1. The Milky Way Galaxy
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3. NGC 4103, 55 million LY away edge on spiral galaxy
dark dust band 500 LY thick
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4. side
top

5. center of galaxy
obscured by dust
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6. 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
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7. 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
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8. 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

9. 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

10. Rotation Speeds Orbital Period: 240 Myr
Inner Parts: Rise from Zero to few 100 km/sec
Outer Parts: Nearly constant at a few 100 km/sec
Orbital Period: 240 Myr

11. Rotation  Spiral Arms
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12. Spiral pattern for billions of years?
Spiral density waves
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14. Spiral Density Waves: Traffic Jam

15. 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

16. Galactic Center

17. 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
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18. Stellar Populations Population II: Spheroid Stars
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

19. 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
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20. 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
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21. Contrast & Compare Population I Disk & open clusters Young & old stars
Metal-rich
Blue M-S stars
Ordered, circular orbits in a plane
Gas-rich environment with recent star formation
Population II
Spheroid & globular clusters
Oldest stars
Metal-poor
No blue M-S stars
Disordered, elliptical orbits in all directions.
Little or no gas & dust, and no star formation
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22. 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
FIND: more mass than we see -- dark matter halo
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23. 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!
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24. Major Parts of the Milky Way
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25. Galaxy Formation