ASTR 1120ASTR 1120General Astronomy:General Astronomy:Stars & GalaxiesStars & Galaxies

!omework #6

on Mas"ring As#onom$due on Tuesday, %ov. 03, by 5p&

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The Milky WayThe Milky Way

Size of the Milky WaySize of the Milky Way

•• 100-400100-400 billion stars

• 100,000 light years in

diameter

• Sun (and us) are

located ~28,000 light~28,000 light

yearsyears from the center,

in the ‘Orion Arm’

Artist’s sketch

Milky Way Milky Way AnatomyAnatomy –– Spiral Galaxy Spiral Galaxy

Disk, Bulge & HaloDisk, Bulge & Halo

•• DiskDisk:: includesincludes

spiral armsspiral arms ----

young, new staryoung, new star

formationformation

•• Bulge & HaloBulge & Halo::older stars,older stars,

globular clustersglobular clusters

Artist’s sketch

Disk is very thin!Disk is very thin!

What Milky Way What Milky Way mightmight look like! look like!

Spiral galaxy

NGC 4414

Galaxy NGC 4565 ! nearly edge-on

What kind of object lie in the What kind of object lie in the halohalo of our of our

Galaxy?Galaxy?

A. O and B stars.

B. Gas and dust.

C. Globular clusters

D. Open clusters.

E. All of the above.

Clicker QuestionClicker Question

What kind of object lie in the What kind of object lie in the halohalo of our of our

Galaxy?Galaxy?

A. O and B stars.

B. Gas and dust.

C. Globular clusters

D. Open clusters.

E. All of the above.

Clicker QuestionClicker Question

Where does most star formation occurWhere does most star formation occur

in the Milky Way todayin the Milky Way today??

A. In the halo

B. In the bulge

C. In the spiral arms

D. In the Galactic center

E. Uniformly throughout the MilkyWay

Clicker QuestionClicker Question

Where does most star formation occurWhere does most star formation occur

in the Milky Way todayin the Milky Way today??

A. In the halo

B. In the bulge

C. In the spiral arms

D. In the Galactic center

E. Uniformly throughout the MilkyWay

Clicker QuestionClicker Question

An important QuestionAn important Question

How do we know all we knowHow do we know all we know

about the Milky Way?about the Milky Way?

Mapping the Milky WayMapping the Milky Way

•• GalileoGalileo "For the Galaxy is nothing else than a congeries of

innumerable stars distributed in clusters."

•• William & Caroline HerschelWilliam & Caroline Herschel (1785): star counts

– Counted stars along 683 lines of sight using their 48-inch

telescope.

Conclusion: Sun is in the center

and MW width is about 5 times its thickness

6,500 ly

30,000 ly

ShapleyShapley’’s s globular clustersglobular clusters

• Harlow Shapely measured distances to globular clusters

– These appeared to be centered on a location tens of thousands oflight-years from the Sun.

Conclusion: Sun not in center, about 2/3 out

How Do Stars Orbit in Our Galaxy?How Do Stars Orbit in Our Galaxy?

Stars in the diskStars in the disk all orbit in the same all orbit in the same

direction with a little up-and-down motiondirection with a little up-and-down motion

• If they get too far above or below the disk, the gravity

of everything in the disk pulls them back in

Orbits of Orbits of stars in the bulge and halostars in the bulge and halo have have

random orientationsrandom orientations

• Evidence points to bulge and halo formed before

the disk existed

– Their orbits not affected (much) by the gravity of the disk

Why spiral arms?Why spiral arms?

““Density wavesDensity waves”” ––stars move in andstars move in andout of denserout of denserregionsregions

Like Like ripples in a pondripples in a pond

In dense regions,In dense regions, starstarformation is moreformation is moreintenseintense, , so so ““armsarms””are brighterare brighter

M51 - WhirlpoolM51 - Whirlpool

Why do orbits ofWhy do orbits of disk stars bob up anddisk stars bob up and

down?down?

A. They’re stuck to the interstellar mediumwhich moves like that because of its density.

B. The gravity of disk stars pulls toward disk

C. The halo stars push them back into disk

D. Their orbits carry them out but then theybounce off the edge of the Galaxy and headback in.

E. The density waves in the spiral arms kickthem out of the disk.

Clicker QuestionClicker Question

Why do orbits ofWhy do orbits of disk stars bob up anddisk stars bob up and

down?down?

A. They’re stuck to the interstellar mediumwhich moves like that because of its density.

B. The gravity of disk stars pulls toward disk

C. The halo stars push them back into disk

D. Their orbits carry them out but then theybounce off the edge of the Galaxy and headback in.

E. The density waves in the spiral arms kickthem out of the disk.

Clicker QuestionClicker Question

Galaxies: Ultimate Galaxies: Ultimate Recyling Recyling PlantsPlants

Which generation of stars do youWhich generation of stars do you

expect to be more metal-rich?expect to be more metal-rich?

A. Older population of stars (i.e. starsformed a very long time ago)

B. Younger population of stars (i.e.formed more recently)

C. No difference

Clicker QuestionClicker Question

Which generation of stars do youWhich generation of stars do you

expect to be more metal-rich?expect to be more metal-rich?

