Nebulae & Island Universes
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text reference BWS pp.169-198
Nebulae of different types
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Diffuse Nebulae
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Planetary Nebulae
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Star Clusters
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100 - 100,000 stars
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gravitationally bound
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Dense Globular Cluster: 47
Tucanae
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Smaller Globular Cluster: M5
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Open clusters: M6
& M7
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Open clusters M11
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Spiral Nebulae
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Island Universes (yes) or protostars (no)?
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Shown to be an external Galaxy by Hubble: M31
(Andromeda)
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Nearby spiral galaxy, face-on: M101
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Nearby spiral galaxy, edge-on: NGC
891, note dust lane!
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Other Galaxies
Great Debate
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BWS pp. 187-194
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Debate between Curtis & Shapley on the size of the Galaxy
& Spiral Nebulae
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Shapley
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The Galaxy is very large and we are at the edge (correct!)
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the spiral nebulae are not external galaxies (wrong)
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Curtis
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We are near the center of a "modest" sized Galaxy (wrong)
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The spiral nebulae are external galaxies (right)
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Evidence
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M101 rotation? (observations wrong)
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Nova or in M31 (Andromeda). Was 1885 event a nova or a Super-Nova?
(SN is right!)
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spectra of spiral nebulae don't look like nearby stellar
spectra (light is dominated by giant stars)
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Spiral nebulae avoid the Galactic Plane (dust blocks the
light)
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Spiral nebulae have higher surface brightness than the stellar
neighborhood (we're not near the Galactic Center).
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Most spiral nebulae are receding at high speed ~1000-3000
km/sec
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Galactic dust blocks the light!
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see NGC
891, for example
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dust obscures the plane of the galaxy
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we can't see much of the bright center or spiral galaxies
in the Galactic plane
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Here's a schematic of our place in the Galaxy:
Cepheid Variables: a Cosmic Yardstick
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Some stars are variable: Cepheid
Variables in M101
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In 1912 "calculator" (female astronomer) Henrieta Leavitt
discovers Period-Luminosity Relation
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Observed Large
Magellanic Cloud
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all stars at the same distance (helpful even if we don't
known the distance
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stars have pulsation instability:
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high temperature- more opaque - radiation pressure lifts
atmosphere
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low temperature - less opaque - atmosphere falls back toward
center
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period determines luminosity
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measure the period, then get distance through b = L/D2
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"Standard Candle"
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Complication: 2 types of cepheids:
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type I (plot of brightness vs. time):
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type II (4 times fainter):
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Here's the relation between Cepheid Period and Luminosity:
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Hubble finds Cepheids in Andromeda
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show that spiral nebulae are external Galaxies
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but gets the distance wrong due to confusion between type
I and II Cepheids
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will soon realize that spiral nebulae are redshifted because
the Universe is expanding!