stars orbiting a distant center.
Modern Observations
- Contemporary tools (radio and infrared) allow astronomers to penetrate galactic dust and understand Galactic anatomy, supporting Shapley’s conclusions while expanding knowledge of its mass and structure.
25.2 Spiral Structure
Learning Objectives
- Describe the spiral structure of the Milky Way Galaxy and the mechanisms behind it.
Overview of Spiral Arms
- Formation of Spiral Structure
- Identification through radio observations indicates two main spiral arms emerging from the central bar.
- Visibility of these features in infrared provides a clearer view of the Galaxy's true shape.
- The Sun resides in the Orion-Cygnus Arm, featuring notable star formation like the Orion Nebula.
Differential Galactic Rotation
- Stars and interstellar matter follow different orbital speeds based on their distance from the center, leading to a differential rotation.
- Spiral arms form as stars in closer orbits move faster than those further out, creating an elongated structure based on the rotational dynamics of the Galaxy.
25.3 The Mass of the Galaxy
Learning Objectives
- Determine the mass of the Milky Way Galaxy and its implications.
Mass Estimation Techniques
- Kepler’s Third Law: Applying Kepler’s law enables calculations of mass through the orbiting speed of the Sun in relation to its distance from the center.
- The Sun orbits at 200 km/s, taking 225 million years for one complete revolution.
- Calculating the total mass of the Galaxy results in approximately 100 billion solar masses.
Dark Matter
- Identified that a vast proportion (up to 95%) of the galaxy's mass is composed of dark matter, which does not emit light and is currently undetectable except via gravitational effects.
- Dark matter extends beyond visible star boundaries up to 200,000 light-years from the center of the Milky Way.
25.4 The Center of the Galaxy
Learning Objectives
- Evidence of supermassive black holes at the center of the Milky Way.
Discoveries About the Galactic Center
- Identification of Sagittarius A* as a supermassive black hole with a mass of around 4.6 million solar masses.
- Descriptions of observational techniques including infrared and radio imaging to bypass interstellar dust limiting visibility.
Structure Observations
- Detailed study of stars nearby Sagittarius A* showing rapid orbital velocities consistent with a dense mass concentration.
- The density of material at the center means that if another type of object had been present, it would collect and collapse into a black hole, providing strong evidence for its existence.
25.5 Stellar Populations in the Galaxy
Learning Objectives
- Differentiate between various stellar populations and their characteristics.
Classification of Stars
- Population I and II Stars
- Population I stars are young, found primarily in the disk, are rich in heavy elements, and several may still be forming.
- Population II stars are older, have low heavy element abundances, and exhibit eccentric orbits across the halo.
- Insights into star formation history illuminate correlations between stellar age and chemical composition, attributing metal-rich environments to the recycling of materials by previous star generations.
Learning Objectives
- Understanding the processes leading to the Milky Way's current architecture.
- Monolithic Collapse Model: Initial formation presumed from a single cloud collapsing under gravity to create a disk-like structure of stars.
- Multiple Merger Events: Evidence of gravitational interactions where smaller galaxies are captured by the Milky Way, altering its structure.
- Detailing the fate of smaller galaxies like the Sagittarius dwarf galaxy.
Expected Future Interactions
- Predictions for the Milky Way's collision with Andromeda galaxy in approximately 3 to 4 billion years leading to a larger elliptical galaxy formation.
Key Terms
- Central Bulge: The rounded, central area of a galaxy.
- Dark Matter: Non-luminous mass inferred from its gravitational influence, composition remains unknown.
- Differential Galactic Rotation: The concept that parts of the galaxy rotate at different speeds.
- Population I Star: Stars rich in heavy elements, typically younger.
- Population II Star: Older stars, low in heavy elements.