Download Galaxies

The Milky Way Galaxy
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• NGC 4103, 55 million LY away
• edge on spiral galaxy
• dark dust band 500 LY thick
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side
top
center of galaxy
obscured by dust
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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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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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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?
• 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
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Spiral pattern for billions of years?
Spiral density waves
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Spiral Density Waves: Traffic Jam
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
Galactic Center
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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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
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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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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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
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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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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
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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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Major Parts of
the Milky Way
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Galaxy Formation