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The Milky Way2
The Milky Way
Almost everything we see in thenight sky belongs to the Milky Way.
We see most of the Milky Way as afaint band of light across the sky.
From outside, our Milky Way mightvery much look like our cosmic
neighbor, the Andromeda Galaxy.
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First Studies of the Galaxy
First attempt to unveil thestructure of the galaxy by
William Herschel (1785), basedon optical observations.
The shape of the Milky Way wasbelieved to resemble a grindstone,
with the sun close to the center4
Determining the Structureof the Milky Way
Galactic Plane
Galactic Center
The structure of our Milky Way is hard to determine because:1) We are inside.2) Distance measurements are difficult.3) Our view towards the center is
obscured by gas and dust.
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Measuring Distances:The Cepheid Method
Instability Strip
•The moreluminous aCepheid variable,the longer itspulsation period.
•Observing theperiod yields ameasure of itsluminosity andthus its distance!
•Cepheids allowus to measure thedistances to starclustersthroughout theMilky Way
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Period-Luminosity Relation• We now know there are 2
types of Cepheids• Prototype is δ Cephei• In-class project:
– determine period of δCephei using observationsof AAVSO
– use period and period-luminosity relation tocalculate M and distance
– next Mon, Wed
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Extra Credit:Heanrietta Swan Leavitt
• Use the web to find out about HeanriettaLeavitt.
• Write a short summary (~2 paragraphs)about her life and work.
• Due Friday 3/31.
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Exploring the GalaxyUsing Clusters of Stars
Two types of clusters of stars:
1) Open clusters = young clusters of recentlyformed stars; within the disk of the Galaxy
2) Globular clusters = old, centrally concentratedclusters of stars; mostly in a halo around the galaxy
Globular ClusterM 19
Open clusters hand χ Persei
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Globular Clusters
• Dense clusters of 50,000 – a million stars
• Approx. 200 globular clusters in our Milky Way
• Old (~ 11 billion years), lower-main-sequence stars
Globular Cluster M80
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Locating the Center of the Milky Way
• Harlow Shapley usedCepheids to measuredistances to globular clusters
• Distribution of globularsclusters was NOT centeredon sun
• Center was in a heavilyobscured location in theconstellation of Sagittarius
• Sun lived on outskirts ofgiant wheel of stars
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Model of the Milky Way
After Shapley
Before Shapley
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The Structure of the Milky Way75,000 light years
Disk
Nuclear Bulge
Halo
Sun
Globular Clusters
Open Clusters,O/B Associations
Sun is ~8.5 kpc from center
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Stellar Populations
Population I: Young stars:metal rich; located in spiral
arms and disk
Population II: Old stars: metalpoor; located in the halo(globular clusters) and
nuclear bulge
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Radio Maps of Milky Way• 21-cm emission
– Hyperfine transition of neutral H
15 16
Structure of Milky Way
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The Structure of theMilky Way Revealed
Distribution of dust
Sun
RingBar
Distribution of starsand neutral hydrogen
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Large MagellanicCloud
Small MagellanicCloud
Sagittariusdwarf stream
Orion star-forming
region
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Infrared View of the Milky Way
Interstellar dust(absorbing optical light)emits mostly infrared.
Near-infrared image
Infrared emission is notstrongly absorbed andprovides a clear view
throughout the Milky Way
Nuclear bulge
Galactic plane
Far-infrared image 20
The Formation ofthe Milky Way
Similar process asformation of solar system
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Modifications of theTraditional Theory
Recently discovered ring ofstars around the Milky Way
may be the remnant of amerger.
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Orbital Motions in the Milky Way (II)
Differential RotationSun orbits aroundgalactic center at
220 km/s
1 orbit takes approx.240 million years.
Stars closer to thegalactic center
orbit faster.
Stars farther out orbitmore slowly.
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Mass determinationfrom orbital velocity:
The more mass there isinside the orbit, the faster
the sun has to orbitaround the galactic
center.
Combined mass:
M = 4 billion MsunM = 11 billion MsunM = 25 billion MsunM = 100 billion MsunM = 400 billion Msun
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The Mass of the Milky WayIf all mass was concentrated in thecenter, Rotation curve would follow amodified version of Kepler’s 3rd law.
Rotation Curve = orbital velocityas function of radius.
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The Mass of the Milky Way (II)Total mass in the disk
of the Milky Way:
Approx. 200 billionsolar masses
Additional mass in anextended halo:
Total: Approx. 1 trillionsolar masses
Most (>80%) of themass is not emitting
any radiation:
dark matter!
One of Vera Rubin's importantcontributions to astrophysics is thecollection of conclusive data pointing tothe presence of dark matter in galaxies.These data are measurements of theorbital velocities of interstellar matter ingalaxies. She studied the variation ofthese velocities with distance from thecenter of the galaxy.
It is assumed that matter orbits aboutthe center of a galaxy owing to acentripetal force which is thegravitational attraction of other matterin the galaxy. Assuming all other matterin the galaxy is luminous,astrophysicists cannot account for thecentripetal accelerations observed.These can be accounted for, however, ifadditional matter is present.
Consequently, Rubin's measurementswere of fundamental importance asempirical evidence for dark matter.
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The Nature of Spiral Arms
Chance coincidence of small spiral galaxyin front of a large background galaxy
Spiral arms appearbright (newly formed,
massive stars!)against the dark sky
background,
but dark (gas and dustin dense, star-forming
clouds) against thebright background of
the large galaxy
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The Galactic Center (I)
Wide-angle optical view of the GC region
galactic center
Our view (in visible light) towards the Galactic center (GC)is heavily obscured by gas and dust:
Extinction by 30 magnitudes
Only 1 out of 1012 optical photons makes itsway from the GC towards Earth!
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Radio View of the Galactic CenterMany supernova remnants;
shells and filaments
Sgr A
Arc
Sgr A*: The center of our galaxy
The galactic center contains a supermassiveblack hole of approx. 2.6 million solar masses.
Sgr A
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Measuring the Mass of the BlackHole in the Center of the Milky Way
By following the orbits ofindividual stars near the centerof the Milky Way, the mass ofthe central black hole could bedetermined to be ~ 2.6 million
solar masses.
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Astrometry of Galactic Center Stars
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Flare of Sag A*
http://www.mpe.mpg.de/ir/GC/index.php
May 9, 2003
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