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Class 20: The Milky Way
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Our own galaxy, the Milky Way.
The debate about other galaxies.
The Milky Way
Credit & Copyright Barney Magrath
Credit: Dave Palmer
I: The Milky Way
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Milky Way – bright band of light across sky.
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Galileo
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First to use telescope to study MW.
Found it was made of millions of faint stars.
What is the Milky Way?
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Thomas Wright (1750) suggested that solar system
was embedded in enormous shell of stars…
Emmanuel Kant (1755) suggested instead that the
MW is a giant disk of stars.
Kant also hypothesized that space was full of other,
similar disks of stars (island universes).
LMC
SMC
Inventory of the Milky Way
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About 100 billion stars…
Disk
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Bulge
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50 kiloparsecs (~150,000 ly) in diameter.
Generally contains younger stars.
Dusty and gassy.
Contains older stars.
Less dust and gas.
Other stuff
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Galactic nucleus in the bulge.
Globular star clusters in the halo.
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If our solar system were 1 mm dot…
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Nearest star would be 100 m away.
MW would be size of continental U.S.
Halo would be size of Earth.
Other cool facts:
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Disk is about 0.6 kpc thick.
Central bulge is 2 kpc across.
Sun is located ~8 kpc from center.
Andromeda Galaxy is 750 kpc away.
Spiral Structure
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MW is a spiral galaxy – looks like a
pinwheel (we’ll see other examples).
Structure difficult to detect because we
live inside galaxy – literally can’t see
the forest for the trees…
Careful measurement reveals spiral
arms, and shows that the disk is
rotating differentially.
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Material closer to center orbits faster than
material farther away.
Evolution of Spiral Structure
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Because of differential rotation, any
radial feature gets wound up very
tightly (see figure).
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But spiral arms are “open”, “loose”…
Modern theory: spiral structure due to
waves in density of stars (& gas).
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Spiral arms should be tightly wound!
Traffic jam analogy (see figure).
Spiral arms are bright because high
density promotes formation of big stars.
• Fast cars slow down to pass paint truck
II : Other galaxies?
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What of Kant’s suggestion of other
disks? Need to look at Nebulae.
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Nebulae
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Fuzzy blobs in the sky.
First systematically catalogued by Messier.
Messier was mainly interested in comets…
Credit : A. Dimai
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What were Messier’s objects?
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Possibilities…
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Glowing clouds of gas in between the stars.
Large collections of stars in our own Galaxy.
Whole other galaxies.
Turns out that all of these possibilities
are represented…
The Orion Nebula (M42)
Glowing gas
cloud
The Hercules Cluster (M13)
Ball of
stars in our
own galaxy
Credit: Yuugi Kitahara
Andromeda “Nebula” (M31)
Another galaxy…
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But, idea of external galaxies was vigorously
debated in early 1900s…
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The Great Debate (1920)
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National Academy of Sciences – downtown DC.
Debate between Harlow Shapley and Heber Curtis.
Shapley argued for nebulae all being local (i.e. within
the Milky Way, like the Orion Nebula).
Curtis argued for “island universe” hypothesis (i.e.,
there are many islands of stars like the Milky Way in
the universe).
Really need a good distance indicator to solve
the problem… (next class!).