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The Milky Way Galaxy NGC 4103, 55 million LY away edge on spiral galaxy dark dust band 500 LY thick side top center of galaxy obscured by dust The Milky Way Galaxy (1) Our galaxy, the Milky Way, is a rotating disk of stars distributed across a region

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the milky way galaxy

The Milky Way Galaxy

AST 2010: Chapter 24

NGC 4103, 55 million LY away
  • edge on spiral galaxy
  • dark dust band 500 LY thick

AST 2010: Chapter 24




center of galaxy

obscured by dust

AST 2010: Chapter 24

the milky way galaxy 1
The Milky Way Galaxy (1)
  • Our galaxy, the Milky Way, is a rotating disk of stars distributed across a region
    • about 100,000 LY in diameter
    • about 1,000 LY thick
  • In addition to stars, gas and dust are also found in the thin disk of the Galaxy
  • The stars around its center form a nuclear bulge

AST 2010: Chapter 24

the milky way galaxy 2
The Milky Way Galaxy (2)
  • It has 4 major spiral arms plus smaller spurs
    • The major arms are Cygnus, Perseus, Carina, and a fourth unnamed one, which is hard to detect (on other side of bulge)
    • The 3 named arms are each about 80,000 LY long
  • The Sun is near the inner edge of a short arm, or spur, called the Orion arm
    • which is about 15,000 LY long and also contains the Orion Nebula

AST 2010: Chapter 24

spiral arms
Spiral Arms
  • Hot blue stars delineate spiral structure – like Xmas lights on a tree
  • The arms are regions where matter is more densely concentrated
  • Cool orange and red stars are found in and between spiral arms
  • Interstellar dust limits our view in visible light to dashed circle
why spiral arms
Why Spiral Arms?
  • Stars orbit around center of mass of galaxy,
    • like planets
  • Kepler’s laws imply
    • stars near the center are faster
    • stars farther out are slower
  • Differential rotation of stars explains curved shape of spiral arms
Rotation Speeds
    • Inner Parts: Rise from Zero to few 100 km/sec
    • Outer Parts: Nearly constant at a few 100 km/sec
  • Orbital Period: 240 Myr
rotation spiral arms
Rotation  Spiral Arms

AST 2010: Chapter 24


Spiral pattern for billions of years?

Spiral density waves

AST 2010: Chapter 24

spherical structure
Spherical Structure
  • Nuclear Bulge
    • Many RR Lyrae stars
    • A little gas & dust
  • Galactic Halo: outer sphere with very few stars
    • Old metal-poor stars
    • Globular clusters
    • Dark matter
    • RR Lyrae Stars
galaxy mapping with radio waves
Galaxy Mapping with Radio Waves
  • Radio waves are the best for mapping the distribution of hydrogen in the galaxy
    • Their wavelengths are large compared with the size of interstellar dust grains and thus the waves pass easily through dust

Center of the Galaxy

AST 2010: Chapter 24

stellar populations
Population I: Disk Stars

Ordered, roughly circular orbits in a plane

All orbit in the same general direction

Orbit speeds similar at a given radius

Population II: Spheroid Stars

Disordered, elliptical orbits at all inclinations

Mix of regular and retrograde orbits

Wide ranges of orbital speeds

Stellar Populations
population i
Population I
  • Location: Disk and Open Clusters
  • Age: Mix of young and old stars
  • Composition: Metal rich (roughly solar)
  • 70% Hydrogen
  • 28% Helium
  • ~2% "metals"
  • Environment: Often gas rich, especially for the young stars

AST 2010: Chapter 24

population ii
Population II
  • Location: Spheroid and Globular Clusters
  • Ages: Oldest stars, >10 Gyr
  • Composition: Metal Poor (0.1-1% solar)
  • 75% Hydrogen
  • 24.99% Helium
  • ~0.01% metals
  • Environment: gas poor, no star formation

AST 2010: Chapter 24

contrast compare
Population I

Disk & open clusters

Young & old stars


Blue M-S stars

Ordered, circular orbits in a plane

Gas-rich environment with recent star formation

Population II

Spheroid & globular clusters

Oldest stars


No blue M-S stars

Disordered, elliptical orbits in all directions.

Little or no gas & dust, and no star formation

Contrast & Compare

AST 2010: Chapter 24

mass of the milky way
Mass of the Milky Way

Observe orbital period, P, of stars or interstellar matter vs. distance, D, from center

Kepler’s 3rd law  period

of orbit determined by mass

within orbit

D3 = (Mgalaxy + Msun)P2

  • earth orbit  sun’s mass
  • farther from the galactic

center, the more mass

within orbit

FIND: more mass than we see -- dark matter halo

AST 2010: Chapter 24

dark matter
Dark Matter
  • Two possible explanations for the observed rotation:
    • Law of gravity is wrong for galaxies
    • There is additional matter that doesn’t emit detectable radiation (dark matter)
  • No evidence that gravity behaves differently
  • The “dark matter” hypothesis is therefore favored
    • It could be brown dwarfs, black holes, or new exotic particles
  • Measurements indicate about 90% of the mass in the universe is dark matter!

AST 2010: Chapter 24

major parts of the milky way
Major Parts of the Milky Way

AST 2010: Chapter 24