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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

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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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33Chandra X ray image of Sgr A*

Supermassive black hole in the galacticcenter is unusually faint in X rays,

compared to those in other galaxies.

Galactic center region contains many black-holeand neutron-star X-ray binaries.

X-Ray View of the Galactic Center