A. Older population of stars (i.e. starsformed a very long time ago)

B. Younger population of stars (i.e.formed more recently)

C. No difference

Clicker QuestionClicker Question

Contents: Contents: Cold stuffCold stuff

• Molecular CLOUDS

– Mostly atomic hydrogen,

some helium and other

molecules

• Dark, dusty, cold

– 10-30K

• Emit molecular

emission lines in far IR,

radio

•• Orion imageOrion image here inhere in

carbon monoxidecarbon monoxide (CO) (CO)

–– colors are Doppler shiftscolors are Doppler shifts

COCO

DopplerDoppler

ImagesImages

Molecular clouds =Molecular clouds =

star forming regionsstar forming regions

Mapping Mapping ColdCold Hydrogen Hydrogen

•• Even the coldestEven the coldest

hydrogen emitshydrogen emits

faint emission linesfaint emission lines

in thein the RADIORADIO

•• Change in Change in energyenergy

levels of nuclearlevels of nuclear

configurationconfiguration

•• Wavelength: Wavelength: 21 cm21 cm

All sky 21 cm radio mappingAll sky 21 cm radio mapping

Semi-Warm stuffSemi-Warm stuff

•• Dust:Dust:

–– absorbsabsorbs visible visible

and UV lightand UV light

–– TransparentTransparent to to

longlong

wavelengthswavelengths

(red, IR, radio)(red, IR, radio)

•• Emits IR lightEmits IR light

Horsehead Horsehead NebulaNebula

Dust+darkDust+dark

molecularmolecular

cloudsclouds

HorseheadHorsehead

in close-upin close-up

VLTVLT

(Very Large(Very Large

Telescope)Telescope)

•• Hot starsHot stars exciteexcite

atomic transitionsatomic transitions

in hydrogen andin hydrogen and

other light elementsother light elements

in the gasin the gas

•• T~ 10,000 K nearT~ 10,000 K near

hot young starshot young stars

Ionization nebulaeIonization nebulae

““O & B star O & B star

associationsassociations””

Lagoon NebulaLagoon Nebula

Hot stuffHot stuff

If we took a spectrum of the gas in anIf we took a spectrum of the gas in an

ionization nebula, what should weionization nebula, what should we

expect to see?expect to see?

A.A. A continuous spectrumA continuous spectrum

B.B. Emission lines of hydrogen plus a few otherEmission lines of hydrogen plus a few other

slightly heavier elementsslightly heavier elements

C.C. Emission lines ofEmission lines of elements all the way up toelements all the way up to

iron andiron and a lot of heavier elementsa lot of heavier elements

D.D. Absorption lines of a number of elementsAbsorption lines of a number of elements

E.E. Absorption linesAbsorption lines from hydrogen onlyfrom hydrogen only

Clicker QuestionClicker Question

A.A. A continuous spectrumA continuous spectrum

B.B. Emission lines of hydrogen plus a few other slightly heavierEmission lines of hydrogen plus a few other slightly heavierelementselements

C.C. Emission lines ofEmission lines of elements all the way up to iron andelements all the way up to iron and a lot ofa lot ofheavier elementsheavier elements

D.D. Absorption lines of a number of elementsAbsorption lines of a number of elements

E.E. Absorption linesAbsorption lines from hydrogen onlyfrom hydrogen only

Trifid Trifid nebula (M20)nebula (M20)

visiblevisible Spitzer infraredSpitzer infrared

Stellar nursery (Stellar nursery (Sharpless Sharpless 140)140)

Spitzer IR image shows deeply embeddedSpitzer IR image shows deeply embedded

O-type stars within dark dust cloud encasing themO-type stars within dark dust cloud encasing them

(10 (10 ly ly across)across)

Really Hot StuffReally Hot Stuff

Hot starHot star

winds fromwinds from

aging starsaging stars

SupernovaSupernova

RemnantsRemnants&&

PlanetaryPlanetary

NebulaNebula

More hot stuffMore hot stuff

Some stuff is Some stuff is REALLY HOTREALLY HOT

•• Bubbles of hot gasBubbles of hot gasblown out byblown out bySUPERNOVAESUPERNOVAE

•• T = tens of millionsT = tens of millionsof degrees Kof degrees K

•• Mixing with rest ofMixing with rest ofgalactic gas galactic gas !!enrichment withenrichment withheavy elementsheavy elements

Superbubbles Superbubbles & Fountains& Fountains

•• WhenWhen multiple bubblesmultiple bubblesjoin (from a cluster) theyjoin (from a cluster) theycan create can create superbubblessuperbubbles..

•• Superbubbles Superbubbles cancan blastblasthot gashot gas eveneven out of theout of theGalaxy!Galaxy!

•• ““EnrichesEnriches”” gas between gas betweengalaxiesgalaxies

•• Some will rain back downSome will rain back downand mix into the Galaxyand mix into the Galaxy

ArtistsArtists’’ conceptions! conceptions!

Edge-on view of spiral galaxy Edge-on view of spiral galaxy NGC 4013NGC 4013

•• Dust/gas in diskDust/gas in diskobscures lightobscures light

•• Plumes and fuzzPlumes and fuzzsticking out aresticking out are““fountainsfountains”” & &““superbubblessuperbubbles””from supernovaefrom supernovae

HSTHST

SuperbubblesSuperbubbles in spiral galaxy NGC 3079in spiral galaxy NGC 3079

HSTHST

Inside our Galaxy: Material isInside our Galaxy: Material is

Constantly Being RecycledConstantly Being Recycled